B.Sc. Semester VI - ChemZone
Transcript of B.Sc. Semester VI - ChemZone
Prepared By: Dipen Shah B.Sc. / MATERIAL / SEM-VI / Chemistry - 602 / Unit-1 Page: 1 of 16
B.Sc. Semester – VI
Subject: - CHE - 602: Polynuclear Aromatic Hydrocarbons
Prepared By: - Dipen Shah
Contents:
Introduction
Synthesis and Chemical Properties of:
A. Biphenyl
B. Diphenyl methane
C. Naphthalene
D. Anthracene
Introduction
Two or more aromatic (benzene or non-benzene) rings are fused together its called
polynuclear aromatic hydrocarbon (PAH).
Polynuclear aromatic hydrocarbon potential health risk due to their inner chemical
stability, high reactivity to different types of degradation and high toxicity to living
organisms.
Physical and Chemical Properties of Polynuclear Aromatic Hydrocarbon
PAHs as pure chemicals exist as colorless, white, or pale yellow-green solids.
They are non-polar, hydrophobic compounds, which do not ionize.
They have a faint odor.
Enter the environment (Air, Water and Soil)
PAHs are introduced into the environment mainly via natural and anthropogenic burning
processes.
PAHs enter air as releases from forest fires, residential wood burning and exhausts from
vehicle.
Some PAH particles can readily evaporate into the air from soil or surface waters.
They can also enter surface water through discharges from industrial plants and
wastewater treatment plants.
Most of PAHs don‘t dissolve easily in water.
They stick to solid particles and settle to the bottoms of lakes or rivers.
PAHs in soils also contaminate underground water.
We are most likely to be exposed to PAH vapors or PAHs that are attached to dust and
other particles in the air.
Sources include cigarette smoke, vehicle exhausts, asphalt roads, coal, etc.
PAHs effect on human and animal body
Breathing or touching mixtures of PAHs and other chemicals for long periods of time have
developed cancer in human body.
Some PAHs have caused cancer in laboratory animals when they breathed air containing
them (lung cancer), ingested them in food (stomach cancer), or had them applied to their
skin (skin cancer).
In the body, PAHs are changed into chemicals that can attach to substances within the
body. Special tests that can detect PAHs in body tissues or blood.
Analytical determination of PAHs
Samples of PAHs are mostly analysed by HPLC using fluorence detection, or by gas
chromatography method with flame ionizazion detection, or mass spectrometry.
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Classification of Polynuclear Hydrocarbon
Polynuclear Hydrocarbons
Benzenoid Non-Benzenoid
Isolated Rings Fused Rings
Linear Angular
Isolated Polynuclear Hydrocarbon
One or more benzene ring which are either isolated from each other or attached to each
other (via one carbon) it’s called Isolated PAHs.
They have independent benzene ring.
They are also known as polyphenyl compounds.
Examples: Biphenyl, Terphenyl, Diphenylmethane, Triphenylmethane etc.
Fused (Condensed) Polynuclear Hydrocarbon
Two benzene ring are fused with each other at two common points (Ortho to each other)
it’s called fused PAHs.
The two benzene rings two carbon atoms have common.
A large number of fused polynuclear hydrocarbon are carcinogenic.
Examples: Naphthalene, Anthracene, Phenanthrene etc.
A. Biphenyl
Introduction
Biphenyl is an organic compound that forms colorless crystals. It has a distinctively
pleasant smell. Biphenyl is an aromatic hydrocarbon with a molecular formula C12H10.
It is notable as a starting material for the production of polychlorinated biphenyls (PCBs),
which were once widely used as dielectric fluids and heat transfer agents.
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Biphenyl is also an intermediate for the production of a host of other organic compounds
such as emulsifiers, optical brighteners, crop protection products, and plastics. Biphenyl
is insoluble in water, but soluble in typical organic solvents. The biphenyl molecule
consists of two connected phenyl rings.
Synthesis
Many reactions have been reported for the biphenyl. Among them some reactions are given below.
By Fitting reaction (extension of Wurtz reaction)
Diphenyl can be synthesized by heating an ethereal solution bromobenzene with metallic sodium.
By the reaction between bromobenzene and hydrazine
Diphenyl is obtained by refluxing an alkaline solution of ethanolic bromobenzene and hydrazine in presence of palladium catalyst supported over CaCO3.
From aryl magnesium halide
Aryl magnesium halides on reaction with aryl halides in presence of a small amount of CoCl2, NiCl2, or FeCl3 give polynuclear hydrocarbon. For example, phenyl magnesium bromide on reaction with bromobenzene in presence of CoCl2 gives diphenyl.
Ullmann reaction
The Ullmann reaction is an organic reaction used to couple two molecules of aryl halide to form a biaryl using copper metal and thermal conditions.
By Suzuki reaction
The Ullmann reaction is an organic reaction used to couple two molecules of aryl halide to form a biaryl using copper metal and thermal conditions.
Physical properties of diphenyl
Colourless crystalline solid compound
Molecular formula C12H10
Molecular Weight 154 g/mole
Melting Point 69-71°C
Insoluble in water but soluble in organic solvent
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Chemical properties of diphenyl or Chemical reaction of diphenyl
Diphenyl can be regarded as phenyl substituted benzene, it gives almost the same
reaction as observed with benzene. The phenyl group cause the substitution to take place
mainly at the para position and a small proportion of ortho substituted product is
obtained. Here one phenyl group act as an electron releasing and the other as an electron
accepting group. The second substitution generally goes to the un-substitution phenyl
group.
Biphenyl shows aromatic character with remarkable stability. It can be represented in
several contributing structures as shown below:
Nitration
Nitration of diphenyl gives mainly 4-nitrodiphenyl and negligible proportion of 2-
nitrodiphenyl. Further nitration gives 4,4’-dinitrodiphenyl main product and negligible
amount of 2,2’-dinitrodiphenyl and 2,4’-dinitrodiphenyl.
Halogenation or Chlorination of Diphenyl
Diphenyl is chlorinated in presences of Fe gives 4-chlorodiphenyl as main product and 2-
nitrodiphenyl as negligible proportion. Further chlorination gives 4,4’-dichlorodiphenyl
and negligible amount of 2,2’-dichlorodiphenyl and 2,4’-dichlorodiphenyl.
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Sulphonation
Sulphonation of diphenyl using concentrated H2SO4 as a sulphonating agent gives
diphenyl-4-sulphonic acid main product (by product diphenyl-2-sulphonic acid is
obtained in negligible amount) which on further sulphonation gives diphenyl-4,4’-
disulphonic acid (by product diphenyl-2,2’-disulphonic acid diphenyl-2,4’-disulphonic
acid are obtained in negligible amount).
Oxidation
Biphenyl is oxidized with chromic acid undergoes oxidation to give mainly CO2 and H2O
along with small amount of benzoic acid. However, ozonolysis of biphenyl at about -20 °C
gives mainly benzoic acid (80 %).
Uses of Diphenyl
Use in production of polychlorinated biphenyls (use as plasticizers).
Widely used as dielectric fluids and heat transfer agents.
Intermediate for the production of a host of other organic compounds such as emulsifiers,
optical brighteners, crop protection products, and plastics.
B. Diphenylmethane
Introduction
Diphenylmethane is an organic compound with the formula (C13H12). The compound
consists of methane wherein two hydrogen atoms are replaced by two phenyl groups.
Diphenylmethane forms a common skeleton in organic chemistry; the diphenylmethyl
group is also known as benzhydryl.
Synthesis
Many reactions have been reported for the biphenyl. Among them some reactions are given below.
By Friedel Crafts Reaction
Friedel Crafts alkylation of benzene on with benzyl chloride in presence of anhydrous AlCl3 yields diphenylmethane.
By the reaction with benzene with formaldehyde
Diphenylmethane can be also synthesized by the reaction between benzene and formaldehyde in presence of concentrated H2SO4.
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From benzyl alcohol and benzene
Diphenylmethane can be obtained by heating a mixture of benzyl alcohol and benzene with H2SO4 and CH3COOH.
From Grignard reagent
Reaction of benzyl chloride with phenylmagnesium bromide gives diphenylmethane.
Physical properties of diphenylmethane
Colourless oil
Molecular formula C13H12
Molecular Weight 168 g/mole
Melting Point 22-24 °C
Insoluble in water but soluble in organic solvent
Chemical properties of diphenylmethane or Chemical reaction of diphenylmethane
The chemical reactions of diphenylmethane are similar to that of biphenyl.
Diphenylmethane may be considered as benzyl substituted benzene. The benzyl group is
an ortho-para (o-p) directing group. The mono-substituted product is mainly 4-
substituted and the second substituent mainly enters p’ or 4’ position.
In diphenylmethane both the benzene rings are electron attracting groups making the –
CH2 group highly reactive. Hence in some reactions, like bromination, instead of
substitution at the phenyl group the hydrogen of the methylene group is substituted.
Nitration
Diphenylmethane on reaction with conc. HNO3 and conc. H2SO4 undergoes nitration. The
first –NO2 group enter the 4 or p position and giving p-nitro diphenylmethane. The second
–NO2 group enter the 4’ or p’ position of the second ring and giving p,p’ dinitro
diphenylmethane.
Bromination
In diphenylmethane both the benzene rings act as negative groups, hence on reaction
with Br2 the –H of the –CH2 group being highly reactive undergoes substitution giving
diphenyl methyl bromide.
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Chlorination
Chlorination of diphenylmethane in presence of Fe gives 4,4’-dichloro diphenylmethane.
But chlorination of diphenylmethane in presence of sunlight gives diphenyl
dichloromethane.
Oxidation
Oxidation of diphenylmethane with chromic acid gives benzophenone. In this reaction
methylene group is converted to a carbonyl group (>C=O).
Cyclization
On passing vapours of diphenylmethane through red hot iron pipe cyclization take place
and converting diphenylmethane to flurorene.
Uses of Diphenylmethane
It has pleasant odour (like orange) and hence is used in making soaps, shampoo etc.
C. Naphthalene
Introduction
Naphthalene is an organic compound with molecular formula C10H8. It has simplest
polycyclic aromatic hydrocarbon, and is a white crystalline solid with characteristic odor.
As an aromatic hydrocarbon, naphthalene’s structure consists of a fused pair of benzene
ring.
Synthesis
Many reactions have been reported for the biphenyl. Among them some reactions are given below.
From 4-phenylbut-1-ene
Naphthalene can be obtained by passing vapours of 4-phenyl-1-ene over red hot calcium oxide.
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From 4-phenylbut-3-enoic acid
On heating with concentrated H2SO4 4-phenylbut-3-enoic acid undergoes cyclization to from 1-naphthol, which on heating with Zn dust yields naphthalene.
By Haworth reaction
This method consists of condensing benzene with succinic anhydride in presence of
anhydrous AlCl3 (friedel craft reaction) to from keto acid which is subsequently
reduced, heated with concentrated H2SO4, again reduced and finally dehydrogenated with
selenium to give naphthalene.
Physical properties of naphthalene
White crystalline solid compound
Molecular formula C10H8
Molecular Weight 128 g/mole
Melting Point 78-80 °C
Insoluble in water but soluble in ether, benzene, and hot alcohol
Characteristic tar like odour and volatile
Consists of two benzene rings fused at ortho position in which like benzene each carbon
atom is sp2 hybridize and forms 3σ bonds in the same plane.
Each carbon atom has an electron in the p-orbital that is not involved in hybridization.
Due to overlapping of p-orbitals a π-electron cloud is formed that is spread equally over
both the rings.
The resonance energy of naphthalene is 61.0 kcal/mole while that the benzene is 36
kcal/mole. Naphthalene fused with two benzene ring so theoretically resonance energy is
72.0 kcal/mole. The deference in the resonance energy is 11.0 kcal/mole. Thus
naphthalene undergoes addition, oxidation and reduction reaction more rapidly than
benzene.
Chemical properties of naphthalene or Chemical reaction of naphthalene
Reactivity of Naphthalene
Naphthalene undergoes electrophilic substitution reaction to form theoretically two
different product α and β. Generally, the α-isomer predominates except in case of
sulphonation and friedeal craft reaction.
Electrophilic substitution reaction depends on two steps:
1. Formation of carbocadions – rate determination step
2. Joining of the electrophile to the carbocation.
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When the electrophile attacks α-position, the carbocation formed is a resonance structure
I and II. In both this structures the positive charged remains on the ring which is
attacked while the other ring maintains its aromatic character. Hence both the structures
are equally stable.
However, when the electrophile attacks the β-position the carbocation formed is a
resonance structure III and IV. In structure III the positive charge remains on the ring
that is attacked and the other ring maintain its aromatic character (hence more stable)
while in structure IV the aromatic character of both the rings is disrupted (hence less
stable).
Orientation
Whether the second substituent would enter the mono-substituted naphthalene ring and
if so at what position depends on the nature and position of the first substituent.
Generally, the first substituent is at the α-position (exception sulphonation and friedeal
craft reaction). On the basis of the nature of the first substituent (electro releasing or
electro withdrawing) the second substituent would occupy certain specific position.
Group present Position of 1st Substitution
Position of 2nd Substitution
Type
-CH3, -OH, -NH2, -Cl, -Br, -NHR, -NHCOCH3
α (1-position) 4th – Major
2nd – Minor Homonuclear
-CH3, -OH, -NHCOCH3 β (2-position)
1-position Homonuclear
If incoming group is –SO3H
than at 6-position Heteronuclear
-NO2, -SO3H, -COOH
α (1-position)
or
β (2-position)
5th and 8th position Heteronuclear
-X, -NH2 β (2-position) 5th and 8th position Heteronuclear
Substitution reaction
Nitration
Nitration of naphthalene at 50-60 °C gives 1 or α-nitronaphthalene. Now since the –NO2
group deactivates the ring further reaction take place in the second ring giving mainly
1,8-dinitronaphthalene along with some 1,5-dinitronaphthalene.
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Halogenation (Bromination and Chlorination)
Naphthalene undergoes bromination on reaction of Br2 in boiling CCl4 or CH3COOH as
solvent to give 1-bromonaphthalene which on further bromination gives 1,2-
dibromonaphthalene (minor product) and 1,4-dibromonaphthalene (major product).
Reaction naphthalene with thionyl chloride (SOCl2) and sulfurly chloride (SO2Cl2) in
equimolar proportion at 25 °C in presence of AlCl3 gives 1-chloronaphthalene. However at
100-140 °C in the ration of 1:2 further chlorination gives 1,2-dichloronaphthalene (minor
product) and 1,4-dichloronaphthalene (major product).
Sulphonation
Sulphonation of naphthalene by concentrated H2SO4 at 80 °C gives mainly naphthalene-
1-sulphonic acid. This reaction is reversible. When naphthalene-1-sulphonic acid is
heated its converted to naphthalene-2-sulphonic acid. However, sulphonation at a higher
temperature of 160 °C gives mainly naphthalene-2-sulphonic acid.
Friedel Craft Acylation
Acetylation of naphthalene gives a mixture of 1 and 2 isomers. When the reaction is
carried out in carbon disulfide (CS2) solvent 75 % 1-actyl naphthalene and 25 % 2-actyl
naphthalene is obtained. When in nitrobenzene solvent it gives 10 % 1-acetyl naphthalene
and 90 % 2-acetyle naphthalene is obtained.
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Oxidation reaction
With HgSO4 and H2SO4
Oxidation of naphthalene with HgSO4 and Concentrated H2SO4 or V2O5 in presence of air
gives phthalic acid.
By alkaline KMnO4
Oxidation of naphthalene by alkaline KMnO4 gives phthalonic acid.
By acidic KMnO4
Oxidation of naphthalene by acidic KMnO4 gives phthalic acid.
By chromic acid
Oxidation of naphthalene with chromic acid gives 1,4-naphthaquinone.
With ozone
On reaction with ozone naphthalene forms diozonide, which on further hydrolysis forms
phthalyldehyde.
Addition reaction
Reaction with Na and ethanol
Naphthalene on reaction with sodium and ethanol undergoes reduction (addition of H2)
giving 1,4-dihydronaphthalene (1,4-dialine) which is unstable and rapidly undergoes
dehydrogenation giving back naphthalene.
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Reaction with Na and isopentanol
Reduction of naphthalene with sodium and isopentanol gives 1,2,3,4-tetrahydronaphthalene
called tetralin.
Reaction with H2/Ni
On reaction with naphthalene in presence of nickel as a catalyst. First tetralin and finally
dehydronaphthalene (decalin) is obtained.
Reaction with dry Cl2
Naphthalene undergoes addition reaction with dry Cl2 giving 1,2-dichloro-1,2-
dihydronaphthalene and 1,2,3,4-tetrachloro-1,2,3,4-tetrahydronaphthalene.
Reaction with Sodium
Addition of naphthalene with sodium 1,4-disodium-1,4-dihydronaphthalene, which react
with CO2 giving sodium salt of 1,4-dihydronaphthalene-1,4-dicarboxylic acid.
Uses of Naphthalene
Important source of phthalic acid and anthranilic acid which constitute the intermediates
of important dyes such as indigo, triphenylmethane and azodyes.Use in production of
polychlorinated biphenyls (use as plasticizers).
Used as moth repellent and insecticide.
The reduced forms naphthalene i.e. decalin and tetralin are used in motor fuel and as
lubricants and also as solvents.
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D. Anthracene
Introduction
Anthracene is solid PAH of formula C14H10 consisting of three benzene rings. It is a
component of coal tar. Anthracene is used in the production of the red dye alizarin and
other dyes. Anthracene is colorless but exhibits a blue fluorescence under ultraviolet
light.
Synthesis
Many reactions have been reported for the biphenyl. Among them some reactions are given below.
By Friedel Crafts Reaction from benzyl chloride
Friedel Crafts reaction of two moles of benzyl chloride in presence of anhydrous AlCl3 gives 9,10-dihydroanthraquinone which readily undergoes dehydrogenation to give anthracene.
By Friedel Crafts Reaction from benzene and methylene dibromide
Anthracene can be also obtained by the reaction of benzene with methylene dibromide in presence of anhydrous AlCl3.
By Fitting reaction
In this reaction o-bromobenzylbromide is heated with Na in presence of ether to give dihydroanthracene which on further oxidation gives anthracene. By this reaction phenanthrene may be obtained as by product.
From phthalic anhydride and benzene
The reaction between phthalic anhydride and benzene gives o-benzyl benzoic acid. On
heating with H2SO4 o-benzyl benzoic acid undergoes cyclization to give anthraquinone which on further distillation with Zn dust gives anthracene.
From phthaloyl chloride and benzene
Reaction of phthaloyl chloride and benzene in presence of anhydrous AlCl3 gives anthraquinone which on distillation with Zn dust gives anthracene.
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Physical properties of anthracene
Colourless solid
Molecular formula C14H10
Molecular Weight 178 g/mole
Melting Point 216-218 °C
Insoluble in water but soluble in organic solvent
More reactive than benzene and naphthalene
Exception reactivity of 9,10 position (points of maximum electron density)
Chemical properties of anthracene or Chemical reaction of anthracene
Isomerism and anthracene derivatives
Anthracene forms three monosubstituted isomers 1 or α, 2 or β, and 9 or γ.
In this disubstituted anthracene, if both the substituents are identical there are 15
isomers but more isomer if the substituents are different.
Addition reaction
Reduction in presence of Na and ethanol
Reduction of anthracene with sodium and ethanol gives 9,10-dihydroanthracene which
on reaction with concentrated H2SO4 give back anthracene.
Reduction with H2/Ni
On reduction with Ni at 200-250 °C anthracene gives gradually tetra, hexa, octa and
finally deca or perhydroanthracene (C14H24).
Addition with 1 mole of O2
Anthracene undergoes addition reaction with one mole of oxygen in presence of sunlight
to give anthracene peroxide.
Dimerization of anthracene
Anthracene in its saturated solution in xylene undergoes dimerization in sunlight giving a
dimer of anthracene.
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Substitution reaction
Halogenation
Chlorination of anthracene: On passing chlorine gas through a cold solution of
anthracene in CS2 anthracene first undergoes an addition reaction forming 1,2-dichloro-
1,2-dihydroanthracene which on heating or on reaction with alkali gives 9-
chloroanthracene. 9-chloroanthracene can be also obtained by the reaction of anthracene
with chlorine at 100 °C.
Anthracene can also undergo bromination on reaction with Br2 the reaction take place in
boiling CCl4
Nitration
Anthracene undergoes nitration on reaction with HNO3 in acetic anhydride at 15-20 °C
giving 9-nitroanthracene and 9,10-dinitroanthracene.
Sulphonation
Anthracene undergoes sulphonation easily giving a mixture of anthracene-1-sulphonic
acid. If the reaction is carried out at mild conditions the 1and 2 isomer obtained in
equimolar proportion but at high temperature the major product is anthracene-2-
sulphonic acid.
If the reaction is carried out using excess H2SO4 at low temperature anthracene-1,8-
disulphonic acid is obtained and at high temperature anthracene-2,7-disulphonic acid is
obtained.
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Acylation
Friedel craft reaction of anthracene with acetyl chloride in benzene or nitrobenzene gives
a complex mixture. However, the main product in nitrobenzene solvent is 1-acetyl
anthracene while in ethylene dichloride solvent the product is 9-acetyl anthracene.
Oxidation reaction
Oxidation with HNO3 and chromic acid
Oxidation of anthracene by concentrated HNO3 or chromic acid or vanadium pentoxide in
air gives anthraquinone.
Uses of Anthracene
Anthracene is used in the synthesis of dyes like anthraquinone and alizarin.
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