• • . • • • • • • S D 0 1 0 0 0 2 1

'University of Khartoum

Faculty of Education

Department of Chemistry

Direct Synthesis of Organic

Silicates

A Thesis Submilled lor llie Degree oI 'MSc. in Chemistry

By

liana Hassan Cisinalla

Supervisor

Dr. Onicr Yousif Onier

June 2000

3.2/. 2 7

SOME PAGES ARE MISSINGIN THE

ORIGINAL DOCUMENT

DISCLAIMER

Portions of this document may beillegible in electronic image products.

Images are produced from the bestavailable original document

ContentsQuran :.-..,.. , .. .... , I

Dedication.. ..... ... IIContents...... IllList of Tables: ....... VI

List of Figures VII]

Acknowledgments.. , IXabstract [Arabic] .,. :,... ..... XAbstract [English] , :....; XI

CHAPTER ONE: LITERATURE REVIEWI. Literature Review

[A. Introduction^............ ...........,.;.. ......... \.:....'.' 1

1.1.1. Objectives of the Study v. , ........... 3

.2. Some Aspects of Silicon Chemistry 4

1.2.1. Elemental Silicon , .:.. <.. 4

1.2.2. The Nature of Bonding in Silicon-Oxygen compouiuls 7

1.2.3. Structure of the Silicates , 10

.3. Organic Silicates..... , ''. 13

•>1.3.1. lThysical Prbpertiesof Organic Silicates

1.3.1.1 .Volatility, Molecular Complexities & Thermodynamic

Data 15

1.3.1.2. Structural Aspects..... W

: •1.3.1.3. Dipole Moments 20

1.3.1.4. Densities, Viscosities and Surface Tensions 20

1.3.2. Characterization Methods 21

1.3.2.1. Infra-red Spectra of Organic Silicates ' 21

1.3.2:2. Nuclear Magnetic Resonance spectra of organic 24

Silicates.,' ; 24

1.3.2.3. Mass Spectra of Organic Silicates 26

1.3.2.4. Gas-Iiquiil Chromatography of Organic Silicates 27

in

1.3.3. Chemical Properties of Organic Silicates... 30

1.3.3.1. Hydrolysis of Organic Silicates. ;• ,. 31

1.3.3.2. Hydrolysis and Condensation of Tetraethqxysilane 33

1.3.3.3. Formation of Double Metal Alkoxides 34

1.3.4. Uses'of Organic Silicates 35

1.4. Preparative Methods 36

1.4.E The Halosilane Route to Tetraalkoxysilanes.......: 361.4.2. The Exchange Route to Tetraalkoxysilanes 411.4.3. The direct Synthesis. 42

1.4.3.1. The Elemental Route Using Metalic & Metal Salt; Catalysts 43

1.4.3. 2. The Elemental Route Using Metal Alkoxide catalysts 44.

CHAPTER Two: EXPERIMENTAL2. Experimental

2.1. Materials :... 46

2.1.1. Ethanol ; ..' :...: 46

2.1.2. Magnesium , 46

2.1.3 Mercury (I) Chloride , 46

2.1.4. Silicon Powder , .: , ............:.. 46

2.1.5. Magnesium Ethoxide 46

2.1.6. Anhydrous Tin Tetrachloride... ; 46

2.1.7. Tin(ll) Oxide..... , 47

2.2. Equipment & Apparatus. 47

2.2.1. General 47

2.2.2. Infra-red Spectrometer 47

2.2.3. Cias-liquid Chromatographic System 47

2.3. Experimental Procedure....' • 492.3.1. General 942.3.2. Preparation of Magnesium Ethoxide catalyst 492.3.3. Direct Synthesis of Tetraetlvoxysilane :..-..- ;. 50

IV

2.3.3.1. Catalysed by Magnesium Ethoxide 5.0

, ' \ 2.3.3.1 Catalysed by Tin Tetrachloride.. .,..*.. - 51

2.3.3.3. Catalysed by Ti.n(II) Oxide..... .........;....... 52

CHAPTER THREE: RESULTS

3. Results

3.1. Theoretical & Experimental Yields «. 54

3.2. Magnesium Ethoxide Catalyst 54

3.2.1. IR Spectrum of Solid Product... .. 54

3.2.2. IR Spectrum of Liquid Product 55

3.3. Si I icon Ethoxide , ... 5 8

3.3.1. Catalysed by Mg(GC21I5)2 • • • • 58

3.3.l.l.IR Spectrum......... 58

3.3.1.2. GL Chromatograph 60

3.3.2. Catalysed by SnCf,. : . . . . . . 62

3.3.2.1. IR Spectrum....- ' 62

3.3.2.2. GI. Chiomatogiaph 64

3.3.3. Catalysed by SnO...; 66

3.3.3.4. IR Spectrum... ., 66

3.3.3.2. GL Chromatograph. '. 68

CHAPTER FOUR: DISCUSSION & CONCLUSION4. Discussion & Conclusion

4.1. Discussion ;-.'. 71

4.1.1. Preparation of Magnesium Ethoxide by the Reaction of

Magnesium & Ethanol 71

4.1.2. Preparation of Telraethoxysilane by the Reaction of Silicon &

1'thanol 72

4.2. Conclusion 75

Re f e re n-c es 76

List of Tables

TABLE

No.

1.1.

1.2.

1.5.

1.6.

1.7.

.8.

.9.

1.10.

2.1.

2:3.

2.4.

3.1.

TITLE

Some Selected Physical Properties of Silicon.

Selected Values of Electronegativities on Pauling Scale.

I'AGK

6

-.-•• f

i Selected Values for the (Si-O) Bond Length in Some

Organosilicon Compounds. 9

Bond Energies of Some Silicon & Carbon Bonds. -

Physical Properties of Some Metal Ethoxide.

Boiling Points and Complexities of Titanium & Zirconium.

Physical Constants of Tetraethoxysilane.

Density & Surface Tension of Some Alkoxysiiane Derivatives.

Vibrational Spectrum of Ethoxypolysiloxanes.

H'NMR Spectra of the Product of the Reaction Between Ethanof

& Silicon.

GLC Analysis of the Product of the Reaction Between Ethanol

& Silicon Metal Usi'ng Magnesium Ethoxide Catalyst.

Summary of the Reaction Between Ethanol & Magnesium

Catalysed by Mercury(l) Chloride.

Summary of the Reaction Between Elhanol and Silicon

Catalysed by Magnesium Ethoxide,

Summary of the Reaction Between Ethanol & Silicon Catalysed

by fin Telrachloride.

Summary of the Reaction Between Ethanol & Silicon Catalysed

by Tin Oxide.

Yields of Tetraethoxysilane Obtained Using Different Catalysts

Infra-red Spectra of the Solid Product of the Reaction Between

Ethanol & Magnesium-Catalysed by Mercury(l) Chloride.

7;T

22

24

28

50

53

"54"

54

1 3 :

3.4.

3.5.

3.6.

3.7.

3.9.

~4~2.

43 .

Infra-red Spectra of the Liquid Product of the Reaction Between• • • ' ' • • • v • ' ' • ; . ' - / • • • • • - • ' : - : '

& Magnesium Catalysed by Mercury(I) Chloride.

Infra-red Spectra of the Product of the Reaction Between4

Ethanol & Silicon Catalysed by Magnesium Ethoxide.

GLC Analysis of the Product of the Reaction Between Ethanol

& Silicon Catalysed by Magnesium Ethoxide.

Infra-red Spectra of the Product of the Reaction Between

Ethanol & Silicpn Catalysed by Tin Tetrachloride.

GLC Analysis of the Product of the Reaction Between Ethanol

& Silicon Catalysed by Tin Tetrachloride.

Infra-red Spectra of the Product-of the Read ion Between

Ethanol & Silicon Catalysed by Tin Oxide. . \

GLC Analysis of the Product of the Reaction Between Ethanol

& Silicon Catalysed by Tin Oxide.

Reported and Obtained IR Characteristic Bands for.Mg(OC;>l I5)2

Reported and Obtained IR Characteristic Bands for Si(OC2l I5)i

Prepared Using MgCOCiHs): Catalyst.

Reported and. Obtained IR Characteristic Bands for SU

Prepared Using SnCi.4"Catalyst.

Reported and Obtained IR Characteristic Bands for Si(O(\I lO

Prepared I Ising SnO Catalyst.

60

62

64

66

68

7 C

72

73

v 11

List of Figures

Fig.

No.

1.1.

1.3.

7A

2.1

3.2.

3.3.

3.4.

3.5.

3.6.

3.7.

3.8.

TITLE

(pn - dn) Bonding in the (Si - O) Bond.

Assigned Infra-red Spectrumof Tetraethoxysi lane. '

H'NMR Spectrum of Tetraethoxysi lne.

G1C of the Product of the Reaction Between Ethanol & Silicon

Metal.

Reaction System for Experimental Apparatus.

Infra-red Spectra of the Solid Product of the Reaction Between

Ethanol & Magnesium. '

Infra-red Spectra the Liquid Product, of the Reaction Between

Ethanol & Magnesium.

Infra-red Spectra of the Product of the Reaction Between

Ethanol & Silicon Using Mg(OC2M5)2 Catalyst.

Gas-liquid Chromatograph of the Product of the Reaction

Between Ethanol & Silicon Using Mg(()El)2 Catalyst.

Infra-red Spectra of the Product of the Reaction'Between

Ethanol & Silicon Using SnCl.t Catalyst.

Gas-liquid Chiomatograph of the Product of the Reaction

Between Eihanol & Silicon Using SnCl.| Catalyst.

Infra-red Spectra of the Product of the Reaction Between

Ethanol & Silicon Using SnO Catalyst.

Gas-liquid Chrbmatbgraph of the Product of the Reaction

Between Ethhnol & Silicon.Using SnO Catalyst.

•I»AC;IC

8

1 3

"25

29

•56

•57

6

65

67

69

v I 11 .•

Abstract

Tetraethoxysilane was prepared using. the direct synthetic

procedure in; .presence of magnesium ethoxide, tin telrachloride and tin

oxide as catalysts.

Magnesium ethoxide was prepared firstly, identified by spectral

analysis and then used in the preparation of tetraethoxysilane.

The method adopted is reliable and significant as far as synthetic

routes are concerned. ,

The product obtained was analysed using infra-red speetroscopy

and gas-liquid chromatography, these indicated that the final reaction

product can be obtained in high yield and. purity. Spectral analysis

obtained are in good agreement with reported data for tetraethoxysilane.

XI

Chapter One

Literature Review

1. Literature Review:

J.I. Introduction: *• . . ' • . ' . ; ' • . •

• - • • " • • • - • ' • • • . • . . • ' • . . » • • • '

Actually, though, silicon chemistry has deep roots in* human/ '

history, dating from the dawn of the race and extending through all of

geology, mineralogy, and the ancient ceramic arts, .

The development of silicone materials is, in perspective, as part of

the fascinating involvement of the element silicon in our daily life, from

the: stuff the earth and the moon aremade of to the modern use of ultra-

pure silicon in transistors and computers, and the use of ordinary

elementary silicon to make silicone rubber, silicone oil, silicon resins,

silicon-containing polishes, drugs and fragrances.

Of course these are not our.only connections with silicon. The

natural compounds of silicon and oxygen (the silicates) are the starting

materials for making bricks, tile, cement, glass, and a host of modern

ceramic products. •

The widespread usefulness of silicon and its compounds conies'"

about for two reasons: First, there is so much of it, and second, it is so

versatile. Its chemical and physical properties are so unusual and so

varied that they just cry out for research into creative and ingenious uses.

Moreover, silicon is a rather friendly element, devoid of a specific

elemental toxicity like thai'of arsenic or lead or plutonium, and so

accustomed to long association with the insides and out sides of living

systems (including the human body)'that silicone polymers are even used .

in cosmetics, medicines, and prosthetic pails for the body. Therefore

silicon and. its compounds are not strangers to us, nor need silicones be.

The interest in organic silicates.(me(al alkoxicles) --subject of this

work - arose due to their tendency to form highly polymeric compounds.

Although metal alkoxides have been known for many years and used in a

number of organic reactions, it is surprising how little systematic work

lias been carried out on these compounds. It is only since a round 1950. :.;•

that a rapid overall development can be discerned in the field of alkoxide

chemistry in general.

In the 1950 the alkoxides of only a dozen elements were known where as

the alkoxide chemistry of almost all the metalic and metalloidal elements

has been investigated'"' during the last two and a half decades.

Metal alkoxides have the general'formula M (OR)X, where M is a

metal of valency x and R is an alkyl group, and can be considered1'1"'1 ^ to .'

be derivatives of alcohols (ROM) in which the hydroxylic hydrogen has

been replaced by a metal (M). Metal alkoxides involve (M^'~()'v-(') bonds,

• which are" •< polarized in the direction shown due to the highly

electronegative character of oxygen.

The degree of polarization in an alkoxide molecule depends upon

the eleclronegativity of the central element (M) and the nature of these

compounds,; varies from essentially covalent volatile monomers as in

cases of electronegative elements like silicon, germanium, phosphorous

and sulphur to more electrovalent polymeric solids in the cases of

electropositive elements such as (he alkali and alkaline earlh metals as

well as the lanthanons. For derivatives of the same element, the covalent

character of the (M-O) bond increases with greater!! inductive effect of

the alky 1 group. 1 he decreasing order of molar conductivities of sodium

methoxide, ethoxide, isopropoxide and tert-butoxide in their parent

alcohol (i.e. 92.0, 45.0, 2.5 and 0.01 mhos respectively) appears to arise

at least in part from the increasing 11 inductive effect of the alkyl group.

The polarity of the. M l ) bond may also be partially offset in cases of

electrophilic metals, which undergo covalency expantion, by

intermolecular coordination through the oxygen atom of the alkoxy

tp\. This type of molecular association appears to be sensitive to stericV » ; ' ' ' •' * • •

)r&vsuch as the ramification of the alky 1 group.

|; In view of the hydroxy derivatives/of elements behaving as bases,

ifoxyacids according to the electronegativity values of the central

llment, there has been some confusion in the literature regarding the

'riienclature of these alkoxy derivatives. The alkoxy derivatives of

iements with electronegativity of 2.0 or less appears to have been

generally termed as alkoxides whilst the others are termed orthoesters. It

ias been sometimes fouild more convenient for comparison to name theL r ' ' • " . > • ' • '

palkoxy derivatives of all the elements as alkoxides.. Further in keeping

with the nomenclature, generally adopted for metal alkoxides by most of

the authors, the common names like meth'oxides, cthoxides, propoxides

(n-and iso-), butoxides (n-, iso-, sec- and tert-). It is only in the cases of

higher alkoxides, e.g: Th (OCMe Et IV) that the nomenclature is derived

strictly from IUPAC conventions.

There has been a noticeable increase in the industrial applications of

metal alkoxides, and these developments have emphasized the

inadequacy of our present fundamental knowledge of these compounds.

l.l.hObjectives oHhe Study:

The objectives of this work are:

(i) To prepare tetraethoxysilane by the direct procedure

avoiding any silanol intermediate,

(ii) • To investigate this facile reaction ol elemental silicon with

alcohol with a view of improving this procedure and to

investigate lavtors affecting reaction process,

(iii) To characterize final reaction products using appropriate

analytical methods.

1.2. Some Aspects of Silicon Chemistry:

1.2.1. Eleni-ental Silicon: .

Silicon, with atomic number 14 (Is" 2s ' 2p6 3s" 3p" 3d") and atomic-

weight 28, is second only to oxygen in abundance (27.2 wt%). Because of

its pronounced tendency to combine with oxygen, about half the earth

crust consist of silica SiO2 and silicate ( l '6). ' ;

Silicon is an analogue of carbon as regards the number of valence

electrons, but its atom is larger, its ionisation energy lower, and its

electron affinity and Polarisability higher. Therefore as an element of the

third period, it differs substantially from carbon, an element of the second

period, in structure and properties. Silicon is active at high temperatures

but sluggish in its reactions at room temperature (has a high activation

energy), its covalent compounds are not stable towards air and water the

way those of carbon are (chemists would point out that silicon16'7'81 has d-

orbitals readily available in its atomic structure and so it can expand its

bonding capacity to six instead of being limited to lour, as carbon is).

Crystalline silicon -- the most stable form - has a diamond type

cubic lattice structure and is a hard brittle substance with high

temperatures of fusion and vaporisation16' :>. :

The element is a semiconductor at room temperature with a distinct

shiny dark-grey metalic luster. Selected values for the physical properties

of silicon are represented'71 in fable 1.1.

Silicon has not. been recognized1 ' as an element until 1823-, when

Ber/.elius reduced potassium fluorosilicate with potassium and obtained a

brown powder:

. I<2 Sil r, M K •-> Okf (soluble in water) I-' Si . /

Still it was another thirty fpiif years before Devi le succeed in melting the

powder and obtained' steel-grey pellets that could be recognized a l

elementary silicon. , • .^

Silicon is a metalloid (neither metal nor non-metal, it looks like

metals, but behave decidedly different both in the physical and chemical

sense).

Elementary silicon is now produced*L6'7> commercially by reducing

the oxide SiO2 with carbon in an electric furnace at 3000C:

Si()2 + 2C : — > S i + 2CO

Silicon in its diamond lattice crystalline form is relatively

unreactive except at high temperatures, this is due to (he formation of a

thin protective layer of silica (SiO2), so oxidation in air occurs at above

95O'C\

Also a catalyst is sometimes needed to. activate elementary'silicon.

A copper-silicon alloy has been found"1 to react with hydrogen chloride

faster••than does pure silicon. Similarly copper was found to be a much

more effective catalyst in accelerating the reaction between silicon and

alkyl or aryl halides to form alkyl or aryl halosilanes as follows:

RX-i-Si . •Cii/>250^ RnSiX.».n ,

(R -- Al.kyl or aryf groii]"), X r : I lalogen)

Silicon is tetrahedraljy coordinated in most of its compounds, like

silica, hydrides and-halkles, (with mainly sp hybridization), but other

cooi'dinalion geometries e.g. tii.agonal bipyi'amidal with sp cl, octahedral

with sp'd" hybridization) are also known.

Table 1.1. Some Selected Physical Properties of Siiieon(7):

Property

'Electronic configuration

Melting point/C

Boiling point

Density (20 C)g/cnr

AMflls/KJmol -i

AHV;||1/Kjntor

Crystal lattice

Lattice constant (25 C)pm

Covalent radius in crystal/pm

Ionic radius in SLO.j" 7pm

Pauling electronegativity

Specific heat/KJmol"

1 (l) /KJmol '

1 (2)"7KJ mof

I (3)/KJmor

ViAj/KimoY1'

Ivntiopy/KJinor1

Thermal conductix it\ KJiuM'sec"

Value

[Ne] 3 s" 3:p"

1420

3280

2.33'6,

506+1.7

383+10

Diamond

ao = 541.99

117.6

26 7 ' '" T

T.8

0.1 135-253 to 1-96

0.7428- lOlo.lOOC

786

1575

3220

4350

18.93 (solid)

168.02 (gas)

83.7

1.2.2. The Nature of Bonding in Silicon - Oxygen Compounds:

r' '•••'-From chemical analysis of vast numbers of rocks and minerals over

a period, of-'a hundred years, a good idea of what is the earth crust is made

of has been obtained01. It is found that tile earth crust is 77.5% silicon and

oxygen, and even the next 22.5 of it consists of those metals whose ions

fit into a framework of silicon and oxygen to make the myriad metal

silicate rocks and minerals. These materials are polymers, with highly.

stable Si-0 backbones'6'.

According to Pauling's original scale of elect rone-gat ivity of

elements'71, /Table 1.2., silicon has a rather low electronegativity

compared with the elements. As a result of this it is expected that silicon

would be the most electropositive in many bondings even for Si-M, Si-C,

Si-1 bonds.

Table: 1.2. Selected Values of Flectronegativities on Pauling Scale,

"; 0(3.5)H(2.1)— —

Si(7.S)--------

As(2.0) • Se(2.4)

(4.0)

Hr(2.3

1(2.5)

Silicon with its electronic configuration of |Ne| 3s~ 3p'~ 3d

resemble carbon in forming predominantly tetracovalent compounds with

main!)' sp ! hybridisation. However, the presence of 3d-orbitals affect the

chemical behaviour of silicon in increasing the coordination number

beyond four.

In all tetracovalent silicon compounds, the silicon - oxygen bond is

caused by the o bonding of tlie hybridised s and p electrons ofthe silicon

atom with the p-electrons of oxygen and the additional ^interaction of

the unshared p-electrons of oxygen with 3d-orbitals of silicon which has

been termed (Pn - d j conjugation, (see Fig. 1.1.).

Vacant Si 3ds

orbital

filled oxygen

Multiple bond

Fig. 1.1. (l\ --tln) bonding in the (Si ()) bond.

Ti-bonding of (Si-O) should also be expected1'1 lo inlluence the

chemical reactivity of Si-0 compounds. The basicity of the .oxygen in

siloxanes was reduced by such 7r-bond'mg.

The (Si-O) bond length of majority of organosilicon compounds

('fable 1.3.) equal to (I64±3pm), this value is much smaller than the

calculated value of the Si-O bond length (183pm), which was calculated

with addition ofthe atomic radii of silicon (117.7pm), and that of oxygen

(66pm), this indicates a partial double bond character which arises as a

result of (IV; d;t) interaction.

Table 1.3. Selected Values for the Si-O Bond'Length in sonic• • • • • . • • , • - • • • • • • • * • . ' • • • • • • • • • • • • • • ' • '

Grganosilicon Coinpounds(9):

Compound

(H3Si)2O

(Me3Si)2O

Si(OSiMe3).,

Et2Si(OH)2

Si(OMe).,

(F3Si)2O '

(Me2Si0j3

Me2Si(OII)2

PhOSiH3

Me3SiOM

SP = Spectroscc

e = Electron dif

X - X-ray diflVf

M = K, Rb, Cs

Bond length pm

163.4

163.0

163.0

163.0

164.0

185.0

166.0

163.0

164.0;

160.0

>pic.

Vaction.

iction.

Method of determination

SP

• . . e .

e

e

e

•• ' e

e

X

e

Structural evidence indicates that it-bonding takes place in silicon -

oxygen compounds, the-'-vSi-.O-Si x - bond angle is larger in disiloxanes

(140-160) than would be expected if only signia bonding of normal

(sp or sp"") h)'bridisai ion were taking place at oxygen.

The Si-0 bond is one oft he most stable bonds'''S) (see Table I A.).

The greater energy of formation of Si-O bond is due to existence of

(p;t (i.[) interaction.

Table 1.4. Bond Energies of some Silicon and Carbon Bonds(8):i

. ft

Bond

Si = Si

1 S i - S i

Si - H

S i - C

S i - 0

S i - F

Si - Cl

Bond Energy

Kcal/mol

78

54 ' ,

76

• ' 7 3

111

143

' 96

Bond

C = Cz^i f \L — L

c-c.0-11

c-o.C-F

C-Cl

Bond Energy

Kcal/mol

200

146

83

99

86

117

78

It is found that the strong 7i-bonding in organosilieon compounds is

a dative 7i-bonding.

The oxygen atom uses only one pair 'ofthe two unshared pair of

Tc-electroils to occupy only a single 3d-orbital of silicon.

Generally speaking, the silicon-carbon bond is reasonably stable11"1

(the order of energies B - O Si-C> Al-C is inversely related to the order

of polarities and susceptibility to attack); because of this certain

organosilanes and their derivatives have found commercial usage.

Probably the- most important class of organosilanes consists o f the

organosiloxanes, commonly known as silicones. These materials are

polymers with highly stable (Si-O) backbones.

1.2.3. Structure of Hie Silicates:

Organosilicon compounds with Si-O-Si bonds may show1'" arrange

of structural features. Silicon, unlike carbon, almost never forms double

bonds -; chemists o n l y recently succeeded in making a double-bonded

compounds of silicon by special and complex methods -.

Id

. The basic chemical .unit' of silicates is ( l l ) the.(SiO.))'tetrahedron

shaped an ionic group with a negative four charge (-4). The central silicon

ion has a charge of positive four while each oxygen has a charge of

negative two (-2) and thus each silicon - oxygen bond is equal to one

halfC/2) the total bond energy of oxygen. This condition leaves the

oxygens with the option of bonding to another silicon ion and therefore

linking one (SiO.4) tetrahedron to another and another etc..

The silicates tetrahedra form complicated structures. They can

form as single units, double units, chains, sheets, rings and framework

structures"""'.So we must picture each silicon atom forming ' single bonds

to four separate oxygen atoms, with each oxygen atom linked to two

separate silicon atoms, thus:

; ' • • ' • • ( ) • . . ' 0 " :

V

Si

" 0 ; ( ) • • •

Actually, in three dimensions the silicon sits at the '. center of a

tetrahedron, 'with'oxygen" at each of the four corners. This is true of

almost all metal silicates. Obviously the silicon oxygen teterhedra can

bond to each other, and indeed must do so in the soikl silicates, for the

oxygen atoms are bivalent and 'nuis t attach to two silicon atoms. This

leads to chains and' rings of linked (Si-O) telrahedra, and even to

continuous sheets of such linked tetrahedra.

As for the mineral silicates [there are also non mineral silicates

Which are volatile liquids; such as ethylsilicate, Si (OC2Il;0i, b.p. 1 68.5 C

I I

fthese are esters of orthosificic acid, Si(OH)4], they are of several distinct

| t y p e s : ' - " - - ' • * • •I ? . • • • • ' • . ' •

' l . Silicates witlr.discrete negatively-charged silicate ions, as in:

a. Those in which there are discrete orthosilicate ions, SiO.i'1" (no

oxygen atoms are shared by other silicate tetrahedra; all four

; .charges are balanced by positively-charged ions of metal such

as Na . K . Ca~\ Mg" etc.). The gem mineral, zircon Zr StOi,

is an example.

b. Those in which there are discrete disilicate ions, SiiO7(>", in

which two silicate tetrahedra share one coiner.

c. Those containing cyclic polysilicate ions in which three or

more tetrahedra share two corners, as in:

• O / O \ O

Si "Si

Q

o 02. Silicate with infinite chains of tetrahedra, each sharing two corners

with the outside oxygen atoms bearing negative charges:

O" O" O' (J

: Si . •• . Si ;

/ \ / \ • •

( ) • ( ) ' O O •'(.) ' 0\

( )"

Si

()"

' ' \,/ N

0 0" (Y

Si/ \/ \

0

12

Such ions and the. ones in 1 .£., have the average composition (SiC).-}),,2""

are all tailed metasilicates.

3. Silicates in which^the tetrahedra share three corners, leading to flat

sheets of alternate silicon and oxygen atoms. This conformation is

typical of the layered minerals. Such as the clays and micas.

The different ways that the silicate tetrahedra combine is what

makes the silicate class the largest, the most interesting and complicated

class of minerals, and also give them their characteristic properties which

allow them to be used widely.

1.3. Organic Silicates:

1.3.1. Physical Properties of Organic Silicates:

Organic silicates exhibit"''great differences in physical properties

(see 'fable 1.5.) depending primarily on the. position of the metal in the

Periodic Table, and secondarily on the alky 1 group.

Table 1.5. P

Alkoxiile

LiOC2II5

NaOC2H5

KOC2H5

Mg(QC2ll5)2Ca(OC2H5)2

Ba(OC2H5)2

U(OC2U5)5

Ce(OC2H5)4 .

Sn (OC2I-I5)4

u(oc2n5)6

hysieal Properties !

Colour & PhysicalForm

White solid

White solid

White solid

White solid

White solid :

White solid

Dark-brown liquid

Yellow solid

White solid

Dark-red liquid

Some Metal VA

ni .p .C

20-24260250270270270—200Un meltable

hoxidcs(l

b.P:c•/pa1'

n.d.

n.cl.

n.d.

n.cl

n.cl.

n.cl.

1 60_.n.d72/0.13

3):

Solubility inOrganic Solvents-t-

+

+—1-

-1-

b = to convert Pa to mml Ig divide by 1 33 .3 .

n.cl. ;^ ncit disti lable.

d = less soluble.

The alkoxyderivatives of metals have at least one (M-.O-C) system.

Due to*the strongly electronegative character of oxygen (electronegativity

Value, 3.5 on the Pauling scale), alkoxides of metalic elements exhibit

strongly polar character'2'. ThulM-0 bond in these derivatives could be

expected to have about 65% ionic character for metals with

eleclronegativity values of 1.5 — 1.3 (e.g. Al) to about 80% for more

electropositive metals with electronegativity values of the order of

1.2 -0.9 (on Pauling scale) (e.g., alkali metals and alkaline earths).

However, most of these alkoxides show a fair degree of volatility and

solubility in common organic solvents; properties which can be

considered as, characteristic of covalent compounds. The two factors

which have been postulated^' ' for explaining the attenuation in the

polarity of the (M-O) bond, are the inductive effect of the alky 1 or aryl

groups at the oxygen atom (this increases with the branching of the alkyl

chain) and the formation of oligomers through-dative association of the

type:

• < /

M ' • *

O

The latter tendency is expected to decrease with the ramification on

the alkyl group due to steric factors.

The .applications of more sophisticated spectroscopic and magnetic

techniques have thrown clearer light on the structures of organic silicates.

Physical characteristic of metal alkoxides are divided into the

following headings:

14

1.3,-1.1. Volatility, Molecular Complexities and Therinoclynamic

Data:

Volatility of m^tal aikoxides depends mainly upon three Factors.

(i) Molecular size and shape of the alkoxide group:

For homologous series of monomeric aikoxides M(OR)X, the

volatility decreases with increase in n-alkyl chain length

whilst in an isomeric series of monomeric aikoxides,

branching of the alkyl chain may lead to small increase in

volatility due to the effect of the shape of the molecule on its

intermolecular forces. In the case of oligomeric aikoxides

•[M(OR)x]n, the volatility decreases due to its greater size and

intermolecular forces.

(ii) Nature of the central metal atom:

•The size of the central metal atom influence volatility and

, •• molecular complexity of the aikoxides. (volatility increase

with the decrease in size). '

(iii) The nature of (M - 0 - C) bond:

The chain branching of the alkyl group affectsnot oim' the

degree of polymerization, but also the electron releasing

tendency which makes the (M-() - - • C) bond less polaj4 and

tend to increase the volatility of particular alk||xide

••derivatives. Table 1.6. show1 ' the influence of branching of

the alkyl group- on volatility and complexity using titanium

and zirconium amyloxides as examples.

Tabie 1.6. Boiling Points and Complexities of Titanium and of Zirconium(13):

- CH--

' -CHr-

-CH;—

i -CH :-

-CH (

RinM(OR)4

-CH-.-CH.-CH.-CH,x/CH3

CH, CH-

/ C H ; C H ,C H "

^ C H v/ CH,

C —CH3

" ^ CH?

^ CH:—CH?

v CH2—CH3

/ C H ; - C H : - C H 3

Titanium

: Bp.C /pad

I . - ' • •175/80

: ;.• 184 /10

154/50

105 •'?

112 '5

i 135/100

Alkoxide: - Molecular:"' complexity! 5.4; 1 . 2 ' . - • •i

1.1

i • • ; ' - . 1 . 3ii •

i 1-0

i . o - '

ZirconiumBp,c7pa

a

255/1247/10

238/10

188/20

178/5

175/5

i Alkoxide .M o l e c u l a r •'•'

- c o m p l e x i t y ••..3.2

• i - i -•• • -

•• J . J •

I

3,7 "

• 2 . 4 '• /

2 . 0 ' •

2.0

-C HCH-,

CH,-CH, - CH?

CH,

• - 9 8 , 1 0 1.0 95/10 .0

a: To convert Pa to mm Hs divide bv

•It .has been establishe(^(l4) that the structure and volatility of the

alkoxides of titanium and zirconium are governed by the configuration of

the alky 1 group.

(A) Alkoxides of the alkali metals:

They are ionic in character due to the strongly electropositive

nature of the alkali metals. Sodium ethoxide has been shown to

behave as a strong base in ethanol,

(B) Alkoxides of group (II) elements:

Their primary alkoxides derivatives are generally non-volatile

compounds. Where as their secondary and tertiary alkoxides tend

to be Comparatively more volatile and soluble in organic solvents.

(C) Alkoxides of group (III) elements: ,

Aluminum alkoxides are thermally stable and even in (he lowest

member of the series. . '

[AL(OMe)i] may be sublimed with difficulty at 240 C under high

vacuum. The higher alkbxides are all soluble, distillable prodiicl

and the "'melting points of the solids increase with increasing

ramification of the alkyl chain.

In view of .'the comparatively lower values of entropies of

vaporization of aluminum alkoxides, il appears (hat they might-be

associated in the vapour': phase also, it has been shown that Al-

isopropoxide is dimeric in the vapour phase while it is trimcric in

solution. This special behaviour of aluminum alkoxides and'the

stability of bridges even in the vapour phase was explained16'by

Mehrotra-on the basis of electron delicient nature of tricovalently

bonded aluminum atoms.

17

Y '• ' AL ' AL

The variation in solubility and melting points ofalkoxides is due to

the extent of hydrolysis which depends on the percentage

composition of the alkoxides.

(D) Alkoxides of group (IV) elements:

Silicon alkoxides are highly volatile and most of their derivatives

distilled unchanged at atmospheric pressure, e.g. tetraethoxide and

tetraphenoxide distilled at 168 C and 41 7 - 420 C respectively.

And the low boiling points of silicon alkoxides are due to(2) their

monomeric nature irrespective of the chain length and branching of

the alkoxy groups.

Metal alkoxides are1 'colored when the corresponding metal ions

are colored, otherwise not.

The alkoxysilanes generally have sweet, fruity odors that-become

less apparent as molecular weight increases.

Tetraethoxysilane is; a colourless mobile liquid, with a plaasant

ester like odor, flammable, irritant, also possesses highly toxic

properties. Some physical constants of tetraethoxysilane are listed

i n T a b l e 1.7.(7).

Table 1*7.,Physical Constants of Tetracthoxysilane(7):

Property

Density gm/cni

Boiling point

Refractive Index

Heat of Combustion

At 25C, p30 atmo.

Specific heat. Capacity at 60C1

Heat of formation

Autogenous Ignition temp

Molar Entropy of vaporisation

Silica Contents w/w%

Value

0.933

1-66C

1.3834

-1322.9±2.2

Kcal/mol

2.41KJKg"K\

-318.9+2.2 Kcal.mol

235 C

121.8KJmoF

28' 1

1.3.1.2. Structural Aspects:

Although direct structural evidence for metal alkoxides is not

available expect in a few isolated cases, important properties like tiegree

of association, volatility and reactivity with different reagents throw

considerable light on the possible structural features of these derivatives.

Bradley, in 1958 proposed'"' a simple structural theory according to

which alkoxides 'derivatives adopt the smallest possible structural unit

consistent with all atoms attaining a higher coordination number.

Furthermore, the coordination number of the oxygen atom should not

exceed four and therefore the sterochemist'ry of the alkoxides was

proposed.

In recent decades, much work has been done on the structure of the

metal alkoxides113'11 A?>. The simple alkali alkoxides have an ionic lattice

19

and a layer-like structure,' but alkaline earth alkoxides show-more .

tova-lent character.

The* aluminum alkoxicfes have been thoroughly studied and there is no • $

doubt as to their covalent nature, the lower alkoxides are cyclic, even in

solution and in the vapor phase.

The aging phenomenon observed*2' was explained by Bradley who

considered it to arise from the tendency of tetrahedral AL to change to

octahedral configuration

1.3.1/3. Dipole Moments:

The ease of hydrolysis of metal alkoxides leaves less scope for (he

measurement of their dipole moments.

The dipole moments of a number of Si tetraalkoxides -were measured1"' by

Bradley who concluded that the free rotation of alkoxy groups a round

M O bond was possible m the case of Si. Also the dipole moment of

variety of Si phenoxides have recently been measured by Bradley, who

concluded that the free rotation of the phenoxy groups a round Si O bond

depend on the steric factors.

1.3.1.4. Densities, Viscosities and Surface'1 ensioiis:

These constants are helpful in identifying the products. It has been

observed that the density, viscosity and surface tension are dependant on

temperature.

fable I.(S. Show'"1 the densih and surface tension of some alkoxy

lienvatives of silicon

20

Table 1.8. Density and Surface Tension of Some Alkoxysihme .

Derivatives(2):

Si(OMe).,

Si(OEt).,

Si(OPrn),<

Density g/ciii

.02804

0.9320

0.9158

Surface Tension dynes/em

21.67

23.58

1.3.2. Charactrizatioii Methods:

1.3.2.1. Infra-red Spectra of Organic Silicates:

Infra-red spectroscopy has been utilised'2"'1 to support the identity

of metal alkbxides by observing bands characteristic of the bonded

alkoxide groups (e.g. M O and C--0 stretching vibrations). ;

Thus the \' (C--O) band in metal alkoxides appears at 1070, 1025, 1375,

1365, 1170, 1 150, 980 and 950; for melhoxides, elhoxides and

isopropoxides. . • .

The vibralional spectrum of (CNI lsO)4Si is well known and the

assignments have been made" '''Sl (see Table 1.9. and l;ig. .12.),

\'as(Si O) and vs(vSi-;O) bands in Si tetraalkoxides appears in the range

720 - 880 and 640 • 780cm'1 respectively.

21

Table 1.9, Vibrational Spectrum of Ethoxypo!ysiloxanes(17):

Frequency cm"1 & Intensity

478 m

656 w

789 s ,

962 s

1080vs

1 102 vs

1163

1291 m

1391 m ,

1 4 4 1 w . •''•

1456 w.

in r:- moderate.

\v -- weak.

s r-r- strong.

vs ^ very strong

Assignment

OSiO, SiOC and CO deformation

SiO4 symmetric frequency

SiO_t asymmetric stretch frequency

C - C stretch frequency

C - O stretch frequeiicy

CH3andCH2 internal

and external deformation

Wave-number (em1)

4000 ,3600 -3200 .2800 2400 2000, . . ; i ;•• , / ' i ... . . . . . . i . • . ' . i i i—!_J

1800 1600 1400 1200 1000 S00 600

Si-O-Cstretch antisymmetric

stretch .

Fig. (1.2.): Assigned Infra-Red Spectrum of Tetraethoxysilane.

1.3.2.2. Nuclear Magnetic Resonance Spectra of Organic

,Silic'ates(NMR):

The NMR spectra of only a few. soluble tertiary alkoxide , '

derivatives of alkyl magnesium have been recently studied12'. .For a

tetrameric methyl magnesium tert-butoxide in benzene, the (wo broad'

signals at 8.06 and 5.94 are due to methyl magnesium and tert-butoxide

protons respectively. The NMR spectrum of molten Al-isopropoxide was

measured at 198 C showing'the presence of a pair of signals.

The PMR spectra of alkyl silicon alkoxides R.^SiCOR),, had been

studied'-3"6' in" detail to substanciate the presence of Vn-dn bonding

between (Si-O) bonds. It has been observed.that when methyl group of

tetramethoxysilane is replaced by an ethoxy group, the " Si resonance

shift down Field from -12 to-27-ppm and the silicon methyl protons from

Zero to 0.005 ppm, indicating that the electron withdrawal being greater

than (he it-bonding contribution. The replacement of two methyl groups

of tetramethoxysilane by two ethoxy groups however, shifted the 29Si

resonance towards high field from -2 to 16 ppm and the methyl protons

from Zero to '-0.022 ppm, indicating the increase in the election density

on silicon atom..

The I l'NMK spectrum of tetraelhoxysilane is well established'7'1^.

The 111 chemical shift of tetraethoxysilane was obtained"''1 (see Fig. 1.3.).

'fable 1.10. show the II'NMR spectra of the product of the reaction

between etliaiiol and silicon1 '.

Table 1.10. H'NINIR Spectra of the -Product of the Reaction Hehveen

Kthaiml and Silicoii(7):

The protons

<CH.,)t.

ici

C h e m i c a l shift p p i n1.19 ~3.75

I n t e g r a t i o n f a c t o r : - 2 : 3 . i : t r iplet , CJ= qua r t e t .

Fig. (1.3.) I OOMEz H NMR Spectrum of Tetraethoxysilane (7)

• .fj f - 1 , i .

0 H -

o(CH3) =

i. !-' ( " ' I • I

1.3.2.3. Mass Spectra of Organic Silicates: •

The "recorded*I7J mass spectra of triethoxy and tetraethoxysilanes

suggested that the common features of the decomposition were loss of

proton, alkyl or alkoxy group from the parent molecule.

In some cases, CH3 is lost' 3) from an ethoxy group of (Col IsO)^Si, then —

CH2 and finally O.

The mass spectra of tetraalkoxysilanes (RO).|Si where R is C| to C5

alkyl have indicated common features. The fragments obtained due to the

loss of a proton or alkyl and alkoxy group from the parent molecule give

the parent molecular ion and a free radical, according to the following

(1-6) decomposition pathways:

(1) (RO).,Si ->(RO).,Si'1 HKX

(2) (RO)3Si - [O(CM2)MMe] -> (RO)3Si O C! U' + Me(CIl2)"n-i

Olefmic and aldehyde elimiiiation occurs after the primary

decomposition:

(3) R, R,

(4)

•I

Si R,

Si — I I ' c = 0

v -.- o

1 2 1 1 . • I

Si Si

OClh

26

(5) Rearrangement of (SiOCH2) to (Sill)

... O

Si C ——>- Si" - H + IICO"

H II

(6) Rearrangement of(OSiOCH2) to (SiO.11)

O 1-1 II

Si : C > Si' 0-H-i-ilCO"

: • • ( ) ' ' .

The principal ion peaks and decomposition products required in

(5) and (6) path ways have been clearly identified.

1.3.2.4. Gas - Liquid Cliromatography of Organic Silicates:

Gas liquid chronatography (GI.C) has been used to separate

mixtures of.tetraalkoxyislanes.

, GI..C may be considered to have replaced the traditional fractional

distillation method in the initial separation and analysis of

tetraalkoxysilanes. Nevertheless, problems with respect to the full

separation of all individual alkoxysilane members still exist especially for

high molecular weight components. Generally the abundance of each

component detected in the mixture analysed progressively decreases as

the molecular weight of the component increases' ~(".

The most important alkoxysilanes separations can be made on

pohdimetliylsiloxauc stationary phase or an organic grease such as a

pie/on I . A standard ..! meter column or fluid (SF.--30 or ()V-~1()1 ) on

chiomosorb \V pro\ ides adequate separation of most common

alkox;si lanes'

11

However, using GLC analysis sometime presents encountering

problems and is not a perfect and reliable method of analysis. For more

accurate.data a comparative .GLG-mass spectra is required.

The GLC results of final reaction mixture, where the catalyst was a

solution of magnesium ethoxide have generally indicated'7'20 the presence

of ethanol, tetraethoxysilane and the lower siloxane oligomers, (see 'fable

1.11. and Fig. 1.4.).

Table 1.11. GLC Analysis of the Product of the Reaction Between

Ethanol and Silicon metal Using Magnesium Ethoxide

Catalyst(7):

Peak Nuiiibei

2

3

4

Assignment

Ethanol

Tetraethoxysilane

1 lexaelhoxydi siloxane

I lexaelhoxytrisiloxane

Hexaelhoxylelrasiloxane

Fig. (1.4.) GLC ol the Product of the Reaction Between Ethauol and

Silicon i\Ietal(7).

29

1.3.3* Chemical Properties of Organic Silicates:

Alkoxysilanes are characterised by the presence of (Si-'-O-C)'

functional groups, which show similar common reactions depending on

the nature of the alkoxy group and substituent.

Alkoxysilanes are very reactive species which may be due to the

presence of electronegative alkoxy groups making the metal atoms highly

prone to nucleophilic attack. The metal alkoxides are, therefore,

extremely susceptible to hydrolysis by atmospheric moisture and require

careful handling.

Metal al'koxides readily react with excess . of hydrogen halides or

acylhalides giving the metal haiides. However, by using stiochiometric

amounts of these haiides, the metal halide alkoxides may, be prepared.

Alkoxides readily react with the protons of a large number of organic

hydroxy compounds such as alcohols'"'"', glycols'""'" ', carboxylic acids,

hydroxyacids, (3-diketones, alkanolamines etc., containing reactive

hydroxy groups with the replacement of the alkoxy group by the new

organic ligand:

:. M(OR)X I xllOX -> M(OX)X + xROH

These reactions are quite versatile, and appear to be subject mainly to

kinetic factors, e.g., the reaction with highly ramified alcohol arc

generally slower and ma)' be even stcrically hindered in some cases.

A l . ( ( ) C 2 l l 5 h ( 2 C 4 I I . / O H ' • a t ' i i t > A L ( ( ) C 2 l l 5 ) ( ( K : , l | i ; ) > i 2 r , I I 5 ( ) I I ,

2 A L ( ( ) C 2 l l 5 ) ( ( ) C 4 i l . / ) 2 - M C \ , H l / ( ) l l i > i o v L > A L 2 ( ( ) C \ I 1 5 ) ( ( ) C ' . , 1 1 , / ) 3 I C 2 1 I 5 ( ) I I .

2 A I . ( ( ) C M I O ( O C , I f.) l),-i ( ' , ( f / O I I - l l ^ ^ ^ [ A I U O C i l I . , 1 ) , ] ' - I - 2 C M | , O I I.

Also the alkoxides are sometimes reactive towards other molecules

having reactive protons such as those having -Nil or-SI I. In these cases,

the reactions are controlled by thermodynamic factors and arc governed

30

by the comparative1 stability of (M-O), (M-N) and (M-S) bonds.

Metalalkoxides sometimes behave as weak Lewis acids forming

coordination compounds .with suitable ligands, although, in general, the

metal atoms in alkoxides prefer to attain the higher coordination state

through intermolecular alkoxy bridge formation rather than by

coordination with an external reagent.

Also the unsaturated substrates like A=B readily insert a cross the

(M-.O) bonds of certain metal alkoxides with molecular.re-arrangement

resulting in the formation of insertion products12'.

It was established*2'^ that Bu3SnH effectively catalyzed the silicon

hydride mediated reductive cyclization of enals and enones. Employing

Bu3SnH as a catalyst for this transformation requires the use of

stiochiometric a mount of a second metal hydride capable of regenerating

Bu3SnH from tributyltinalkoxide.

C a t . H113S11I I, 0 . 5 P h S i l l. •w

r ad ica l in i t i a to r 0 1

I ' t O H , Phi l o r t o l u e n e , A

S i l i c o n h y d r i d e s reac t w i th a l k o x i d e s to afford tin h y d r i d e s and silyl

ethers, provide (lie basis for a new catalytic process1"""*"26'.

1.3.3.1. Hydrolysis of Organic Silicates:

All metal alkoxides- so far investigated1" have been characterized

by the ease with which they are hydrolysed.

In many cases the alkoxides are so sensitive even to traces of

water, that very special precautions have to be adopted in order to study

their properties. When restricted amounts of water are added, these metal

alkoxides undergo partial -hydrolysis reactions yielding in some cases

products of • definite composition called metal oxide alkoxides

.11

MOn (OR)X. Arid when an excess of water is present, the ultimate product

is the metal hydroxide or more commonly the hyclraled metal' oxide, and

these compounds, which are polymeric, are extremely interesting, from-

the structural point of view.

Hydrolysis of metal alkoxides using pure reagent is slow, therefore,

and acid or base are usually used(2'2930) as catalysts.

M(OR)X + 112O H7OIT ^ M(OR)X., 011 + ROM

Catalyst |[H20

• • ' . • . , . . M(Ok)x.2--(Ol-I)2 i RO11

Hydrolysis is usually followed by condensation polymerization. So

higher temperatures, longer times, higher acid/base concentrations and

higher 1120/M(0R)x ratios, all tend to shift the molecular size to higher

values'311. Also on the basis of the water consumed during the hydrolysis

reaction of Ti(OHt).t, it was concluded'2' that the alkoxide derived from

aromatic alcohols are more resistant to hydrolysis than the corresponding

normal aliphatic alkoxides. The resistance to hydrolysis increases with

the increase in length of the alky! chain.

The proposed mechanism of hydrolysis of metal alkoxides involves

the coord ina t ion <.if water molecu le •through its oxygen to the metal in a

facile nucleophilic process: '

11 11

\ • • v \

( ) : - > M ( 0 R ) , • > . (.): - * M(()RK-i •-> M(OI I)(OR)X . | -i-ROI 1

11 11 :0-R

32

£.. Acid Catalysed Mechanising '.

= M - OR * iI3OB:.-4- = M - OR *-> = M ~ Oil + NOR I 1 IB

, V ; HOHHB

I HB

or: H,O + = M - OR + MB -> HoO: M-OR -*= M-OIH1IOR + 1 IB

. / \

B. Base Catalysed Mechanism*32':

H O - M =

R O " ' • »

One of the protons on the water molecules interacts1" with the oxygen of

an alkoxide group through hydrogen bonding and, Ibllowing an electronic

rearrangement a molecule of alcohol is expelled. The hydroxy metal

alkoxides formed may react further to form the oxide alkoxide by either

of reactions (2) or (3):

(2). M(OI D(RO)V| i Mf OR)X - > (Up),., MOM(OR)vl -I- RO11

(3). •2M(0il)(R0)v,->(RO)V, MOM(OR)X., .-i-N2O

1.3.3.2. Hydrolysis and Condensation ()t'rretraethoxysilane:

The stability of some tetraalkoxysilanes to atmospheric moisture

was studied'7'.. It is found that tetraethoxysilane (TEOS) undergoes

hydrolysis after two months. Hydrolysis of tetraethoxysilane occurs

readily in tire presence of acidic or basic catalysts, but due to the

immiscibility of water and IT-OS, a mutual solvent such as alcohol and

ketones is often used to obtain homogeneous mixture.

Base catalyst: r

I.!,('f:2H5n)38j7.OC2H5 < OH / N [(C2H5O)3-SiOH -OC2H5.P

V * \ p =^(C 2H 5O) 3 SiOM + C2H5O"

2. C2U5O- + H2O -> C2H5OM + 011"

Acid catalyst:

1: (C 2 H5O)3Si -OC 2 H5+H 3 O + ^ = ^(C2H5O)3 Si l l ' - OC2MS+ M 20

- = ^ ( C 2 I - I 5 O ) 3 S i — 0112 -i- C2115Oil

2: (C2I-I5O)3 Si-OH2 + H 2 0 -» (C2I-f50).i SiOH +11.,O'

Condensation of Hydrolysed Species:

l:(C2ll5O)3SiOIJ+C2H.5OSi(OC2H5)f==^(C2IIsO)3SiOSi((X;2Il5)rK12lI5

2: 2(C2 lf5O)3SiOri,—^ (C2I {5O).vSi O - Si(OC2l l5)., 1 -I I2O

When (Oil) is more than one in the hydrolysate molecule, gelation

occurs129'32' through the cross-linking bfthe groups at each silicon atom.

1.3.3.3. Formation of Double Metal Alkoxides:

A large number of double metal alkoxides involving more than one

metal atom within the molecular species were synthesised(2J'" during the .

late twenties by Mcerwein and Bersin.

However, the apparently covalent behaviour ofdouble alkoxides of

•strongly " .electropositive elements (e.g., alkali, alkaline earths and

lanthanide elements) has attracted special attention. Alkoxides-of strongly

electropositive elements like alkali metals has been reported to behave as

strong bases particularly in their parent alcohols, similar to the reactions

of alkalies like caustic soda with amphoteric hydroxides like;Zn(OH)2 in i

aqueous medium to give (hydroxo salts) of the type Na2/n(011).|.

Meerwein and Bersin reported that the titrations of strong bases like alkali

alkoxides with alkoxides of less electropositive elements, like Zn and AL, '

carried out in' parent alcohols, benzene or nitrobenzene gave end points

(using thymolphthaiein as indicator) corresponding to the formation of

derivatives of types Na2fZn(OR)4], or Na[AL(OR).]] which were termed,

"alkoxosalts" corresponding to "hydroxo slats" in aqueous systems.

The formation of double alkoxides may be considered to arise

partially from the mutual neutralisation of acidic and basic alkoxides and

partially from the tendency of the metal to form coordination complexes.

Thus if two metal halides or alkoxides having different electropositive

character, but capable of increasing their coordination number are

allowed to react under suitable conditions in the presence of the parent

alcohdl, the formation of double alkoxide derivatives may be achieved.

A novel yttrium-copper double alkoxide clusters - which act as

good candidates for sol-gel processes - have been synthesized'2-' recently.

1.3.4. Uses of. Organic Silicates:

The uses of metal alkoxides depends on their chemical reactivity in

common organic solvents'2'. The chemical'reactivity is manifest in the

variety of catalytic applications of the alkoxides ranging from redox

catalysts (Aluminium alkoxides), to accelerators for the drying of paints

and inks. Ultimately the alkoxicles are valuable precursors of high purity

metal oxides through hydrolysis, pyrolysisor combustion.

Aluminum alkoxides and other organic aluminum compounds are used as

dryers in paints.

Silicon alkoxides copolymers are used as protective film forming media

and in thermally stable inorganic polymers. Dihydrido carbonyltris

(triphenylphosphine) ruthenium (Ru) catalysed copolymerisation of

disiloxane compounds'1''. .

Alkoxides .of aluminum could be used for water-proofing of textiles'""0'.

Monomeric organometalic precursors may be converted through

hydrolysis reaction into gels and ultimately glasses or ceramic* "'(>' '.

Tetraalkoxysilanes are used for the preparation of ceramics by the sol-gel

process^ . . . . ' ' • . '

Tetraethoxysilane is an important industrial material, it can be used as

heat transfer fluid, as an electric, coolant, and can be hydrolysed under

controlled conditions to form hydrolysates which act as binders for

refractory grains. This latter applications is particularly important in the

production of precision cast material for ceramic and foundry

applications1"' '. ,

A variety of carbocyclic derivatives of silicon' which was

biologically active have found a medicinal interest'V)|.

Purified methyl chlorosilane are used to prepare the various methyl

silicplie resins, oils and elastomers.

Trichlororsilane HSiCL.i is the preferred source of hyperpure silicon for

the transistor and integrated circuits which go into every radio, television

and telephone appliance and which are the hearl of every computer

system"1. • '

1.4. Preparative Methods:

•Organic silicates can be prepared by a number of selected methods

These include the following: the halosilane route, the exchange route and

the elemental route.

1.4.1. The Ilalosi lane Route to Tetr . inlkoxysihines:

Tetraalkoxysilanes can be prepared by (he reaction of halosilane.

with the corresponding alcohol as follows:

' Sf X i i KOI 1 ->-.S.i(OK).i i -t NC'lt

Where R is an alkvl or aryl group, and X is a halogen.

When anhydrous ethaiml is used 'the product is telraethoxysilane, bu!

when1 industrial spin! or aqueous ethanol is used, the product is technical

36

ethylsilicate(7'13'l9'29). Technical ethylsilicate is a mixture of

.ethoxypolysilbxanes, comprising hexaethoxydisiloxane and. the higher

oligomers. . T-he reason that ethylpolysilicates are formed is that the

condensation-polymerization reactions occur in the presence of small•i

amounts of .water in the reaction mixture which are catalysed by (he

hydrogen chloride by-product.

Lower yields of alkoxysilanes, particularly with secondary and

tertiary alcohols have been related to various side reactions, for example,

lower aliphatic alcohols are rapidly attached by the IIX by-product:

ROH + HX-^RX + M:,0

Where as water in the reaction medium leads to polymerization of the

product: , . • , .

( R O ) , : - Si - ORi- RO Si (OR)., __JJ2O____.._ _ •;IIX act as catalyst

OR ORI I

RO - S i - O S i - O R I 2 ROM, etc.' • • • I - I

OR OR

l,n order to obtain higher yields -of monomer product, there have

been many attempts concerned with the removal of hydrogen chloride, for

example, hydrogen chloride can be driven oil by blowing a stream of dry

air or nitrogen through the reaction- mixture, alternatively hydrogen

chloride acceptors such as pyridine or dimethyamine can be introduced

causing the hydrogen chloride to be precipitated and then removed by

filtration. Also the yield is increased by addition of small amount of an

organic solvent such as chlorinated hydrocarbon and followed by

neutralisation with sodiuni ethoxide (C\l IsONa) before distillation'729'10'.

Tetramethoxysilane has been prepared'7'4" by treatment of an

ethered slurry of sodiuni methoxid.e (which reduce the solubility of MCI

37

in the reaction mixture) with an alky] chlorosilane and for which yields of

50 - 70% have been reported'7*. Partial hydrolysis products are formed in

considerable a mount due to, hydrogen chloride which liberated in the

primary step.

Other alkylorthosilicates including ethyl, n-propyl, n-butyl, n-amyl,

iso-butyl,; secrbutyl and 2-chloroethylortho-silicates were prepared using

tetrachlorosilane and the corresponding alcohol and their properties were

studied'7'. It was noted that the vigor of the reaction was greatest for

alcohols containing +1 or —I inductive effect and decreased for normal

alcohols as carbon lengthened.

Due to steric reasons it was found that1'12' it is difficult to synthesize

tetra-t-butoxysilane from chlorosilane and the corresponding alcohol, but

in the presence of pyridine, tri-butoxychlorosilane results;

A similar process was reported using silicon tetraehloride and

sodium-t-butoxide in refluxing petroleum ether, again the end product

was tri-butoxychlorosilane. However, the fourth chlorine can be replaced

by an ethoxy or isopropoxy group. Also tetra-t-butoxysilane has been

prepared,1'13', using tetrafluorosilane and sodium-t-butoxide or tertbulyl

alcohol. .

Other tetraalkoxysilanes were prepared by using tetrafluorosilane instead

of tetrachloro, according to the equation:

4ROH-I Sir, :>Si(OR), -I 41II-

(R - C21U, Bu, C(111,,, HLIC21 I3 CM CII2 and ph).

The synthesis of tetraethoxysilane by the reaction between ethanol

and tetrachlorosilane was a perfect method* "' for over a century in

laboratory and industry as illustrated by the following equation:

Si Cl., i-.4C\I Is ()l 1 T'y'M11^ Si (OC2I Uh -I- 411C1

38

The yield of tetraethpxysilane was 79% or 85% when the reactants

were added at very low temperature (0 - 2 C). It was found that MCI can

react at the beginning of the reaction with the product which can lower

the yield, this reaction is catalysed by higher temperature not by higher

concentration of HC1(741);

Si(OQ>H5j., + 1 ICl -» Si(OC,H.0.i CI+C21 -f5OI I

Tetraethoxysilane can be produced by a continuous method. The

continuous feeding of ethanol and tetrachlorosilane in automatic column

was described1451, the resulting product was 85-87%). Similarly vaporised

ethanol (82 - 85 C), and tetrachlorosilane were introduced in the glass

column at opposite points, the hydrogen chloride escaped through the

reflux, system, tetraethoxysilane is collected, in a receiver (82% yield).

Also the reaction can be done in a stream of nitrogen gas<7>.

The purity of tetraethoxysilane produced by the continuous method was

described. Tetraethoxysilane having < 10 ppm I ICl, or with no halogen

contents, with < 5%'di, tri and tetra content was produced.

The resulting product depends on the ethanol-tetrachlorosilane

ratio, thus esterification of SiCl4 with ethanol in 1:1, 1:2, 1:3 and 1:4

ratios at 100, 130, 145 and 155 C respectively, gave 90%, C2Il50SiCI:,,

95%). (C2H5O)2 SiCU 80% (t\II,OJ., SiCl and 82% (C2II,O), Si

respectively'' '*.

A procedure in which cthyltetraethanoate is used insteatl of ethanol

to react with tetrachlorosilane is described1''. This procedure gives a.high

yield of telratilhoxysilane (.88 - 91%) with high purity (0.02 - .0 .1

chlorine content, and with 3.1 -4.6%) polysiloxanes). Studies have been

.V)

carried out to elucidate the mechanism of the reaction:

, .i" .. SiCl4.t4RQH->S.i(OR)4 + 4HClt

Two mechanisms appeared to be possible, both involving the

addition of the lone pair of alcoholic oxygen to tetrachlorosilane, which

increases the electron density at silicon atom, and weakens the silicon

halogen link. These two mechanisms are shown as:

R Cl Cl II

\ \ x ,xO >S\< O ->Si(OR)4 + 4HCl

X . x \ \H Cl Cl R

R Cl Cl II

X . \ x x '^ O — > Si< O 4 Si (OR)4 I SiO, -I 4RCI -I- 211,0

/ ; • • X \ A . ;

II ' Cl Cl R

Both mechanisms occur when normal alcohols are used.

The preparation of alkoxysilanes from glycol monoethers

(IIOCH.CTbpR) has been reported1719' as shown below:

41IOCII2CII2OR * SiCL, ~> Si(OCI I2CH2OR)t ^ 411CI

(R = Me, Pr1 and n Hu).

Similarly phenols react with tetrachlorosilane to produce

tetraphenoxysilanes, phenol do not react with hydrogen chloride like

ulcohols.

A methocl for the preparation of polyhalophenoxysilanes has been

described'7'191 by reacting jihehol in the presence of amiiie catalyst, the

reaction can be illustrated as follows:

n c . r M . . . . . . . , , , , C a t a l y s tR n S i ( 1 , , , 1 - 4 - 1 RnS. o-<0

X X

(R - alkyl or aryl, X - halogen, n ~ 0, 1, 2).

The reaction is carried out at elevated temperature with reactants in the

molten state or, dissolved in tin inert solvent such as diethyl .ether or

t o l u e n e . •' < J' i: .' '' ;'.'' : ' .

1.4.2. The Exchange Route to Tetraalkoxysilanes:

The, name implies an interchange of an alkoxy group of a lower

alcohol with an alkoxy group of a higher alcohol or phenol.

Tetraalkoxysilanes are often prepared by exchange reactions between an

alkoxysilanes and alcohols(f7V

Si(OR)4 + nROI 1 -> (RO).,.llSi(OR)n + nROI 1

Where R and R - alky!, aryl, R ± R :. ,

The reaction is known as alcoholysis or displacement and some times

transesterifi.cation. Usually higher alkyl or aryl alcohols are used together

with tetramethokysilane or (etraethoxysilanes so that the volatile alcohol

liberated can be distilled out of reaction vessel to assist in forming the

desired product.

The reaction is usually accelerated by heat and/or catalyst, for

example acids or bases. The reaction is useful for the synthesis of

tetraalkoxysilanes containing reactive groups such as tertiary alcohols

which are rapidly attached by hydrogen chloride liberated in the initial

step of the reaction. The process is also suitable for the preparation of

letraalkoxysilanes from alcohols containing a group which reacts with

hydrogen chloride such as aminoalcohols. The alkoxy-exchange reaction

can also occur between two different alkoxysilanes.

Thus in a study of the reaction between tetramethoxysilane and'

tetraethoxysilaiie all possible exchange products were detected'7 ':

(CI hO),Si + (C21 l5O)4Si ^ - = ^ (Cl hOh SiOC2l I3 i

• • ' ' • ( C l h O ) : Si '(CM I^O)5+ CM

41

Some alkoxysilanes were prepared by using tetraethoxysiiane, also

the mechanism of. the reaction is postulated1"12'which indicates that the

Si - O link is broken as shown below:

R H8+

H5C2O O .OC2H5\ 4; / •

' . S i , • • ' • • . • • • .

. / \

The alkoxy-exchange route to tetraalkoxysilanes is significant,

especially with those reagents, which react with alcohols. :•

1.4.3. The Direct Synthesis:

The halosilane route to tetranlkoxysilanes is a source of many

difficulties concerning control of pollution'291, chlorine which is used in

the manufacture of silicon tetrachloride, is a hazardous material. So also

is silicon tetrachloride. Both can cause severe pollution. The hydrogen

chloride by-product decreases the overall yield by initiating a series of

side reactions,

However, an alternative recent route has been developed for the

production of tetraalkoxysilanes, by a direct method which is based on

reacting elementary silicon with a corresponding alcohol using suitable

catalyst. The reaction can be represented as follows:

Si t- 4ROII _ -_£ iU^L> Si(OR)i + 21 i2ts o l v e n t • ' ••

•If the alcohol used.is elhanol, the product is telraethoxysilane, which can

be converted to elhylpolysilicafes by hydrolysis and condensation-

polymerisation, using a limited a mount of water' to control the

hydrolysis •• . .

With the diScoxcry in 1940 of the direct synthesis, the problems of

large-scale production of organosilicon halides have been solved to great

, A 1.50.51)

extent

42

1.4.3.1. The Elemental Route Using Mctalie and Metal Salt Catalysts:

•Alkali, alkaline earth metals, and aluminum react(l3) with alcohols

to jgive metal alkoxides. The speed of the reaction depends both on the

metal and on the alcohol, increasing with increasing electropositivity and

decreasing with length and branching of the chain. Thus sodium reacts

strongly with ethanol, but' slowly with tretiary butanol. The reaction with

alkali metals is sometimes carried out in ether, benzene, or xylene. Some

processes use the metal amalgam or hydride instead of the free metal.

Alkaline earth metals and aluminum are almost always covered wilh an

oxide film. Slight etching with iodine or mercuric chloride breaks the film

and facilitates the reaction. So a metalic and metal salt catalysts is usually

needed to activate the metal surface. The reaction of alcohols with silicon

metal using metalic or metal salt catalysts wasstudied"9 '^^ ' .

It was found that of the several lower alcohols, only mcthanol'

reacted readily to form recognizable product using copper metal as

catalyst at elevated temperature in the range of 250-300 C,

( ' u • • ^

4011,011 I Si —--; > S i ( ( )n i , ) r t 21 h t250-300 ('

Telramethoxysilane was found to be in 40 •-• 45% yi.eld where as

tetraethoxysilane is produced in only. 10% yield'7'.

Similarly tetramethoxysilane in a high yield (85%) was produced using

CuCl2 as catalyst solvated in alky! benzene' '.

•The lower primary alcohols'such • as ethanol, l-propanol and iso-

butyl alcohol react to give small yields mainly of triaikoxysilane, where

as isopropyl, n-butyl, sec-butyl and t-butyl alcohols did not react'7'"'.

Reaction of elhanol with silicon at 23OC catalysed with Cu.(l) chloride

using aromatic hydrocarbon having 1-4 alkyl groups as solvent was

* described, giving (75%) HSi(OC2H5).i. (C2H5O).,Si was produced using. . * • • • • • • • • • . • • • • • • • . ' • ,

, CuCl, NiCL and mixed polyeyclic hydrocarbons (280 450 C).

. - • . - . , " . . *

Also tetraethoxysilane was prepared by contacting Me2Nl'l witli

activated silicon.

1.4.3.2. The Elemental Route Using Metal Alkovides Catalysis:

The use of metal alkoxides as catalysts iiv production of

tetraethoxysilanes from silicon metal and the corresponding alcohol has

been reported"9'. The preferred metal alkoxides are those of alkali metals

particularly those of sodium and potassium, alkaline-erath metals, and

even alkoxides of aluminum. Other possible catalyst is the reaction

product of sodium ethoxide and 2-ethoxyethanol. Tetramelhoxysilane

.with high purity and high yield (95%) was prepared by the reaction of

silicon with Cil.iOII and NaOCH, as catalyst'7'. Si(QR)., (R = C,..,) are

prepared from the corresponding alcohol ROM and silicon in the presence

of NaOR and Si(OR).| as a solvent. In another experiment KO2CII,

NaO2CHt, N'aOBu or K( )/\c are used as catalysts'7 '".

A method is described for production of tetraethoxysilane as

follows: Stir suspension of silicon powder in a large volume of catalytic

solution pre-heated to 1.50- 160C, dry elh'anol is then added batch-wise.

The catalytic solution has1""'1 sufficient thermal capacity to maintain the

temperature Catalysed the reaction, and to,discharge tetraethoxysilane as

vapour together with ethanol and hydrogen gas. The thermal capacity can

be maintained by step-wise addition of reactants.

Also BuOCI LCI LOCI LCI I.?OK has been used""1 as a catalyst for the

production of tetraelhox) silane.

'I'etraethoxysilane-iSU.7% yield'was prepared by the reaction of

ferro-silicide usins.' K'.()(' .1 Is or NaOl\l l.s 35% solution in CNH.sOI I, but a

44

15% NaOC2H5-C2HsOH solution gives 39.5% and 5% LiOC2H5-C2H5QI 1

•'•gives only 25% (C2H5Q).,Si(7). •

Telraethoxysilane 98% pure was prepared from silicon powder or

metalsilicide slurried with Si(OC2H5).( then C2II5OCH2CI I2ONa was

added, the mixture was heated to 1 30 C then ethanol was added.

Tetraethoxysilane 70.2% yield was prepared by the reaction of

ethanol with a mixture of silicon or sjlicide in the presence of Si(OC2l 15)|,

NaOC2H5 and C2H5OCH2CH2OH to act as 3-component catalyst.

The reaction between elemental silicon and ethanol does not take place

without using any catalyst. Magnesium ethoxide was found(7) to be an

effective catalyst for the reaction.

Also the reaction of elemental silicon in presence of tin ethoxide

was described'" '. It is found that tin ethoxide can act as a catalyst for this

reaction.

However, the direct synthesis of tetraethoxysilane by a direct

reaction of silicon with alcohol has many advantages compared lo the

previous methods, so a considerable effort has been expended1 9) to

develop the direct synthesis for production oftetraethoxysilane due lo its

industrial uses or applications.

Other methods lor the production of organic silicates which have

specialized significance were described' ', these include alcoholysis or

oxidation of organometalic compounds and carbides and reduction of

esters. . •

A new route which offers a path to alkoxysilanes that is simple and '

versatile has been developed1'''1". It enables the production of Si(()C2I I0i

from Ca,(SiO))O and C'a2Si0.|. Also it opens a way to alkoxysilanes that

are not easily available by-other means, it uses readily accessible starting

materials and gives non-toxic by-products.

45

Chapter Two

Experimental

E Experimental:ter* • • - , . •

§Lt. Materials:

Absolute eihauoi v v s - ^ / W ) Jus lDecr» t.voo. V»MV\V\\*V. <*\ . V , .

Cliemicals, and industrial spirit was dried using calcium chloride which

was put in the oven at 100- 120 c for two days - which was added to and

closed in a flask and left for a week in the hood, then dried ethanol, after

confirmation, was decanted and protected from atmospheric moisture

until used.

2.1.2. Magnesium:

•The grey ribbon (Supplied by Hopkin and Williams) was cleaned

by scratching to give a bright ribbon which was then used.

2.1.3. Mercury (I) Chloride:

A white powder (Supplied by BDI1 Chemicals) was used as a

catalyst,

2.1.4. Silicon Powder:

The silicon pure blue-grey powder (Supplied by Hopkin and

Williams) was used. .

2.1.5. Magnesium Ethovide:

The white solid magnesium ethoxide was prepared and identified

(see section 2.3.2.) then used immediately in the reaction.

2.1.6. Anhydrous Tin I etrachloride:

The volatile colourless liquid (Supplied by BD11 Chemicals) was

used as a catalyst.

2.1.7. Tin(ll) Oxide: r

% Pure black crystals^:(Supplied by Hopkin and Williams) was used

as a catalyst. : ' .

2.2. Equipment and Apparatus:

2.2.1. General:

Quick-fit Apparatus were used all through the study.

Figure 2.1. represents the reaction system for experimental apparatus*719'.

2.2.2. Infra-red Spectrometer:

'•Infra-red spectra were recorded on A Perkin-Blmer (157 sodium

chloride) spectrometer at ambient temperature with spectra range

4000cm"1 to 600cm"1. Liquid sample was spotted on two sodium chloride

plates which were pressed together to give a thin film, solid sample were

mulled with potassium bromide to give a thin disk. Absorption

frequencies are given in cm" .

2.2.3. Gas-Liquid Chromatographic Systein(GLC):

A pye unicam gas chromatography wi(h (lame ionisation detector

(Hydrogen was 30cm7min., air at 300em7niin.) 1'itted at 150 C was used

to obtain chromatograms. The carrier gas was nitrogen at 20cm7min, and

the column used is OV1 7 glass which is 2.0m long X4mm (I.D.), lilted at

140 C. Chart-speed was lem/min. '

Also A Hewlett Packard (series 5848) gas chromatograph with

(lame ionization detector was used.

The carrier gas was nitrogen at 30cm /min, and the column used is () V I 7

glass which is 6ft long x 2mm (1.1).). Injection temperature is 150 C and

oven temperature 140 (\

47

Water inlet

hermometer

Guard lube

(Containing CaC

Water Outlet

Condenser

Round-bottom iiask

1 lot plate

magnetic stirrer

rij». 2.1. Reaction Sysleni for experinRMital Apparatus

2.3. Experimental Procedure:* • - •

2.3.1: General: . ,

Chromic acid was used for cleaning apparatus, rinsed with tap

water, distilled water and dried in the oven at 120 C the most important

features of these reactions are:

(i) Moisture must be excluded.

(ii) Dry ethanol or commercial redistilled is used.

(iii) The reaction system is closed carefully and left inside the

hood during the night.

The experimental section includes two parts:

Part one: includes preparation of magnesium ethoxide using dry ethanol,

and mercury (I) chloride as a catalyst.

Part two: includes preparation of silicon ethoxide, using the direct

reaction between ethanol and silicon in presence of magnesium ethoxide,

tin tetrachloride and tinnous oxide as catalysts.

2.3.2. Preparation of Magnesium Ethoxide Catalyst:

Using the reaction system (Fig. 2.1.) lor experimental apparatus;

ethanol (100cm', 1.72mol) was put in a two-necked round bottom flask,

then cleaned magnesium metal (1.2.g, 0.05mol)-was:cut into small pieces,

and added in one portion-in addition to a tinny amount of mercury(l)

chloride. In one side neck a thermometer was fitted, and at the main neck

a vertical condenser ended with a guard tube containing calcium chloride

to avoid atmospheric moisture. ;

The reaction mixture was relluxed for about 14 hours and stirred

magnetically using effective magnetic stirrer where the leading of the

thermometer was steady at 78 c. .

, At the end a, white precipitate \vas formed which was separated by

filtration. :1 The. product obtained was analysed using IR spectra. This experiment

was summarised.in Table 2.1.

Table 2.1. Summary of the Reaction Between Ethnnol and

Magnesium Catalysted by Mercury(I) Chloride:

Reflux time/hours

Ethanol /cm

Magnesium /g

Merciiry(I) chloride

Magnesium ethoxide /g :

14.0

100

1.2

Tinny a mount

2.9

2.3.3. Direct Synthesis of Tetraethoxysilane:

2.3.3.1. Catalysed by Magnesium Ethoxide:

Using similar apparatus and procedure as in experiment (2.3.2), dry

ethanol (100cm, 1.72mol) was added to silicon powder (lg, O.()4mol) in

a two necked round bottom flask, then the catalyst magnesium ethoxide -

prepared in experiment 2.3.2. - (0.1 g, 0.001 niol) was added to.

In the side neck, a thermometer was fitted, and at the main neck a vertical

condenser ended with a guard lube containing calcium chloride to avoid

atmospheric moisture was fitted.

The mixture was refluxed .for a bout 4.6 hours continued-with stirring

using effective magnetic stirrer. The reading of the thermometer was

steady at 78 c. •

At the end of the experiment, the final reaction mixture was filtered to

separate unreacted silicon. •

A colourless'•'liquid remained was then fractionated to separate the

product at its boiling temperature. The product was analysed using 1R

spectra arid.GLC.

This experiment was summarised in Table 2.2.

Table 2.2. Summary of the Reaction Between Ethanol and Silicon

Catalysted by Magnesium Ethoxide:

Time /hours • 46.0

Ethanol / cm3

Silicon/ g

Magnesium ethoxide / g

Product/cm"

100

0J

1.9

2.3.3.2. Catalysed by Tin Tetrachloridc:..

Using similar apparatus and procedure as in experiment (2.3.2), dry

ethanol (100cm , \J2mo\) was added to silicon powder (lg,0.04mol) in

a two necked round bottom flask, then the catalyst tin tetraehloride (lew

drops) Was added using a dropper.

In the side neck, a thermometer was fitted, and at the main neck a vertical

condenser ended with a guard tube containing calcium chloride to avoid

atmospheric moisture was fitted.

The mixture was refluxed for a boul 46 hours continued with stirring

using effective magnetic stirrer. The reading of the thermometer was

steady at 78 c.

At the end of the experiment, the final reaction mixture was filtered to

separate uiireaeted silicon. ; ' •

A pale yellow filtrate which,was obtained, was then fractionated to

separate the product at iis boiling temperature. The product was analysed

using IK, spectra ami ( i l \ . \

This experiment was summarised in Table 2.3.

51

Table 2.3. Summary''of the Reaction Between Ethanol and Silicon

Catalysted by Tin Tetrachloride:

Time / hours

Ethanol / cnv

Silicon / g

Tin Tetrachloride /drop

Product/cm

46.0

100

1

5

2.7

2.3.3.3. Catalysed by Tin(ll) Oxide:

Using similar apparatus and procedure as in experiment (2.3.2), dry

ethanol (100cm, 1.72mol) was added to silicon powder (0.5g, ().02mol)

in a two necked round bottom flask, then the catalyst tin oxide (tinny

amount) was added.

In the side neck a thermometer was fitted, and at the main neck a vertical

condenser ended with a guard tube containing calcium chloride - to avoid

atmospheric mojsture - w a s fitted. •

The mixture was re fluxed for about 46 hours continued with stirring

using effective magnetic stirrer.

The reading of the thermometer was steady nt 7<S e. At the end of the

experiment, the final reaction mixture was filtered to separate unreacted

silicon.

A colourless filtrate which was obtained was fractionated to separate the

product at its boiling temperature. The product was analysed using IR

spectra and Gl.('.

,52

This experiment was summarised in Table 2.4.

i,- • • - . ' •

• * * • • • , ' • •

Table 2.4.. Summary'of the Reaction Between Ethanol and Silicon

Catalysfed by Tin oxide:

Time / hours

Ethanol / cm

Silicon/g

Tin oxide

Product / cm

46.0

100

0.5

Tinny a mount

1.3

53

Chapter Three

Results

3. Results: *. • • • • • > j

3.1. Theoretical and Experimental Yields:• i

Yields of tetraethoxysilane obtained using different catalysts were

summarised in the following table:

Table 3.1. Yields of Tetraethoxysilane Obtained Using Different

Catalysts:

The catalyst

••Mg(OC2H5)2

SnCl4

SnO

Experimentalyield /g

1.7872.4831.192

Theoreticalyjeld /g

2.9713.7142.229

Yield %

60.1566.8653.48

3.2. Magnesium Ethoxide Catalyst :

3.2.1. IR Spectrum of the Solid Product:

The white solid magnesium ethoxide obtained was analysed using

the equipment and procedure given.in section (2.2.2.). '

Results are- shown in fable (3.2.). and Fig. (3.1.).

Table 3.2. 'Infra-red Spectra of the Solid Product of the Reaction

Between Ethanol and Magnesium Catalysted by Mercury(I)

Chloride:

Frequency/em'1

3350-3020 b

2790\v1 8 5 0 s . •' •• :i -,

1470s1430s1275w '95 3 w

,i = broad.s = sharp.w = weak.

Assignments

C - 11 Stretch

C - 11 StretchC r-11 StretchC - O - M StretchC - O - M Stretch

• C - C - 0 StretchC-O Stretch

3.2.1. IR Spectrum of the Liquid Product:

IR .spectra -ot the filtrate of reaction 2.3.2. w -,\s oruiued

Results are shown m Table (,3.3.) and Fie. i3'.2\.

Table 3.3. Infra-red Spectra of the Liquid Product of the Reaction

Between Etiianot and Magnesium Catalysted by Mercury (I)

Chloride:

Frequency/cm"

3350-3030 b

2800-2700 b

1350-1330 b

1060 s

875 s

b = broad.

s = sharp.

vv = weak.

Assignments

O - H Stretch

C - 11 Asym. Stretch

C — H Bending

C - C T T T TClh Rocking

55

.4000

106

tot-

2.0 ooW a v e n u m b e r (cm" )

1SCG- " sooo SCO

Wavelength (Microns)

Fig. (3.1.) Infra-Red Spectra of the Solid Product of the ReactionBetween Ethanol and Magnesium (see Table 3.2.)

2OO0Wavenumber (cm" )

19 00 v 1000 900 &D0 loo

r-

. . . Wavelength (Microns)

Fig. (3.2) Infra-Red Spectra of the Liquid Product of the ReactionBetween Ethanol and Magnesium (see Table 3.3. ;

•33. Silicon Ethoxide: '

3.3.1. Catalysed by Mg(OC2H5)2: * v

3.3.1.1. IR Spectrum:

The IR spectra results for tetraethoxysilane obtained using

magnesium ethoxide catalyst are shown in Table (3.4.) and Fig. (3.3.).

Table 3.4. Infra-red Spectra of the Prodiict of the Reaction Between

Ethanol and Silicon Catalysted by Magnesium Ethoxide:

Frequency / cm"

3400 b

2900 m

1500-1350 b

110s

1065 s

895 b

ssiunments

O - H Stretch

C - II Asym Stretch;

C - H Bend

Si - O Asym stretch

•C - O Stretch

SiO.| Asym stretch

b = broad.

s = sharp.

in = moderate.

.58

2000Wavenumber (cm'1)

[coo SCO 700

•o

7 S ? toWavelength (Microns)

iz

Fig. (3.3.) Infra-Red Spectra of the Product of the Reaction BetweenEthanol and Silicon Using Mg(OC2H5)2 Catalyst (see Table 3.4.)

3.3.1.2. GI Ghromatograph:

Results' obtained using GL chromalography are summarised in

Table (3.5.) and Fig. (3.4.).

Table 3.5. GLC Analysis of the Product of the Reaction Between

Ethanol & Silicon Catalysed by Magnesium Ethoxide:

(On Hewlett Packard)

Retention Time (RT)

0.58

0,63

10.15

12.80

Area

29120000

359,90000

20280

37020

Area %

44.685

55.227

0,03 1

0.057

Assignment

Rthanol

Tetrnelhoxysilane

1 Iexaethoxydisiloxane

60

- )—-I I—

2 1.3 0

Fig. (3.4.): Cas-liquid Chromafogrnph of the Product of.the

Reaction Between Ethanol and Silicon Using

I\'ig(OEt)2 Catalyst (On Apye IJnicain).

61

3.3.2. Catalysed by S11CI4: »v. , ' • • • • • • . ; , . . , •• • ;

3.3.2.1. IR Spectrum:

T h e IR spect ra results for t e t rae thoxys i lane obta ined us ing tin

te t raohlor ide ca ta lys t are shown in Tab le (3.6.) and Fig. (3 .5 . ) .

Table 3.6. Infra-red Spectra of the Product of the Reaction Between

Etha'nol and Silicon Catalysted by Tin Tetrachloride:

Frequency/cm"'

3150s

2870 s

1 4 5 0 - 1 4 3 0 b ••

1410-1370 b

1350 m

1210 m

1100-1080 b

1050s

890 s

718 in'

D =.broad.

s s= sharp.

nl ^ moderate.

Assignments

O-

C-

Si-

c -c -

Si

~sT-

-H Stretch

- H A Sym.Stretch

- O Stretch

-H Bend

-II Bend

- () A sym stretch

-OStrelch

62

\z \l- <r S 6 : 7 . S ;• T ^ . ^

Wavelength (Microns)

Fig. (3.5.) Infra-Red Spectra of the Product of the Reaction BetweenEthanol and Silicon Using SnCl* Catalyst (see Table 3.6.)

15

3.3.2.2, GL Chroniatograph

,*. Results obtained using GL chromatography are summarised in

Table (3.7.) and Fig. (3.6.). • • • , . .

Table 3.7. GLC Analysis of the Product of the Reaction Between

Ethanol & Silicon Catalysted by Tin Tetrachloride;

(On Hewlett Packard)

Retention Time (RT)

0.06

0.91

1.35

18.10

Area

121200

481500000

207200

32100

Area %

0.025

99.925

0.043

0.007

Assignment

—

Ethanol

Tetraethoxysilane

1 Iexaethoxydisiloxnne

64

0.91

0

Fig. (3.6.): Gas-liquid Chronialograph of (he Product of (he Keaclion

Behveen Ethanol and Silicon Using S11CI4 Ca(alys(.

(see Table 3.7.).

33.3. Catalysed by SnO: '

3.3.3.1. IR Spectrum::.

T h e IR s p e c t r a resu l t s for t e t r a e t l i o x y s i l a n e o b t a i n e d u s i n g t in ( I I )

o x i d e c a t a l y s t a re s h o w n in T a b l e (3 .8 . ) a n d Fig . ( 3 .7 . ) .

Table 3.8. Infra-red Spectra of the Product of the Reaction Between

Ethanol and Silicon Catalysted by Tin Oxide:

b = brand.

s = sharp,

w = weak.

Frequency cm"'

3 3 5 0 s ; : ; • / •••;•..

2880 w

1450-1370 1)

Tf[0w~

1060 s

890 w 7 '

Assignments

C-

c-

Si-

"si"-

Si-

- H Stretch .

- 11 Stretch

- 0 Stretch

0 A sym Slrelch

- O Stretch

C) Stretch

66

Wavenumber (cm"1)

0 3 7 T 9 io

Wavelength (Microns)

Too

15

Fig. (3.7.) Infra-Red Spectra of the Product of the Reaction BetweenEthanol and Silicon Using SnO Catalyst (see Table 3.8.)

3.3.3.2. GL Chroniatograph:

Results obtained using GL chroniatography are summarised in

Table (3.9!) and Fig. (3.8.)..

Table 3.9. GLC Analysis of the Product of the Reaction Between

Ethano! & Silicon Catalysed by Tin Oxide:

(On Hewlett Packard)

Retention Time (RT)

0.05

0.53

0.62

0.81

7.88

Area

13850

396

456

563200000

392400

Area %

0.002

0.000

0.000

99.928

0.070

Assignment

__

—

lilhanol

Tetraethoxysilane

68

O.Si

- f -

0

Fig. (3.8.): Gas-liquid Chromatograph of Hie Product of the

Reaction Between Elhanol and Silicon Using SnO

Catalyst.

(see Table 3.9.).

:< However*' results obtained from all these experiments are

sequentially discussed in details in the following discussion section.

70

Chapter Four

Discussion & Conclusion

4. Discussion and Conclusion:

4.1. Discussion;This discussion section is concerned With tire .preparation of some' ''

metal alkoxides, magnesium ethoxide and tetraethoxysilane using the

direct synthetic procedure in presence of different catalysts: magnesium

ethoxide, tin tetrachioride and tin oxide.

This procedure was carried out to confirm results and gain experience and

scope of the reaction.

This discussion chapter contains two main parts, firstly the

preparation of magnesium ethoxide by the direct method. The product

obtained have been used as a.catalyst in the second part, which deals with

the synthesis of tetraethoxysilane by the direct method using dry ethanol

and silicon powder in presence of different catalysts.

4.1.1. .Preparation of Magnesium Ethoxide by the Reaction of

Magnesium and Ethanol:

Magnesium ethoxide in 50.9% yield was obtained as a white solid

using the direct synthetic procedure as represented by the equation'7':

Mg + 2CH3a-I2OH .JiM^i ^ Mg(OC2Hi)2 -I- I J2t

The IR characteristic bands of magnesium ethoxide compared with

literature values are shown in the following table:

Table 4.1. •Reported and Obtained 1R Clmraeterislic Bands for

Mg(OC2H5)2:

Clccc-

Assigiiinenls

\)-C a sym. def.-O-lvl stretch- 0 stretch- 0 stretch

Frequency/em"1

Obtained

147014301275

?53_ :

Literature

1470-14351430.1275950

Reference

IK57167

71

These IR results and physical characteristic (state, colour and in.p.)

confirm the .formation of magnesium ethoxide.

4.1.2. Preparation of Tetraethoxysilane by the Reaction o f Silicon

and Ethanol:

Tetraethoxysiiane in 60.15%, 66.86% and 53.48% yields was

prepared using the direct synthetic procedure in presence of Mg(OC2I I.O2,

SnCl.) and SnO catalysts respectively as represented by the equation'7':

Si + 4CH3CH,OH - C a t a i y lL> Si (OCHOi + 2M,tA

The IR characteristic bands of tetraethoxysiiane obtained in these

reactions were compared with the reported values in the following tables:

Table 4.2. Reported and Obtained IR Characteristic Bands for

Si(OC2H5)4 Prepared Using Mg(OC2H5)2 Catalyst:

Assignments

Si - O a syin. Stretch

• C - 0 stretch

Si - O a syni. Stretch

Obtained

Frequency/em"

Literature

1110 .

1065 •

895

1135- 1090 .

1110

065

900

Reference

57

72

Table 4.3. Reported and Obtained IR Characteristic Bands ton

.Si(OC2Hs).» Prepared Using SnCI4 Catalyst:

Si

Si

Si

Assignments

- O Stretch

- O a sym. stretch

- 0. Stretch

Frequeney/cnf'

Obtained

1450-

1100-

1050

890

718 •

-1430

-1080

Literature

1430- 1425

1135-1090

1130- 1000

1090 - 1020

880 - 720

780 - 640

'Reference

57

57

7

17

Table 4.4. Reported and Obtained IR Characteristic Bands for

Si(OC2H5)4 Prepared Using SnO Catalyst:

Si -

Si -

S i -

S i -

Assignments

O Stretch

(.) a sym. stretch

O. Stretch

O. Stretch

Obtain

1450-

1110

1060

890

Frequency/cm '

ed

1370

Literature

1430- 1425

1 1 3 5 - 1090

1 130- 1000

1090 - 1020

800 720

Reference

57

57

17 •

18

7

These 111 results and boiling point confirmed the formation of

letraethoxysilane using Mg(OC\I h)i, SnCl.i and SnO catalysts.

Also the, (iL ehromatograph main beaks - agree with the reported

ones""'1 '.- confirmed the formation of tetraethoxysilane mixed with

hexaethoxydisiloxane which may formed due to hydrolysis by

atmospheric moisture or water which present as minor contaminant in

ethanol.

73

Tetraethoxysiiane could be,obtained using Mg(OC7H5)2, SnCl.i and Sn()

catalysts. . ' '

The efficiency of theses catalyst in accelerating-this reaction is

attributed to the availability of the empty 3d-orbital of silicon and

the unshared pair of electrons at the oxygen of Mg(OC2ll5)2 and SnO

and chlorine of S11CI4 which was in agreement with the proposed

mechanism for this reaction*7'.

S11CI4 was found to be the most effective catalyst, because it is the

most electrons dense so if easily attacks (lie empty 3d- orbital of silicon.

74

4.2. Conclusion

' . . ' • • * '

Tetraethoxysilane has been prepared from, the reaction of elemental

silicon and absolute ethanol using the direct synthetic procedure

according to the following equation:

Si + C2H5OH J ^ l ^ L ^ Si (OC2H5)4 + 21 l2tA

The reaction does not proceed without using any catalyst due to the silica

layer, which covers the silicon surface.

The efficiency of different catalysts was examined under the standard

conditions which was described.

Magnesium ethoxide, tin tetrachloride and tin oxide were found to be

effective catalysts in the direct synthesis of tetraethoxysilane.

The products obtained were characterised using infra-red spedroscopy

and gas-liquid chromatography.

The direct synthesis of tetraethoxysilane from elemental silicon and

ethanol permits the isolation of a relatively high purity product in

comparison with other methods.

The route offers a path to alkoxysilanes and alkoxysiloxanes that is

simple and versatile. In addition, it opens a way to alkoxysiloxanes that '•

are not easily available by "other methods. In many instances it uses

readily accessible starling materials and gives non toxic by products.

Some of the alkoxysilanes and alkoxysiloxanes to which it leads may be

of interest as ceramic precursors.

75

References* T

L. Rochow, E.-G.; Silicon and Silicones; P.I, 40 - 50; Springer

Verlag; 1986.

2. Bradley, D. C , Mehrotra, R.C. &. Gaur, D.P.; Mela! Alkoxides,

P. 1 - 55, 74 - 81; Academic Press INC, (London) Ltd.; 1978.

3. Kirk - Othmer; Encyclopedia of Chemical Technology; Vol. 20;

3rtl Edition; P. 912; John Wiley & Sons, (New York); 1982.

4. Bradley, D.C.; Progress in Inorganic Chemistry, Vol. 2, P. 3, 357;

Interscience, (New York); 1960.

5. Finar, I. L.; Organic Chemistry; Vol. 1, 6lh Edition; P. 181 182;

Continental Printing Co. Ltd., (Hong Kong); 1973.

6. Akhmetor, N.S.; General and Inorganic Chemistry, P. 412 - 4 1 3 ; .

Mir Publishers; 1987.

7. Sulayman, N.M.; M.Sc; Thesis; P. 3 -33 , 78; U. ofK. (Sudan);

1995.

8. Purccll, K.F. & Kotz, J.C.; Inorganic Chemistry; P. 3 19, 477; 1 loll

- Saimders International Editions; 1977.

9. Cotton, F.A. & Wilkinson, G,; Advanced Inorganic Chemistry; 3"'

Edition; P. 309 ~ 333; John Wiley & Sons (New York); 1972.

10. Aylett, B.J.; Organometalic Compound's; Vol. 1, Part Tow;'4 t h

Edition; P. 8; John Wiley & Sons, (New York); 1970.

11. Mineral Gallery -The Silicate Class; P. 1; Amethyst galleries Inc.;

1998.

12. Chang, P.S. & Ikiese, M.A.; .1. Am. Chcm. Soc; Vol. 115; P.

11475 84; 1993,

13. Brel/.inger, • I.)... ^ Josten, • W.; Encyclopedia of. Chemical

Technology.; Vol. 2; 3rd Edition; P. 1-17; John Wiley & Sons,

(New York'); 1 ^82.

. 76

14. Bradlecy, D.C., Mehrotra, R.C., Swanwick, J.D. & Wardlaw, W.;J.

' Chem.Soc.; Vol. 2; P. 2025-30; 1953.

15. Bradlecy, D C ; Advanced Inorg.Chem. Radio Chem.; Vol. 15;

P. 259; 1972.

16. Aleksa, V., Klaeboe, P., Nielsen, C.J., Tanevska, V. & Guirgis,

G.A.; Vib Spectrsc; Vol 17(1); P. 1 - 30; 1998.

17. Smith, A.L.; Analysis of Silicones; Vol.,41; P. 217-242,271;

Wiley & Sons, (London); 1974.

18. Jones, R.A. & Cross, A.D.; An Introduction to Practical Infra-red

Spectroscopy; 3rd Edition; P. 66 - 67, 72 - 96; Butter Worth,

(London); 1969.

19. Omer, O. Y.; Ph.D. Thesis; P. 40; University of Manchester; 1989.

20. Peetre, I.B;J. of Chromatography; Vol. 90( 1); P. 35 - 55; 1974.

21. Ahmed, A.Y.; M.Sc. Thesis; P. 81; U. of K. (Sudan); 1997.

22. Gpel, S.C., Chiang, M.Y. & Bulwo, W.E; Inorg. Chem.; Vol. 29;

•',.., P. 4640-46; 1990.

23. Bidell, W., Doring, J., Hans, W.B., Hund, U., Plappert, E. & Berke,

IT; Inorg. Chem.; Vol. 32; P. 502 - 507; 1973.

24. Ingham, R.K. & Hurtstman, W.D.; CA; Vol. 79; P. 508-509;

1973.

25. I lays, D.S. & Fu, G.C.; J. Org. Chem.; Vol. 61; P. 4; 1996.

26. Shah, G.B.; J. Appl. Polym. ScL; Vol. 70(1 1); P.2235 - 3 9 ; 1998.

27. Lammcrtink, R.G., llempenius, M.A. & Vancso, G.J.; Macromol.

Chem. Physics; Vol. 199(10); P. 2141 - 4 5 ; 1998.

28. Yang, IT, Aspiund, M.C., Kotz, K.T., Wilkens, M.J., Frei, 11. &

. Harris, C.B.; J. Am. Chem. Soc; Vol. 120(39); P. 10154 65;

1998.

29. Emblem, H.G.; Materials Chemistry & Physics; Vol. 8; P. 2 6 5 -

277; 1983.

.. '• 7 7 ' ' '•

Aelion,-R.,. Loebel, A. & Eirich, F.; j . Am. Chem. Soc.; Vol. 72,

P. 5-705- 11; 1950. •

Yoldas, B.E.; J. of 'Polymer. Science; Vol. 24( 12); P. .3475 - 78;

1986.

Paul, A.; Chemistry Of Glasses; 2"d Edition, P. 52 - 53; Champan

& Hall Ltd. (London); 1990.

Wijk, ML, Norrestam, R. & Westin, G.; Inorg. Chem.; Vol. 35;

P. 1077-79; 1996.

Kepler, C.L., Londergan, T.M., Lu, J.Q., Pa'ulasaari, J. & Weber,

W.P.;J. Polymer; Vol. 40(3); 1999,

Greenwood, N.N. & Earnshow, A.; Chemistry of the Elements;

P. 3 i 7 - 319, 380 - 382; Butler Worth - I Icinemann 1 ,td.; 1984,

Wideman, T., Fazen, P.J., Su, K., Remesen, E.E., Zank G.A. &

Sneddon, L.G.; Appl Organometai Chem; Vol. 12; P. 681 - 693,

725-734; 1998.

Hench, L.L. & -West,. J.K.; Chem. Rev.; Vol. 90(1); P. 33 - 72;

1990.

Good, G.B. & kenney, M.E.; Inorg. Chem.; Vol. 29; P. 1216; 1990.

Ojima, .1., Fracchiolla, D.A., Donovan, R.J.& Banerji, P.; J. Org.

Chem.; Vol. 59; P.7594 - 95; 1994.

Saucier, R.; CA; Vol. 56; P. 3357c; 1962.

Seylerth, 1). ^. Rochovv, E.G.; J. Org. Chem.; Vol. 20; P. 250 -

255; 1955. ' •. , ' '.

Ridge, D. & 1odd,M.; J; Chem. Soc'.; Pt. 4; 2637; 1949. :

Hyde, -J.F. & Curry, J.W.; .1. Amr. Chem. Soc; Vol. 77;

P. 3140-41;; 1955.

Okavvara,. P.; CA; Vol. 47;'P. 4478h; 1953.

Okawara, P.'/Numa, V. & Wastase, T.; CA; Vol. 49; P. 1154b;

1955.

78

46. Fukiikawa, S.; C M Vol. 49; P. 11315; 1953.

47. Seif, C.A.V -Bosch^i, R.H. & Holder, J.'P.; CA; Vol. 72; P. 78391 j ;

1970. , , .. . '

48. Bhule, R.S. & Swasamban, M.A.; CA; Vol. 76; P. 85383u; 197).

49. . Ivanoi, V. 1., Monsalyskii, V.N., Samolilov, S.JVI. & Tsvetkov,

O.N.; CA; Vol. 83; P. 590213; 1975.

50. Newton, W.E. & Rochow, E.G.; Inorg. Chem.; Vol. 9(5); P. 1071 -

76; 1970.

51. Rochow, E.G.; J. Amr. Chem. Soc; Vol. 67; P. 963 - 965; 1945.

52. Baumgarten, R.L.; Organic Chemistry - A Brief Survey; P. 1 6 8 -

169; John Wiley &'Sons (London); 1978.

53. Rochow, E.G.; J. Amr.Chem. Soc; Vol. 70; P. 2170 - 2179; 1948.

54. Montle, J.F., Markouski, II.J., Lodewyck, P.I). & Schneider, D.F.;

CA; Vol. 97; P. 56006b; 1932.

55. Magee, W.L.•& Telshow, J.E; CA; Vol. 95; P. 21977x; 1982.

56. Okawara, R., Hatta, S. & Shirma, T.; Bull. Chem. Soc. Japan; Vol.

28; P. 541; 1955.

57. Man;, G. & Pockett, B.W.; Practical Inorganic Chemistry.; P. 97;

Van Nastrand Reinhold Company (London); 1972.

58. The Perkin Elmer Catalogue of Gas Liquid Chromatography;

Perkin Elmer Supplies; P. 56; 1984.

79