CHAPTER 1

INTRODUCTION

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1.1 Introductory:

Manganese is the member of first transition series and belongs

to the VIIth

A group of the periodic table. Manganese atom has 3d5 4s

2

configuration and Mn(II) has 3d5 structure, hence manganous

compounds are stable. The 3d5 electrons of manganese can be

removed by successive oxidation and due to this reason it can form

variable oxidation states. Manganese has a wide range of oxidation

states from +2 to +7. In the +2 oxidation state compounds are known

as manganous compounds, in +3 oxidation state they are designated as

manganic compound and in +4 oxidation are known as manganatied.

In +6 oxidation state its salts are known as mangnates whereas in +7

state the salts are known as permanganates. Though the highest

oxidation state of manganese corresponds to the total number of 3d

and 4s electrons, but it occurs only in the oxo compounds like Mn04─,

Mn207 and MnO3F [1]. The most common oxidation state of

Manganese are +2, +4 and +7 but +3 and +6 can also be readily

obtained. Mn(III) can be prepared by the reaction of manganese(II)

with manganese(IV) oxide in either acid or alkaline medium. The

reaction can be given by the following equations:

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(i) Acidic medium:

MnO2 + 4H+ + e

─ Mn

3+ + 2H2O E

0 = +0.95V .....1.1

Mn3+

+ e ─ Mn

2+ E

0 = +1.15V .....1.2

(ii) Basic medium:

MnO2 + 2H2O + e ─ Mn(OH)3 + OH

─ E

0 = +0.20V .....1.3

Mn(OH)3 + e ─

Mn(OH)2 + OH─ E

0 = -0.10V .....1.4

The manganese(III) oxidation state is known, in compounds

such as manganese(III) acetate, but these compounds are quiet

powerful oxidizing agents. The electrolytic oxidation of

manganese(II) acetate results in the formation of manganese(III)

acetate [2].

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1.2 Nature of manganese(III):

Manganese(III) has attracted much attention with regards to the

oxidation of various biological substrates. Mn(III) por-phyrins have

been studied as possible models for closely related biologically

significant systems [3,4]. In solutions the oxidation potential in

several oxidation states depends upon the hydrogen ion concentration

and ions present in solution. The relevant experimental data for the

principle oxidation states (zero, +2, +3, +4, +6 and +7) are as given

below [5,6,7]:

The nature of the oxidizing species present in manganese(III)

solution also depends upon the nature of the media. Several studies

have been reported on the kinetics of manganese(III) oxidation of

Mn2+

+ 2 e ─ Mn E

0 = -1.18V …..1.5

Mn3+

+ e ─ Mn

2+ E

0 = +1.51V …..1.6

MnO2 + 4H+ + 2 e

─ Mn

2+ + 2H2O E

0 = +1.23V …..1.7

MnO4─ + 8H

+ + 5 e

─ Mn

2+ + 4H2O E

0 = +1.51V …..1.8

MnO4─ + e

─ MnO4

2─ E

0 = +0.56V …..1.9

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substrate in perchlorate [8,9], sulphate [10,11,12], acetate [13,14,15]

and pyrophosphate [16] media. The redox potential Mn(III)-Mn(II)

couple depends upon the acid media and the complexing agent like

persulphate, pyrophosphate, fluoride and chloride [12].

1.2.1 Stability of Mn (III):

Mn(III) solutions are not very stable in aqueous perchloric acid

media, but relatively more stable in sulphuric acid. It has been

reported that Mn(III) ion undergoes hydrolysis even in fairly strong

acid. The hydrolysis of Mn(III) ion was investigated by different

workers [17,18,19]. In these studies the estimated hydrolysis constant

of Mn(III) was 5.0-mol dm─3

at 23°C, 1.5 mol dm─3

and 0.93 mol

dm─3

at 25°C. These hydrolysis constants were estimated for the

reaction given by equation 1.10.

Mn3+

+ H2O

kh

MnOH2+

+ H+

….. 1.10

The sulphate ion affects the disproportionation equilibrium or

the hydrolysis of Mn(IV) and stabilizes the Mn(III) species in aqueous

acetic medium. The presence of manganese(II) ion with common

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anion also gives the more stability to the Mn(III) species present in

acid medium.

1.2.2 Species of Mn(III):

In presence of acetic acid the formation of trivalent manganese

in equilibrium with acetate ion has been proposed [20] by equation

1.11.

Mn(OAc)3 + OAc─ [Mn(OAc)4]

─ ….. 1.11

The absorption spectra of both Mn(aq)3+

and Mn(OH)(aq)2+

have

been reported to be similar in both the visible and UV region.

Formation of dihydroxo species Mn(OH)2(aq)+ is another possibility

[21].

Mn(OH)(aq)2+

+ H2O

Mn(OH)2 (aq)+ + H

+ ….. 1.12

In sulphuric acid the formation of Mn(aq) 3+

, Mn(OH)(aq)2+

and

MnSO4+ have been suggested as the species of Mn(III) species [10].

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In the presence of F ion the species of Mn(III) in aqueous solution

consists of hexaaquamanganese(III) {[Mn(H2O)6]3+

}, Mn(aq)3+

,

hydroxopentaaquamanganese(II) {[Mn(OH)(H2O)5]2+

}, Mn(OH)(aq)2+

and MnF(aq)2+

[20].

In acidic medium manganese(II) and manganese(III) has been

considered as chelates, Mn(H2P2O7)22─

and Mn(H2P2O7)33─

with

H2P2O72─

ion as chelating ligands [22].

Depending upon the experimental conditions absorption

maxima for the Mn(III) species occurred at different wavelengths that

are summarized in Table 1.1. The values of wavelengths also depend

on complexation of the ion and pH of the media.

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Table 1.1:Absorption maxima wavelength of Mn(III) in different

media:

1.3 Oxidations by manganese(III):

Initial work on the oxidation by manganese(III) has received

considerable attention, a large number of substrates with solution

properties have been reviewed by Davies [8]. It is the stability of the

manganese(III) species in presence of different ions namely

perchlorate, pyrophosphate, acetate and sulphate in acidic solution,

S. No Media λ max

(nm)

Reference

1. Sulphate 490nm

500nm

510nm

12

11

24

2. Acetate 400nm

470nm

350nm

20

16

23

3.

Pyrophosphate

500nm

16

4.

Perchlorate

470nm

25

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which has gained the attention of several workers in studying the

oxidation of various substrate [12,26,27,28,29,30,31,32,33]. However

few oxidations have been found successful in understanding the

biological systems. On this account an attempt has been made only to

discuss the oxidations useful to biological system and related to the

present study. Beside this the oxidations discussed here are indicative

of continuous development of the subject in every decade.

1.3.1 Oxidation of quinol:

The oxidation of quinol has been reported in aqueous

perchlorate media by use of the stopped flow technique [25]. In the

study the mechanism of oxidation of quinol by Mn(III) was compared

with both inner and outer-sphere oxidations of other ligands by

manganese(III). The following scheme was proposed.

Mn3+

aq + XH+ Mn

3+X H

+aq …..1.13

MnOH2+

aq + XH+ Mn

3+Xaq …..1.14

Mn3+

X H+

aq Mn3+

Xaq + H+

aq …..1.15

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Mn3+

HXaq Mn(III)

…..1.16

Radicals Products

Mn3+

Xaq …..1.17

1.3.2 Oxidation of glycolic acid:

The kinetics of oxidation of glycolic acid in perchlorate media

has explained by assuming the Mn3+

aq ~ MnOH2+

aq as reactive species

[34]. The disproportionation equilibrium which explains the

stabilizing of manganese(III) ion in the presence of Mn(II) also given

as equation 1.18.

Mn(III) + Mn(III) Mn(IV) + Mn(II) .....1.18

1.3.3 Oxidation of tellurium(IV):

In sulphuric acid medium the kinetics of oxidation of Tl(IV) has

been studied and mechanism has been proposed where in both Mn3+

and MnOH2+

were reactive species of Mn(III) [35].

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1.3.4 Oxidation of methyl mandelate:

Kinetics of oxidation of methyl mandelate by manganese(III)

pyrophosphate has been reported. In the study manganese(III)

coordinate with H2P2O72─

and the solvent composition has been

explained on the basis of following equilibrium [29].

CH3COOH + [Mn(H2P2O7)3]3─

k′′

[Mn(H2P2O7)2(CH3COO)]2─

+ (H2P2O7) ─

….. 1

.19

1.3.5 Oxidation of amino acids:

Amino acids are biologically important substances whose

microbiological and physiological activities largely depend on

oxidizing agent and their redox behaviour.

In recent years the kinetics of oxidation of amino acids, their

derivatives and peptides have been studied using Mn(III) as oxidizing

agent [10,13,20,22,36,37,38]. The studies have been reported in

different media.

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1.3.5.1 Oxidation of neutral amino acid in pyrophosphate

medium:

Kinetics of oxidation of L-serine, L-phenyl alanine, DL-alanine,

DL-valine and DL-threonine have been reported as a function of pH

[32]. The complex species [Mn (H2P2O7)3]3─

has been considered as

the kinetically active species. The following sequence has been

proposed for the reaction.

S +

H+

k1

k-1

SH+

(Fast)

..…1.20

[Mn(H2P2O7)3]3─

+SH+

k2

k-2

X + H+

(Fast)

..…1.21

X

k3

k-3

Y+{Mn(H2P2O7)2}2─

+

(H2P2O7)2─

(Fast)

..…1.22

Y

k4

Z + H+

(Slow)

..…1.23

[Mn(H2P2O7)3]3─

+ Z

R-CHO + (H2P2O7)2─

+ NH4+

+

Mn( H2P2O7)22─

(Fast)

..…1.24

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Here S and SH+ represents the neutral and mono cationic form of the

sulphate, X is the manganese(III) pyrophosphate-substrate complex

anion, Y is the free radical cation and Z is the free radical of amino

acid.

1.3.5.2 Oxidation of glycine and DL-alanine in sulphate

medium:

The oxidation of glycine and DL-alanine in sulphuric acid in the

presence of MnSO4 has been studied [24] assuming the intermediate

complex formation. The result obtained shows the first order

dependence on amino acid, second order on Mn3+

and an inverse order

dependence on H+ and manganese(II). The following mechanism has

been reported

Mn3+

+ A

k1

k-1

MnA3+

+ H (fast)

…..1.25

MnA3+

k2

k-2

Mn2+

+ A0 (slow)

…..1.26

A0

+ Mn3+

k3

Product

+Mn2+

(moderately fast)

…..1.27

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Where A and A0 stands for the amino acid molecule and free radical

respectively.

1.3.5.3 Oxidation of L-Histidine:

In this study the rate of oxidation of amino acid decreased with

increase in H+

ion concentration [12]. MnOH2+

has been considered as

reactive species, which may further hydrolysed [39]. The three

scheme has been given on the basis of kinetic result the mechanism

involves Mn3+

, MnOH2+

as reactive species. The scheme for

involvement of MnOH2+

has been given below.

Mn3+

aq

+ H2O

kh

MnOH2+

aq

+ H+

aq

…..1.28

MnOH2+

+ LH+

k1

k-1

X2+

+ H+

…..1.29

X2+

k2

k-2

Mn2+

+ L•

+ H2O

…..1.30

MnOH2+

+ L•

k3

Mn2+

+

Product

(slow)

…..1.31

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Here LH+ stands for mono protonated histidine, X for metal substrate

complex and L• for the free radical.

1.3.5.4 Oxidation of peptides:

Protein based polymer comprised of amino acids has been

synthesized based on the increasing order of hydrophobicity and

subjected to metal catalysed oxidation to identified the amino acid

residue [20]. In this study the Mn(OAc)3 in acetic acid medium or

[Mn(OAc)4]─ were considered as reactive species. Addition of sodium

acetate enhances the rate of reaction by the formation of active

species. The mechanism for the oxidation of amino acid in acetic acid

[40] has been found similar in the study.

In the oxidation of dipeptide in aqueous sulphuric acid medium

has been studied [11]. The mechanism proposed is based on the

following scheme.

MnOH2+

aq

+ DP

k1

X (slow and rate determining step)

X

k2

Product

(fast)

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Rate = k1 [MnOH2+

] [DP] …..1.32

Where DP is the dipeptide.

1.3.5.5 Oxidation of L-aspartic acid and glutamic acid in

sulphuric acid, acetic acid and pyrophosphate

media:

A detailed study [16] has been carried out in three medias for

same substrates L-aspartic acid and glutamic acid using

manganese(III)

as oxidizing agent. The results obtained were

discussed for each media.

Sulphate media:

It has been assumed that Mn(III) species present in aqueous

sulphuric acid medium are Mn3+

(aq) , Mn(OH)+2

(aq) and MnSO4+

(aq).

Since the effect of HSO4─ ion on the reaction is negligible, the role of

MnSO4+

(aq) as the active oxidizing species is ruled out.

Acetate media:

The observed kinetics shows the first order dependence on

[Mn(III)] and [S], fractional order dependence on [OAc─] and inverse

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fractional order in both [Mn(II)] and [H+]. Mn(OAc)4

─ has been taken

as reactive species.

Pyrophosphate media:

In acetic medium manganese(III) pyrophosphate has been

considered as a metal chelate having the composition

Mn(H2P2O7)33─

which is the reactive species in the study. In this

report kinetic data for oxidation of some amino acids by

manganese(III) species in sulphuric, perchloric, acetic and

pyrophosphate has been also summarized in a tabulated form.

1.4 Micellar catalysed oxidation:

One of the most important properties of micellar system is the

ability to affect the rate of chemical reaction. The effect of surfactant

on reaction kinetics is called micellar catalysis. There are various

reports on micellar catalysed redox reactions [41,42,43]. The reactions

of amino acids are more sensitive towards surfactants. Extensive work

has been reported on the reaction of amino acids in presence of

surfactant like reaction of ninhydrin [44,45,46]. Several kinetic

studies have been reported on the oxidation of amino acids in

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presence of surfactant by various oxidant, acid permagnate [47,48]

chloramines-T [49,50] and N-bromopsthalimide [51] but no efforts

seem to have been made to investigate the oxidation of amino acids by

manganese(III) in micellar systems.

1.4.1 Surfactant:

Depending upon the nature of the group present in the surfactant

molecule there are following classes of surfactant:-

(i) Non-ionic surfactant: These molecules have no appreciable

ionic charge.

(ii) Ionic surfactant: These molecules consists appreciable ionic

charge. The active portion beers negative charge, the surfactant

belongs to the class of anionic surfactant. The class of anionic [52]

surfactant consists carboxylic acid salt of straight chain fatty acid,

sulphonic acid salt, sulphonic acid ester salt, phosphoric and

polyphosphoric acid ester and perfluorinated anionics.

The molecule, which consist active portion with positive charge,

the surfactant belongs to the class of cationic surfactant. This class

consists long chain amines and their salt, diamines and polyamines

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with their salt, quaternary ammonium salt, amine oxides,

polyoxyethynated long chain amines.

There is a class of ampholytic structure, which act as zwitter

ionic surfactant like sulphobetaines, sultaines, N-alkyl-beta-

iminodipropionic acids and imidazoline carboxylates.

Linear alkyl benzene sulphonate is used as an important

surfactant among the most widely used anionic surfactant. The

structure of linear alkyl benzene sulphonate is given below:-

H3C

H2C

CH2

H2C

CH2

H2C

CH2

H2C

CH2

H2C

C

CH3

SO3

Linear Alkyl Benzene Sulphonate (LABS)

1.4.2 Micellar catalysis:

In water at very low concentration, but above a certain

concentration the surfactant molecules have a tendency to associate or

to form larger micelle aggregates. This concentration for each

surfactant is termed as critical micelle concentration, cmc. Micellar

catalysis of reaction in aqueous solutions is usually explained on the

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basis of distribution of reactants between water and micellar “pseudo

phase”. The micellar catalysis can be applied theoretically by making

certain simplification, including assumption that only one substrate is

incorporated into a micelle and the aggregation number N of the

micelle is independent of the substrate [53]. On this basis a

mathematical model given by Bruice et al [54], the reaction can be

represented by scheme 1

nD + S DnS

kw product km

Scheme 1

The rate expression for the system shown in scheme 1 may be given

by equation 1.33.

Where n is the number of detergent molecules, D; DnS is the catalytic

micelle; KD is the dissociation constant of the micelle and km and kw

log {

( kobs - kw) }= nlog[D] – logKD ..... 1.33

(km - kobs)

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are the rate constant in the micellar and aqueous phase respectively.

Various models have explained the micellar-catalyzed reaction.

Menger and Portnoy model [53], Piszkiewicz model [55] and Hill

model [56] are some of them.

1.4.3 Principles of micellar catalysis:-

Micellar catalysis of reactions in solutions is based on the

distribution of the reactants between water and micellar “pseudo

phase”. Here the elucidation of micellar-catalysed reaction can be

given according to the enzymatic catalysis.

M

+ S

K

MS

k0

km

..... 1.34

P P

Where M is the micelle, S is the substrate, MS is the micelle-substrate

complex. The rate constants for the formation of product in solvent

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and micellar phase are k0 and km respectively. The rate expression for

the above reaction can be given below, where [P] represents the

concentration of the product formed.

d[P] = k0[S] + km[MS] …..1.35

dt

The observed rate constant for the product formation kΨ can be given

by the relation 1.36

kΨ = d[S]t / dt

= k0F0 + kmFm …..1.36 [S]t

where [S]t is the stoichiometric concentration of the substrate at time t

and F0 and Fm are the fractions of the uncomplexed and complexed

substrate. Under the condition [M] >> [MS] the equilibrium constant

K can be given by the following relation

K = [MS]

– Fm

…..1.37 ([S]t – [MS] [M]) [M] (1- Fm)

or [M] = (CD – cmc)

…..1.38 N

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Where CD is the total concentration of detergent, cmc is the critical

micelle concentration and N is the aggregation number. Combination

of equation 1.36 and 1.37 and rearrangement leads to

kΨ =

k0 + km K [M] …..1.39

1 + k [M]

Combination of equations 1.38 and 1.39 and rearrangement gives

1 =

1 + [

1 ] [

N ] .....1.40

k0 - kΨ k0 – km k0 – km K(CD – cmc)

The equation 1.40 is known as Menger and Purtnoy model.

1.5 Present work:

In view of existing literature, it was therefore thought to

undertake oxidation kinetics of amino acid by manganese(III) in

aqueous micellar solutions. Since the study involves both

micellization behaviour of surfactant and oxidation of amino acids in

micellar solutions. There are reports on oxidation by manganese(III)

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acetate in presence of acetic acid but there is no any report on

oxidation of manganese(III) acetate in aqueous sulphuric acid. In this

thesis we have studied the micellization of linear alkyl benzene

sulphonate, LABS, in the presence of amino acids. The specific

concentration and interaction present between amino acid and

surfactant were studied through the micellzation process

conductometrically. The oxidation kinetics has been studied in

presence of surfactant, linear alkyl benzene sulphonate by taking the

account of micellization behaviour obtain in the study. The objective

of the study was to investigate the interaction between amino acid and

surfactant micelles, feasibility of oxidizing agent, manganese(III)

acetate in sulphuric acid and the catalytic effect of surfactant on

oxidation of amino acids.

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