“kinetics and mechanism of sulphuric acid oxidation of glycolic (ga) by selenium dioxide in...

“Kinetics and Mechanism of sulphuric acid oxidation of glycolic (GA) by selenium dioxide in aqueous-acid medium”

CHAPTER - IINTRODUCTION

Research provides one an opportunity to pit's wits with the seminal

principles underlying this creation. Since the emergence of nature, everything

changes in universe. It is an inescapable fact, which from time to time

immemorial, has moved poets, exercised metaphysicists and excited the

curiosity of natural philosophers, chemists and scientists towards exploration

of nature in search of something a new for their knowledge and purpose.

Chemistry has been an integral part of the natural phenomenon. Many reactions

move at snail’s pace whereas others occur instantaneously.

Chemical kinetics is a branch of chemistry concerned with the velocity of

chemical reactions and the mechanism by which chemical reactions occur [1]. It

deals with the rates of chemical reactions and how rates depend on factors, such

as concentration and temperature. Ideally, a complete reaction mechanism

would require the knowledge of all the molecular details of the reaction. In most

of the reactions, it is only the disappearance of starting materials and the

appearance of final products that can be detected. In general, however, the net

reaction is not the whole story, but simply represents a summation of all the

changes that occur. The net change may actually consist of several consecutive

reactions each of which constitutes a step in the formation of final products [2].

The mathematical models that describe chemical reaction kinetics provide

chemists and chemical engineers with tools to better understand and describe

chemical processes such as food decomposition, microorganism growth,

stratospheric ozone decomposition, and the complex chemistry of biological

systems[3]. These models can also be used in the design or modification of

chemical reactors to optimize product yield, more efficiently to separate

products, and to eliminate environmentally harmful by-products[4]. Kinetics

aims fundamentally at the details of the process in which a system converts

from one state to another and the time required for the transformation. Hence,

chemical kinetics provides information about the rate of reaction on possible

pathways, by which the reactants are transformed into products. Thus, the

fundamental of objective of the study of the kinetics of chemical The kinetics of

oxidation reactions and the investigation of the reaction mechanisms from the

kinetic data have been always the most interesting subjects in chemistry. In any

kinetic investigation, one may be interested to arrive at (i) the relationship

between the rate and the various factors like concentrations of the reactants,

temperature, reaction medium etc., and, (ii) interpretation of the empirical rate

laws in the light of the mechanism proposed [5].

The pioneering work engineered by German chemist L.F. Wilhelmy

(1812-1864) on the rate of inversion of sucrose [6] has originated a revolution by

opening a new chapter of kinetics in chemistry. Since then the significance of

chemical kinetics came in existence. Chemical kinetics is a branch of chemistry,

which deals with the measurement of the rates of chemical reactions. An ideal

theory of chemical kinetics would start with the time dependent equation,

which could be solved to predict the rates of such simple physical and chemical

processes as change in the energy state of a molecule and energy transfer

reactions in which no net chemical changes occur but energy is transferred

between molecules.

Livingston [7] called the special attention in signifying in the field of

reaction mechanism as “No reaction mechanism can be considered to me more

than a temporary working hypothesis until it is supported by kinetic data”.

The chemical kinetics remains one of the most important tools even this

day in finding out the undetermined mechanism of the reaction. Thus due to

developments of modern physical techniques viz, n.m.r, i.r, u.v, visible,

2


absorption spectroscopy, mass spectra, epr, thermogravimetry, colorimetric,

polarography, chromatography etc. and wide and vast applicability of hi-tech,

super-tech, has shed a new light and horizons on reaction mechanism and

provide the complete picture of the reactions.

The elucidation of reaction mechanism is still one of the most fascinating

problems in inorganic and organic chemistry. Chemical Kinetics has furnished a

pool of precious wealth of information about the nature and in course of a

reaction [8] viz. molecularity, concentration, reaction path, frequency of activated

complex, mass, temperature and other properties such as influence of

substituent groups and structural alterations, rate equation, salt effect, isotopic

effect, activation parameters and various environmental changes etc, like

solvent polarity, pH and catalytic changes in a reaction. The above study leads to

work at stoichiometry, identification of intermediates and isolation of end-

products as an indirect support to reaction mechanism.

Modern trends in kinetics

Pauling L., Franklin [8] introduced the electron transfer that has created a

new era in the field of chemical kinetics. The oxidants based on redox reactions

are of considerably academic interest and of technological importance. In 1969,

A. Broido developed T.G. techniques and employed to study the kinetics of

chemical reaction based on the Arrhenius equation. Recently this valuable

phenomenon of kinetics is fully utilized to carry out the reaction exhibiting

radioactivity [9] with half-life less than a second does. The radioactive decay of

29Cu64an unstable nucleus is an important example of a process that follows a

first-order rate law:

29Cu 30Zn + 64 , = 12.8 hrs.64

The laser technology, Flash photolysis, rate of growth of malignancy in

cancer, rate of blood circulation in body, and rate of tissues movement in

bioplants [10], etc. applied to kinetic measurements within the range of pico

second. Yalman[11] applied electro-negativity and well defined oxidation state to

kinetics. In 1970, Goldstein[12] (U.S.A.) has utilized molecular orbital theory to

provide a strong evidence of changes in order of electro negativities based on

redox reactions.

During the recent era, it has become interesting to investigate the

mechanistic pathway of redox reactions. Originally, this field was little probed

as the mechanism often varied greatly with the oxidizing and/or reducing

agents employed. Oxidation of organic compounds may be represented as

electron transfer, hydride transfer, H atom transfer and addition-elimination

and displacement mechanism.

Oxidation- Reduction

The present study deals with the kinetics of oxidation reactions involving

oxidation-reduction. It will be, therefore, necessary to give a brief account in this

regard. The process in which there is de-electronation from reacting species is

called oxidation and conversely, reduction implies the addition of electrons

(electronation) to the reacting species.

Oxidant is one which gains electrons and reductant is one which loses

electrons. Oxidation- reduction reactions occur simultaneously in solution.

Hence in all oxidation-reductions, there will be a reactant undergoing oxidation

and one undergoing reduction. In these reactions, electrons are transferred

from reductant to the oxidant.

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Selenium Dioxide as an oxidant

The potential of selenium dioxide as an oxidizing agent for organic

compounds was first realized in the early 1930's by Riley[13]. Since this initial

discovery, selenium dioxide has found wide application as a selective reagent in

organic synthesis [14]. Selenium dioxide most commonly oxidizes carbon-

hydrogen bonds attached to various activating groups such as olefins,

aldehydes, ketones, acetylenes, esters, amides, carboxylic acids, anhydrides, and

aromatic nuclei. Aldehydes, ketones and olefins are oxidized in good yields

under relatively mild conditions. Alcohols, amines, phenols, and mercaptans are

oxidized in poor yields under vigorous conditions. Alkanes, ethers, and alkyl

halides are usually not attacked by selenium dioxide, and when they are, only

under severe reaction conditions. Selenium dioxide is a colorless solid. It exists

as one dimensional polymeric chain with alternating selenium and oxygen

atoms. It sublimes readily and hence the commercial samples of SeO2 can be

purified by sublimation. SeO2 is an acidic oxide and dissolves in water to form

selenous acid, H2SeO3.

THE SURVEY OF LITERATURE PERTAINING TO THE OXIDATION OF VARIOUS COMPOUNDS BY SELENIUM DIOXIDE

In the present work, selenium dioxide has been employed as an oxidant.

It has been, therefore, thought worthy to give a brief account regarding the work

done with this oxidant.

Oxidation kinetics with Ketones

Riley[13] introduced selenium dioxide as an oxidant to oxidize some

ketone. They obtained glyoxals as the product of the oxidation of these

compounds. They pointed out that the active species of oxidant was selenious

acid not the selenium dioxide. They observed that when a mixture of acetone

with a little water was added to selenium dioxide in cold, the reddish color due

to libration of Se developed more repidly than the case when dry acetone was

used.

Mel’nikov and Rokitskaya[14] have studied the kinetics of oxidation of

some ketones. They have observed that the oxidation by SeO2 is a bi-molecular

reaction and the rate is measured based on degree of enolization of the carbonyl

group of these compounds. Different types of ketones such as Me2CO, Et2CO,

Pr2CO, Bu2CO,(isoPr)2CO, MeCOEt, MeCOPr and methyl hexyl ketones were

studied. They have found that the rate of oxidation retards gradually with rising

molecular weight of the substrates.

Duke[15] studied the kinetics of oxidation of acetone by seleniuos acid in

each of the experiment, the reactants concentrations were kept such that only

the concentration of seleniuos acid changed. While that of other reactants

remained unchanged. The plots of log [H2SeO3] vs. Time were found to be

straight lines, the slope being first order in oxidant.

The selenium dioxide-catalyzed hydrogen peroxide oxidation of ketones

was discovered by Payne and Smith1 in 1957. The reaction involves a migration

of an alkyl or aryl substituent, which is to a ketone carbonyl group, to the

other available position, with subsequent oxidation of the carbonyl group to a

1 G. B. Payne and C. W. Smith, J. Org.. Chem., 2_2, 1680 (1957).

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carboxylic acid. For example, cyclopentanone 1 is oxidized to

cyclobutanecarboxylic acid 2.

The reaction was carried out in tertiary-butyl alcohol for two hours at 80°

using equimolar quantities of ketone and hydrogen peroxide. Selenium dioxide

was only present in about 2 mole percent.

Sonoda and Tsutsumi[16] have oxidized many ketones by the method of

Payne and Smith in an attempt to elucidate the mechanism of this novel

reaction. Their results have been summarized by White2 and a few examples are

shown below.

Since selenium dioxide does not promote this rearrangement directly, Sonoda

and Tsutsumi have suggested that peroxy selenious acid or some other higher

oxidized state of selenious acid attacks the carbonyl group in the same manner

as selenious acid. The resulting intermediate rearranges to their proposed

2 N. Sonoda, T. Yamaguchi, and S. Tsutsumi, J. Chem. Soc. Japan, Ind. Chem. Soc., 63, 737 (1960).

intermediate 3, which decomposes as shown to the rearranged carboxylic acid

4.

White has followed up the work of Sonoda and Tsutsumi with an

investigation of the hydrogen peroxide-selenium dioxide oxidation of

desoxybenzoin to diphenylacetic acid. Yields as high as 68% (based on

unrecovered desoxybenzoin) were obtained along with small amounts of benzyl

benzoate and its hydrolysis products, when 0.1 mole of ketone, 0.2 mole of

peroxide, 0.03 mole of selenium dioxide and 0.1 mole of sodium acetate were

refluxed in 80% acetic acid for three hours. Obviously the Baeyer-Villiger

oxidation is a competing reaction here. When aliphatic ketones were oxidized by

his method, White only reported the isolation of tarry products. White[17] also

has shown that the reaction is not affected by selenic acid or a combination of

selenic acid and hydrogen peroxide and that it may proceed through an enol

selenite ester 5, as does the normal selenium dioxide oxidation of ketones. He

proposes the following mechanism for the reaction. White concludes that the

reaction is catalyzed by acetate ion, and that it may proceed through an enol

selenite ester 5, as does the normal selenium dioxide oxidation of ketones. He

proposes the following mechanism for the reaction.

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Schaefer[17] also studied the sodium acetate catalyzed kinetics of

oxidation of p-nitrobenzyl phemyl ketone and p-methoxy benzyl phenyl ketone

by SeO2 in 80-90% acetic acid. The proposed mechanism involved the base

catalysed formation of an enol selenite ester, which subsequently rearranged

and decomposed to yield -diketone, se and water. To explain the acetateα

catalyzed SeO2 oxidation of, it was suggested that acetate plays several

conflicting roles, but overall effect was accelerated one.

Oxidation kinetics of ketones by selenium dioxide also been studied by

Sharpless and Gorden[18]. According to them -ketoseleninic acid formed by anβ

electrophillic attack of selenious acid on the enol is the key intermediate. They

suggested that pummerer like decomposition of this intermediate yield -αdiketone.They pointed out that the peroposed mechanism of Corey and

Schaefer[19] was widely accepted but it did not involve an organo-selenium

intermediate. The principal objection to the mechanism was the selenium

(II)keto esteris hydrolysed very rapidly to alcohol. Through these workers could

not isolate such an organo-selenium intermediate, but made an attempt to give

experiemntal evidence regarding its formation in the reaction mexture.

Singh and Anand[20] have studied the kinetics and mechanistic

investigation of cyclic ketones viz. cyclohexanone, cycloheptanone and cyclo-

octanone by selenium dioxide in 50% (v/v) acetic acid water miture. Reactions

were found to be first order both in [oxidant] and [substrate]. The acid was

found to be catalyzed rate of the oxidation odf cyclic ketones. The rate vof

reaction further increased with increase in the the concentration of acetic acid

in the reaction mixture. Plot of log k1 vs. 1/D was lenear with a positive slope,is

suggested that the slow step involved the neutral molecule and positive ion on

the basis of solvent isotopic effect; an attack of reaction species of selenium

dioxide on the keto form of cyclic ketone was suggested. The mechanism

proposed by them involved concerted attack of electrophile RH2SeO3+ and

nucleophile, H2O on cyclic ketone to form enol selenium(IV)ester in the acid

catalysed oxidation.The enol selenium(IV) ester on internal rearrangement

gives selenium(II)ester, which on acid catalysed 1,2-elimination gave the

oxidation products. The reported order of reactivity among cyclic ketones were

Cyclohexanone > cycloheptanone > Cyclooctanone

Valechha and Pradhan [21] have studied the oxidation of acetyl aceton by

selenium dioxide in 50% (v/v) acetic acid –water mixture at 500C. The reaction

follows first order kinetics with respect to xidant and substrate. The presence of

H2SO4 incresed the reaction rate, which was found to further increase with,

increases [H2SO4]. These workers studied the solvent polarity on the reaction

rate by varying the percentage of composition of acetic acid They observed that

the increase in the percentage of acetic acid in reaction mixture decreased the

10


rate of oxidation. Plot of log k1 vs. D-1/2D+1 was found linear, on the basis of

this, it was suggested that the reaction is a dipole-diple type. The primary salt

effect on the reaction rate was found to be normal retarding. The mechanism

proposed for the oxidation process involved an electrophile attack of selenious

acid on acetyl acetone to gives enol selenium (IV) ester in slow step. The latter

on fast internal rearrangement give selenium (II)keto ester and finally 2,3,4-

pentanetrione as the end-product . They ruled out the probability of biselenite

ion, HSeO3-, as being the effective oxidizing species. The operative mechanism

summarized as below-

Sanjay and co-workers[22] have studied kinetics of oxidation of 2-

alkanones by selenium dioxide in aqueous acetic acid medium in presence of

perchloric acid. The reactions exhibited first-order dependency in [substrate],

[SeO2] and [H+]. The reaction was fully acid catalyzed. Decreasing in the

dielectric constant of the medium, slightly accelerating effect. The soichiometric

studies revealed 1:1 mole ratio of oxidant and substrate. They have found 2,3-

diketoneas oxidation product. Selenium dioxide–ketone system suggested the

following mechanism-

12


Sewanne[23] has studied the kinetics of oxidation of acetophenone by selenium

dioxide in aqueous acetic acid sulphuric acid medium. The reaction follow first

order kinetics in [SeO2],[Substrate] and [H+] ion. Reaction is fully acid

catalyzed .Primary salt effect was found to be negligible effect on the reaction

rate. The solvent polarity on the reaction rate by varying the percentage of

composition of acetic acid .Sewanee has observed that the increase in the

percentage of acetic acid in reaction mixture increased the rate of oxidation. Plot

of log k1 vs. D-1/2D+1 was found linear, on the basis of this, he was suggested

that the reaction is a dipole-diple type. It was observed that the electron with

drawing group retard the reaction rate while the electron releasing groups

accelerate the reaction rate. The mechanism involving an electrophile attack of

seleniuos acid,H2SeO3 on the keto form of substrate in rds.

Oxidation of alcohols

The oxidation of alcohols to aldehydes by selenium dioxide has been

studied by Mel'nikov and Rokitskaya[24] They investigated a series of saturated,

low molecular weight alcohols and proposed that at low temperatures an alkyl

acid selenite 7 is formed, and that at higher temperatures 7 can react with more

alcohol to form a dialkyl selenite 8, which decomposes at higher temperatures

to the aldehyde or ketone, selenium, and water.

The alkyl acid selenites can be obtained as crystalline solids from methanol[25-26]

or ethanol[27,28] solutions of selenium dioxide by dehydration with calcium

chloride in a desiccators. They are decomposed by water to the alcohol and

selenious acid. The lower dialkyl selenites may be distilled at reduced pressure

without decomposition[29]. The yields of these oxidation products were always

low and temperatures of 300°C or more were needed to obtain any oxidation at

all. Riley and co workers[30]. report 40-50% conversions of benzyl alcohol to

benzaldehyde with selenium dioxide at the reflux temperature of the alcohol. No

traces of benzoic acid could be detected. Allylic alcohols also show a tendency to

be oxidized in fairly good yields to the aldehyde by selenium dioxide. Thus, -αmethylallyl alcohol has been oxidized to -methylacrolein in 62% yieldα [30] and

-methylallyl alcohol oxidized to crotonaldehyde in 60% yieldβ [30].

Austin[31] et al. also used selenium dioxide as an oxidant in the oxidation

kinetics of some alcohols. They have found that aliphatic alcohols reduced

selenium dioxide at higher temperature than their boiling points. They have

observed that ethyl, propyl and butyl alcohols yield glyoxal on oxidation with

selenium dioxide. It was suggested that reactions with much more complex as

well as sensitive to temperature changes than the aldehyde and ketone in case

of benzyl alcohol, they found that the oxidation occurred but not completely, at

the boiling point of the alcohols under investigation. The products of oxidation

were corresponding aldehydes.

Mel’nikov and Rokiskaya[32] have studied the oxidation of various alcohols

with selenium dioxide. They observed that alcohols on reaction with selenium

dioxide to form to dialkyl selenites,which on heating decomposed in to

corresponding aldehydes, Se, H2O.

Valechha and Pandey[33] have also reported the kinetics of oxidation of

allyl,crotyl and cinnamic alcohols by selenium dioxide in 80%(v/v) acetic acid –

water medium. The order of reaction with respect to each substrate and

14


selenium dioxide was found to be unity. Reactions were fully catalyzed by the

mineral acid is reported. The observed reactivity among the alcohols were

reported as-

Crotyl > allyl > cinnamic alcohols. One scheme of mechanism is proposed as-

Allyl alcohol[34] is reported easily oxidized into the correspondig aldehyde by

selenium dioxide. Weygand prepared cyclic selenite ester from orthophthal

alcohol and decomposed it thermally to ortho phthal aldehyde.

Oxidation of aldehydes

Khan[35] et al have investigated the kinetics of oxidation of some aliphatic

aldehyde such as acetaldehyde,propanaldehyde, n-butanaldehyde and -αchloroacetaldehyde by selenium dioxide in aqueous acetic acid and sulphuric

acid medium. They have found that reaction exhibit first-order kinetics in each

[SeO2], [aldehyde] and [H+] ions. The reaction was acid a catalyzed. A positive

effect of acetic acid is observed. The stoichimetry of each reaction under

investigation has determined by equilibrrating the reaction mixture containing

an excess of selenium dioxide over aldehyde in varying ratio at 400oC for 48 hrs.

The stoichimetry studies revealed that 1:1 mole ratio of substrate and oxidant.

The oxidation product was glyoxals reported. Single rate expression

–d/dt[SeO2]= k1[aldehyde][H+].The application of steady-state treatment with

reasonable approximation yields the rate law capable of explaining and

justifying the observed kinetic.

Oxidation of substituted paraffin

Schaefer and Horvath[36] studied the oxidation of 1,3-diphenyl propane by SeO2

in 99% acetic acid at 115 0C,1,3-diphenyl-2-propane-1-ol acetate was obtained

as the product of oxidation under reaction conditions in high yield.

N.D. Valechha and A .Pradhan[37] have reported kinetics of oxidation of

diphenyl methane by SeO2 in 80% (v/v) dioxane-water mixture in the presence

of perchloric acid. The order of reaction with respect to the [SeO2] is one. The

16


reaction pseudo first order constant in [DPM]. They have found benzophenone

as the oxidation product.

Oxidation studied of desoxybenzoin

Corey and Schaefer[38] have made a most critical study selenium dioxide

oxidation. The Oxidation of desoxybenzoin by selenium dioxide carried out in

70% acetic acid –water mixture at 89.2 0C. It was found that the reaction was

second order in [desoxybenzoin] and [SeO2]. The added strong acid in the

reaction mixture was found to catalyze the reaction rate. They have also

observed the oxidation of various p- substituted desoxybenzoin with

substituents in benzoyl group moiety. It have been reported that the electron

repelling substituents groups in the benzoyl group, increases the rate of

reactions, while the effect in the benzyl groups to decrease the rate. According

to them, this fact is expected for an acid catalyzed oxidation of desoxybenzoin by

selenium dioxide.

Schaefer[39] has also reported the selenium dioxide in 90% of acetic acid.

He has suggested that acetate play several conflicting roles but overall effect

was accelerated one.

With Esters

Riley[13] et. al. applied selenium dioxide to oxidized acetone dicarboxylate,

ethyl mandelate, ethylmalate,ethyl phenyl ropinate and ethyl acetate etc.

Refluxing the compounds with selenium dioxide for 2 to 5 hrs. At

temperature120-200 OC.

Benerji, Borton and Cookson[40] have investigated a series of sterio

isomeric methyl-3,6-dioxoeduues monoates by selenium dioxide.

Valechha[41]have reported that the kinetic study of oxidation of ethyl

acetoacetate by selenium dioxide in aqueous acetic acid water medium. The

reaction is first order in both SeO2 and [EAA]. In presence of sulphuric acid and

percloric acid, reaction rate were found catalyzed.

Oxidative Degradation of Some -α substituted mandelic

The kinetics of oxidation of some substituted mandelic [42] such as mandelic and

para-chloro substituted mandelic acid by selenium dioxide in aqueous acetic

acid medium in the presence of sulphuric acids has been studied. The reaction

follows identical kinetic being first order in each SeO2, substrates and H+

concentrations. The reaction is acid catalyzed. A positive effect of acetic acid is

observed. Various thermodynamic parameters have been computed. A single

rate expression – d/dt= k1[ -hydroxy acid] [Hα 2SeO3] [H+] and one scheme of

18


mechanism are proposed . H3SeO3+ and AcH2SeO3

+ are postulated as the prime

profile species. Stoichiometric study revealed 1:2 mole ratio of oxidant and

substrate.

Where, X stands for –H and –Cl, for mandelic and p-chloromandelic acid

respectively.

Singh[43] et. al.also studied that the kinetics of oxidation of some -αhydroxy acids such as p-NO2 and p-Br substituted mandelic acids by selenium

dioxide in aqueous acetic acid medium in the presence of sulphuric acids has

been carried out. The reaction follows identical kinetics being first order in each

SeO2, substrates and H+ concentrations. The reaction is acid-catalysed. A

positive effect of acetic acid is observed. Various thermodynamic parameters

have been computed. A single rate expression kobs = k1 [ -hydroxy acid] [Hα 2SeO3]

[H+] and one scheme of mechanism is proposed. H3SeO3+ and AcH2SeO3

+ are

postulated as the prime profile species. Stoichiometric study revealed 1:2 mol

ratio.

Oxidation of olefins

Guillemonat[44] made the first serious attempt to explain the mechanism

of olefin oxidation by selenium dioxide. His scheme is outlined in the following

three reaction steps.

The R group is a radical containing an ethylenic linkage adjacent to the

indicated CH2-H. Guillemonat mechanism, although rather general and

crude,does explain why one always recovers starting material, why the product

is dependent on the solvent, why the occasional formation of dienes results, and

why organoselenium compounds are isolated from these reactions. However,

this mechanism does not predict the site of predominant attack in the oxidation.

Guillemonat studied the selenium dioxide oxidation of a number of branched,

straight chain, and cyclic olefins in the C5-C9 range. From this study he

formulated a set of empirical rules, which are listed below and illustrated by

examples from the literature. These rules are listed as summarized by

Trachtenberg[45].

(a) Trisubstituted olefins[46] are oxidized preferentially on the disubstituted side

of the double bond, if there is non-bridgehead allylic hydrogen there. For

example,

20


(b) Reaction at an endocyclic position in 1-alkyl cyclohexenes is preferred to

exocyclic attack[46,47] Thus, carvomenthene yields carvotanacetone as the major

product. Phellandral is obtained as a minor product. '

(c) Oxidation never occurs at bridgehead positions in bicyclic systems falling

within the limits of Bredt's rule[49]. For example, α-pinene is oxidized only to

myrtenol and myrtenal, with no oxidation-taking place in the six-membered

ring.

-pinene myrtenol myrtenal α

(d) When the allylic position favored is tertiary, dienes can result. 2,3-

dimethylcyclohexene[50] is oxidized to a mixture of diene and o-xylene which

probably results from dehydrogenation due to adjacent activated allylic

positions.

Olson[51] has studied the oxidation of ethylene with selenium dioxide in acetic

acid at 100-125° and 50 psi of ethylene. Compounds 11, 12, and 13 were

isolated. When a mineral acid such as hydrochloric or perchloric was added, the

yield of 11 was increased six fold, but without acetic acid present, there was no

reaction. When sodium acetate was added, only 12 and 13 were obtained. The

increased rate on addition of strong acid is consistent with Schaefer and

Horvath's mechanism in which selenium dioxide is converted to its conjugate

acid, which is the reacting species.

Olson concludes from his study that the reaction is specific acid catalyzed by

acetic acid and that acetates are not formed from esterification of alcohols, but

is primary reaction products.

Oxidation of amines with selenium dioxide

The reaction of amines with selenium dioxide was first investigated by

Hinsberg[52-55] in 1889. He found that o-phenylenediamine reacted with selenium

dioxide to give a compound, called a piaselenol. A series of these types 46 47 of

compounds was investigated, Hinsberg also isolated a compound with structure

7 from reaction of 1,8 diaminonapthalene with selenium dioxide Hinsberg also

isolated a compound with structure 8 from reaction of 1,8 diaminonapthalene

with selenium dioxide.

22


Heterocyclic amines having a tertiary nitrogen atom, such as pyridine or

quinoline, may be refluxed with selenium dioxide without change. However, the

-N=CH- group in these compounds is comparable in activating ability to the

carbonyl group. Thus, 2- and 4-picoline 48 are easily oxidized to the

corresponding aldehydes and acids. In addition, the hydroxymethyl pyridines,

as discussed above, serve as good examples. Hydroxy groups allylic to an imine

function are oxidized to the ketone readily. For example, hydroquinine is

oxidized to hydroquinone. Selenious acid combines with aliphatic and aromatic

amines at low temperatures to form crystalline, substituted ammonium salts.

Alkyl ammonium selenites result when alcoholic solutions of selenium dioxide

are added to amines. With increasing temperatures, both aliphatic and aromatic

amines are oxidized rapidly to yield resinous or tarry products. The reaction

between most amines and selenium dioxide is highly exothermic and usually

proceeds with explosive violence. Riley showed that aniline reacted with an

equimolar amount of selenium dioxide in methanol, to give a white crystalline

substance, C7H11O3NSe, M.P. 56°, that slowly decomposed to a dark violet

substance, with a faint odor of nitrobenzene.

THE SURVEY OF LITERATURE PERTAINING TO THE OXIDATION OF

GLYCOLIC ACID WITH OTHER OXIDANTS

By Manganese(III)

The kinetics of oxidation of glycolic acid, Glycolic a by manganese (III) in

sulfuric acid solutions at 293 K has been studied[56]. A solution of the mild

oxidant, Mn(III) sulfate, in aqueous sulfuric acid medium was prepared by a

standard elec-trochemical method. The oxidation reaction was monitored by

spectrophotometry at a fixed wavelength ( max = 491 nm), varied temperature,λ

and varied solution conditions. The Mn(III)-GA reaction stoichiometry of 4:1

was determined and the products were characterized. The experimental rate

law is: rate = kobs [Mn(III)][GA]x[H+]y, where x, and y are fractional orders. The

effects of varying ionic strength, solvent composition (dielectric constant), acid,

and the reduction product, Mn(II), on the rate of the reaction were investigated.

Activation parameters evaluated using Arrhenius and Eyring plots suggest the

occurrence of an entropy controlled reaction. A mechanism consistent with the

observed kinetic data has been proposed. A rate law has been derived based on

the mechanism.

With acid permanganate

The initial oxidation stages of glycolic acid by acid permanganate[57] were

investigated. The rate of the induction period was slow and then gradually

increased. The kinetics of oxidation were second order, first order with respect

to both glycolic acid acid and Mn(VII). The reaction was acid catalyzed. Addition

of Mn(II) ions largely increased the rate of the initial stages and decreased the

rate of the following stages. The oxidation rate was decreased by the addition of

F- or P2O4-4 ions. The Arrhenius equation was valid for the reaction between 16.5

and 34°C. Activation parameters were evaluated and a mechanism consistent

with the results obtained was proposed.

Glycolic acid acid by pyridinium fluorochromate

The kinetics and mechanism of the oxidation of glycolic acid (GA) by pyridinium

fluorochromate[58] (PFC) has been studied by spectrophotometric method in

50% acetic acid – 50% water (v/v) medium in a temperature range of 298 K-

313 K. Under the conditions of the pseudo-first order, the reaction follows first

order with respect to [GA], [H+] and [PFC]. The reaction is catalysed by

perchloric acid. There is no salt effect. The rate increases with increase in the

percentage of acetic acid and the plot of log kobs versus 1/D is linear with a

positive slope indicating the positive ion – dipole nature of the reaction.

24


Activation parameters have been evaluated. Based on the experimental results,

a probable reaction mechanism of oxidation was proposed.

The recent literature survey also revealed that glycolic acid have been

oxidized by various oxidants viz. Pyridinium chlorochromate[59] (PCC),

tripropylammonium fluorochromate (TriPAFC)[60] Vanadium (V)[61] and Ce (IV)

[62] etc. However, literature survey reveals that no work seen on above

topic/entitle, all these facts are inspired to me to explore mechanistic path of

this problem. The substrate is chosen for the kinetics and mechanistic

investigation is:

Glycolic acid

CHAPTER - II

MATERIALS AND METHODS

In the kinetic investigation of glycolic acid by selenium dioxide acetic

acid-water medium in presence of sulphuric acid, different chemicals were used

in the form of solutions. The procedure employed for the preparation of these

solutions and for the kinetic study is mentioned in the following sections:

PREPARATION OF SOLUTIONS AND THEIR STANDARDIZATION

A. Preparation of selenium dioxide solution and its

standardization

Selenium dioxide (Loba) solution was prepared by dissolving a

weighed quantity of pure selenium dioxide in distilled water. Solution

was standardized iodometrically as 2-ml. of selenium dioxide solution

was taken with a graduated pipette in a conical flask, 10ml. of 2N

H2SO4 and one gram of solid KI were added. The iodine liberated was

titrated against standard sodium thiosulphate solution using starch as

an indicator.

B. Preparation of substrates solution

The stock solution of glycolic acid (sigma 98%) was used to solution,

prepared by dissolving a calculated quantity of the glycolic acid in

glacial acetic acid.

C. Iodine solution

3.32 gram of KI were weighed and transferred to a 500 ml. volumetric

flask. About 10 ml. of water was added to it. Now about 5 to 5.2 grams

of iodine were weighed and transferred to the sane volumetric flask.

When iodine was completely dissolved, the solution was diluted with

distilled water and makeup to the mark. The iodine solution thus

prepared was standardized as-

26


Standard sodium thiosulphate solution was taken with a graduated

pipette in a conical flask. To this 10ml. of 4 N HCl and about 2ml. of 1%

starch solution were added. This was titrated with iodine solution

until light blue color is developed.

D. Solution of sulphuric acid

Stock solution of H2SO4 (Analar E. Merck) of desired strength was

prepared by diluting the calculated volume (from specific gravity) of

acid with distilled water and finally its concentration was determined

by titrating it against standard NaOH solution using phenolphthalein

as an indicator.

E. Sodium thiosulphate solution

Sodium thiosulphate solution was prepared and standardized by the

method prescribed in the literature.

F. Starch solution

1% Starch solution was prepared as per procedure given in the

literature; this starch solution was used as an indicator.

Kinetic measurements

Kinetics of oxidation of glycolic acid under study by selenium dioxide in aqueous

acetic acid as solvent has been followed iodometrically as follows:

The glass stoppered reaction flask made of Pyrex glass containing

substrate, acetic acid and other reagents if any, was kept together with a stock

solution of selenium dioxide in a thermostat maintained at a desired

temperature with an accuracy ±0.1. When the two flasks attained the

temperature of thermostat, a required volume of selenium dioxide was pipette

and transferred to reaction flask. At the instant half of selenium dioxide solution

was added to the reaction flask, zero time was noted. Immediately 2 ml. aliquot

was withdrawn into a flask containing 10ml. ice cold water and 10 ml. of 0.01

sodium thiosulphate solution along with 5ml. of 4N HCl. About 2ml of starch

solution was added to it and then un-reacted sodium thiosulphate left was

titrated against standard 0.01N iodine solution until a light blue color is

developed. Aliquots were with drawn at known intervals of time and

concentration of selenium dioxide left un-reacted was estimated iodometrically.

These readings are the values of (a-x) at time “t”. The experimental data were

fed into the integrated form of equation for first-order reactions. The values of

pseudo first-order rate constant obtained from the rate equation -

Were found fairly constant within the experimental error suggested that each

reaction obeys first-order kinetics.

In order to study the effect of varying concentration of sulphuric acid on

the reaction rate, kinetic runs have been carried out at varying concentration of

acid, but at fixed substrate and oxidant concentration, solvent composition and

temperature.

Effect of temperature

The rate of reaction was studied at different temperatures to evaluate

various activation parameters such as temperature coefficient, frequency

factors, and energy of activation, free energy of activation and enthalpy of

activation and entropy of activation.

1. Temperature coefficient

The temperature coefficient of the reaction for 50 and 100C rise in

temperature was calculated by following expression:

28

k =2.303

tlog a

(a-x)k =

2.303

tlog a

(a-x)


Temp .coefficient=k2

k1………… ..(1)

Where, k1 is rate constant at temperature t and k2 is rate constant at 50 or 100C

higher than ‘t’.

2. Energy of activation

The effect of temperature on the rate of reaction given by Arrhenius equation –

k=Ae

−EaRT¿

…………………(2)¿

Where,

k is rate constant at absolute temperature T, A is frequency factor, Ea is

the energy of activation, and R is the gas constant. From equation (2) we obtain-

log k=log A− Ea2.303 RT

…………..(3)

Therefore, a plot of log k against inverse of T should be straight line having a-

Slope= −Ea2.303R

………………(4)

The energy of activation was determined graphically and also by calculating

from equation:

Ea=2.303 RT 1T 2

T 1−T 2log

k2

k1………. (5)

Where k1 and k2 are constant at temperature T1 and T2 respectively.

3. Frequency factor

By rearranging equation (3) frequency factor “A” can be obtained as

log A=log k+ Ea2.303RT

………… ..(6)

The values of frequency factor obtained from equation (6) have been reported.

4. Free energy of activation (G#)

The free energy of activation (G#) is obtained using equation-

–G¿=2.303 RT log k¿ …………….(7)

Where, k# = kr h / KBT

Therefore, –G¿=2 .303RT log krhKBT

…………(8)

5. Enthalpy of activation (H)

The enthalpy of activation (H) of the reaction was obtained from the

Eyring’s equation by plotting log krh/KBT vs. 1/T graphically. The slope of the

plot is H / 2.303R.

6. Entropy of activation (S#)

The entropy of activation (S#) is calculated from the equation

G# = H# – TS# --------- (9)

Thus the values of the activation parameters viz. Ea, A, H#, G# and S# are

calculated for each reaction.

(D) Stoichiometry and product analysis

The stoichiometry of each reaction understudy was determined under

experimental conditions. The oxidation products of the reaction were identified

chromatographically[63-64] and by spot test qualitatively[65].

(E) Test for free radicals

The formation of free radicals during the course of reaction was tested

using the solution of acrylonitrile (monomer) by trapping method[66-67].

30


CHAPTER - IIIEXPERIMENTAL KINETIC DATA

In the chapter III, various experimental kinetic data are recorded in

various tables, were computed. Various Graphs were plotted and facts have

been made according to experimental data and graphs.

SECTION: IIIA

TYPICAL KINETIC RUN

The preliminary studies reveal that the oxidations of Glycolic acid

undertaken are measurable temperature at 308 to 323 K. The results so

obtained were used to calculate the rate constant using integrated rate

equation. The following typical kinetic runs representing the kinetic findings of

the studies are carried out as:

a) typical kinetic run for the effect of Selenium dioxide,

b) typical kinetic run for the effect of substrates,

c) typical kinetic run for the effect of sulphuric acid,

d) typical kinetic run for the effect of dielectric constant of the medium,

e) typical kinetic run for the effect of temperature.

Fig.IIIA-1

32


Table: IIIA-1

TYPICAL KINETIC RUN

Effect of selenium dioxide

[SeO2] : 2.50X10-3 (mol.dm.-3)[GA] : 2.50X10-2 (mol.dm.-3)[H+] : 1.25X10-3 (mol.dm.-3)HOAc-H2O : 30%(v/V),Temperature : 313 K.

S. No. Time

(sec.)Vol. of

N1000 hypo

(ml.)

105k1(s-1)

1. 0 5.00 -

2. 1500 4.15 12.42

3. 3000 3.45 12.37

4. 4500 2.90 12.11

5. 6000 2.40 12.23

6. 7500 2.00 12.22

7. 9000 1.65 12.32

8. 10500 1.40 12.12

9. 12000 1.15 12.25

10. 13500 0.95 12.30

Average k1 =12.25X10-5(s-1)Graphical k1 =12.23X10-5(s-1)

Fig.IIIA-2

34


Table: IIIA-2

TYPICAL KINETIC RUN

Effect of concentration of Glycolic acid


S. No. Time

(sec.)Vol. of

N1000 hypo

(ml.)105k1(s-1)

1. 0 5.00 -

2. 2100 4.15 8.87

3. 4200 3.50 8.45

4. 6300 2.90 8.65

5. 8400 2.45 8.49

6. 10500 2.00 8.73

7. 12600 1.70 8.56

8. 14700 1.45 8.42

9. 16800 1.20 8.50

10. 18900 0.95 8.79

Average k1 =8.69X10-5(s-1)

Graphical k1 =8.64X10-5(s-1)


Fig.IIIA-3

Table: IIIA-3

TYPICAL KINETIC RUN

Effect of concentration of sulphuric acid

38


S. No. Time

(sec.)

Vol. of N

1000 hypo

(ml.)

105k1(s-1)

1. 0 5.00 -

2. 1800 4.15 10.35

3. 3600 3.40 10.71

4. 5400 2.80 10.74

5. 7200 2.30 10.79

6. 9000 1.90 10.75

7. 10800 1.60 10.55

8. 12600 1.30 10.69

9. 14400 1.05 10.84

10. 16200 0.85 10.94

Average k1 =10.68X10-5(s-1)



Fig.IIIA-5

Table: IIIA-4

TYPICAL KINETIC RUN

Effect of Dielectric constant of the medium

40


S. No. Time

(sec.)

Vol. of N

1000 hypo

(ml.)

105k1(s-1)

1. 0 5.00 -

2. 1800 4.05 11.71

3. 3600 3.25 11.97

4. 5400 2.6 12.11

5. 7200 2.15 11.72

6. 9000 1.75 11.67

7. 10800 1.35 12.12

8. 12600 1.15 11.67

9. 14400 0.95 11.53

10. 16200 0.80 11.31

Average k1 =11.81X10-5(s-1)



Fig.IIIA-6

Table: IIIA-5

TYPICAL KINETIC RUN

Effect of concentration of Temperature

From the above tables it is clear that under different conditions the pseudo first

order rate constants obtained are constant for Glycolic acid. The plot of log a/

(a-x) against time are obtained linear passing through origin (Fig. IIIA-1 to IIIA -

5). The rate constant evaluated from plot is good agreement with the respective

calculated value. Therefore, therefore, It is, concluded that system, under study

obeys first order kinetics concluded, that each system, under study obeys first-

order kinetics.

SECTION: III B

The experimental studies reveal that the oxidation of Glycolic acid

undertaken is measurable temperature at 313 K. The results so obtained were

42


S. No. Time

(sec.)Vol. of

N1000 hypo

(ml.)

105k1(s-1)

1. 0 5.00 -

2. 900 4.15 20.70

3. 1800 3.45 20.62

4. 2700 2.90 20.18

5. 3600 2.40 20.39

6. 4500 1.95 20.92

7. 5400 1.75 19.44

8. 6300 1.40 20.21

9. 7200 1.15 20.41

10. 8100 1.00 19.87

Average k1 =20.30X10-5(s-1)Graphical k1 =20.24X10-5(s-1)


used to calculate the rate constant using integrated rate equation. The following

kinetic data i.e. rate constants; recorded and representing the kinetic findings of

the studies are carried out as:

Section III B-1: Dependence of rate on initial concentrations of

SeO2,

Section III B-2: Dependence of rate on the variation in the concentrations of Glycolic acid,

Section III B-3 Dependence of rate on the variation in the concentrations sulphuric acid,

Section III B-4: Dependence of rate on the variation in the % of composition of dielectric constant of the medium,

Section III B-5: Dependence of rate on the variation in temperature

Section III B-6: Thermodynamics parameters.


SECTION: III B -1Dependence of rate of oxidation reaction on the initial

concentration of oxidant (SeO2)The dependence of rate on the initial concentration of oxidant was

investigated by the varying in the concentrations of oxidant i.e. selenium dioxide

(SeO2), while, the concentrations of other reactants are kept constant at their

respective temperature. The summarized results recorded in the Table: III B-1

Table: III B-1

Summary: Dependence of rate of oxidation reaction on the initial concentration of oxidant (SeO2)

[ SeO2]103

(mol.dm.-3)Glycolic acid

1.00 12.35

1.25 12.22

2.00 12.18

2.50 12.25

4.00 12.22

5.00 12.18

Perusal of Table III B-1 and the plots of log (a-x) vs. time are linear with

nearly uniform slope (Fig. III B- 1). Therefore, it is, concluded that the order of

reaction is one with respect to oxidant.

[SeO2] = 2.50X10-3 (mol.dm.-3) [GA] = 1.25X10-2 (mol.dm.-3)[H+] = 1.25X10-3 (mol.dm.-3)HOAc-H2O = 30%(v/V),Temperature = 313 K.

Fig.IIIB-2

46


Fig.IIIB-2(a)

SECTION: III B -2

Dependence of rate on the variation in the concentrations of Glycolic acid

The effect of the concentration of Glycolic acid on the reaction rate was

investigated by varying their concentration, while the concentration of other

reactants was kept constant on their respective temperature. The results are

recorded in the Table III B -2

Table III B -2

[SeO2] = 2.50X103 (mol.dm-3);[H+] = 1.25X102(mol.dm-3);HOAc-H2O = 30% (v/v)Temperature = 313 K .

102[GA](mol.dm.-3) 105k1(s-1)

1.00 8.69

1.25 12.25

2.00 17.77

2.50 22.66

4.00 35.14

5.00 42.53

6.25 53.81

Perusal Tables III B-2 and B-2a that the rate constants are increases with increase in

the concentrations of substrate. The plot of k1 versus [LA] is obtained linear passing

through origin (Fig.IIIB-2). The double reciprocal plot between 1/k1 and 1/

[substrate] is also obtained linear with passing through origin (fig. III B(2a)). Based

on above results it is concluded that-(i) the plot of k1 vs. [substrate] is initially linear

48


passing through origin, hence, the reaction follows first-order behavior with respect

to the concentrations substrate. (ii) This evidence ruled out the formation of a

complex during the reaction.

Fig.IIIB-3

SECTION: IIIB-3

Dependence of rate on the concentration of Sulphuric acid

The effect of the concentration of sulphuric acid on the reaction rate was

investigated by varying their concentration, while the concentration of other

reactants was kept constant on their respective temperature. The results are

recorded in the Table III B-3.

Table: IIIB-3

Summary: Dependence of rate on the variation of the concentration of sulphuric acid.

[SeO2] = 2.50X103 (mol.dm-3);[GA] = 1.25X102(mol.dm-3);HOAc-H2O = 30% (v/v)Temperature = 313 K .

[H+]103 (mol.dm-3) 105k1(s-1)

1.00 10.30

1.25 12.25

2.00 15.56

2.50 16.79

4.00 20.36

5.00 21.28

6.25 21.92

Perusal of Table IIIB-3 shows that the first-order rate constant increases

with increase in concentration of H2SO4 acid i.e. reactions is fully acid catalyzed.

The plot of k1 vs. [H+] and log k1 versus log [H+] was obtained linear with positive

slope (Fig. III B-3).

50


Fig. III B-4

SECTION: IIIB-4

Dependence of rate on the dielectric constant of the medium

In order to study the effect of solvent polarity, on the rate of reaction

between Glycolic acid with selenium dioxide, experiments were carried out by

taking varying composition of binary mixture of acetic acid water; maintaining

the concentration of other reactants constant. The kinetic investigation was

made at constant temperature the results are summarized in Table III E.

Table: IIIB-4

Summary: Dependence of rate on the variation of the composition of binary solvent polarity.

[SeO2] = 2.50 X10-3 (mol.dm-3)[GA] = 1.25 X10-2 (mol.dm-3)[H+] = 1.25 X10-3 (mol.dm-3)Temperature = 313K

# Data of Dielectric constant

of the medium

are taken from N.

venktasubramanian: J. Sci., Indian res., 1961, 20 B, 542.

Perusal of Table IIIB-4, shows that the first-order rate constant increases with

increase % of composition of acetic acid i.e. rate increases with increase in

dielectric constant of the medium. The plot of log k1 versus 103/D was obtained

linear with positive slope (Fig. III B-4).

52

HOAc-H2O % (V/V) 103/D 105k1(s-1)20 17.17 11.8130 19.15 12.2540 21.98 12.5350 25.64 12.7860 30.36 13.1070 38.04 13.35


Fig.IIIB-5

Fig.IIIB-6

54


SECTION: III B-5Dependence of rate on variation of temperature

The dependence of rate on temperature was studied at four different

temperatures for Glycolic acid with selenium dioxide while keeping the

concentration of other reactants constant. The summarized results are recorded

in Table III: B-5.

Table: IIIB-5Summary: Dependence of rate on temperature

[SeO2] = 2.50X10-3(mol.dm-3);[H+] = 1.25X10-3(mol.dm-3), HOAc-H2O = 30% (v/v) ,

Temp. K 308 313 318 325 0C 35 40 45 50

[GA] 102 GLYCOLIC ACID (mol. dm-3)

1.25 8.93 12.25 15.95 20.30

2.00 11.14 15.28 19.61 24.61

SECTION: IIIB-6

THERMODYNAMIC PARAMETERS

Various activation parameters namely temperature coefficient, energy of

activation, frequency factor, free energy, enthalpy of activation and entropy of

activation for each reaction are calculated and presented in the following Table.

Table: IIIB-6

Temp. Coefficient

[GA] 102 GLYCOLIC ACID

(mol. dm-3)

1.25 1.371 1.302 1.272 1.786

2.00 1.317 1.452 1.366 1.914

The Arrhenius plot had drawn between log k1 vs. the reciprocal of

absolute temperature (Fig. III B-5). The value of energy of activation (Ea) is

calculated from the slope of Arrhenius plot. The frequency factor (A) is

calculated using graphical value of Ea. The enthalpy of activation (H#) is

evaluated from the slope of the plot between the krh / kBT and 1/T (Fig. B-6).

Where, kr is specific rate constant. The value of free energy of activation (G#)

and entropy of activation (S#) is calculated by using Erying equation. These

parameters are summarized in the (Table III B-7).

56

[SeO2] = 2.50X103(mol.dm-3);[H+] = 1.25X103(mol.dm-3), HOAc-H2O = 30% (v/v) ,


Table: III B-7

THERMODYNAMIC PARAMETERS

[SeO2] 103 (mol.dm-3) = 2.50(1-3);

[H+] 103(mol.dm-3) = 1.25(1);HOAc-H2O % (v/V) = 30(1-3),Temperature K = 313(1-3).

Substrate Ea

KJ mol-1

A

s-1

∆H#

KJ mol-1

∆G#

KJ mol-1

∆S#

JK mol-1

GLYCOLIC

ACID

48.03

±0. 66

2.36x107

±0.76

49.83

±0.97

-87.68

±0.58

-99.45

±0. 57

STOICHIOMETRY AND PRODUCT ANALYSIS

The stoichiometry of the reaction of oxidation of Glycolic acid with SeO2

in presence of sulphuric acid, in aqueous-acetic acid medium was determined in

duplicate at their experimental temperature by following procedure.

In stoichiometric determination, the experiments were planned and

designed, in which the oxidant concentration was in excess (10 times) over

the concentration of substrate. The binary composition of H2O and CH3COOH

were taken similar to their respective runs. The calculation volume of the

reactants were mixed and maintained in a thermostat at the experimental

condition of temperature, for sufficient time that is until there is no change in

SeO2 concentration. The SeO2 un-reacted in each reaction mixture is, then

estimated separately, periodically by titrating a definite volume of the reaction

mixture iodometrically3. Thus, the quantity of SeO2 used up to oxidize a definite

quantity of each substrate understudy completely is calculated. The results are

recorded in Table: IV-1.

3 Vogel’s text book of practical organic chemistry,5th edn. pp. 1235, Johon wiley & sons inc., 605,avenue, New York

58


Table: IV-1.

Summary: Stoichiometry of the oxidation of Glycolic acid–SeO2

system

[H+] 103(mol.dm-3) = 1.25(1);HOAc-H2O % (v/V) = 30(1-3),Temperature K = 313(1-3).

Sr.No.

[substrate]103

(mol. dm-3)Initial

[SeO2]102

(mol. dm-3)

Final[ SeO2]103

(mol. dm-3)

Consumed[ SeO2]103

(mol. dm-3)

Mole ratio[ SeO2 ]

[substrate]

Glycolic acid1. 4.00 4.00 31.89 8.11 2.02

2. 5.00 5.00 39.99 10.01 2.00

From these stoichiometric data, it is found that for complete oxidation of one

mole of Glycolic acid, one mole of SeO2 is required. The stoichiometric equations

empirically can therefore, represented as:

The oxidation product of Glycolic acid presented in Table- IV-2

Table: IV-2 Oxidation products of Glycolic acid- SeO2 system

Substrate Mole ratio Substrate

SeO2

Products

Glycolic acid 2:1

PRODUCT ANALYSIS

The final oxidation products of the reactions, under investigation were

qualitatively identified by existing conventional methods.4,5,6,7

1. Test of free radicals

The reactions of Glycolic acid with SeO2 showed an induction period. The

presence of free radicals in the system understudy was tested qualitatively by

addition of 1-2 ml of acrylonitrile (monomer) in about 5-6 ml of the reaction

mixture employing trapping method. The non-occurrence of turbidity and white

precipitate clearly indicates the absence of free radicals in the system8.

2. Identification of oxidation main end-product

A 0.25 M solution of 2,4-dinitrophenylhydrazine, may be used for the

preparation of derivatives of keto compounds. Dissolve 25 g of 2,4-

dintrophenylhydrazine in 300 ml of 85 % perchloric acid in a 600 ml beaker on

a stream bath , dilute the solution with 200ml. 95% ethanol, allow to stand and

filter through a sintered glass funnel. it must be emphasized that this reagent is

not suitable for the routine detection of carbonyl compounds since it also gives a

precipitate in cold with certain amine, esters, and other compounds; if, however,

a dilute solution of the keto compound in ethyl alcohol is treated with a few

drops of the reagent and mixture diluted with water and heated, the precipitate

produced with keto compounds generally not dissolves. Collect the crystals of

2,4-dinitrophenylhydrazone of keto and re-crystallized again wash, dried it,

then determine the melting points to compare with reported melting points as

following Table : IV-3

4 Feigel, F.: "Spot Test in Inorganic Applications", Elsevier, NewYork, Vol. I, 341 (1954).5 Feigel, F. : "Spot Test in Inorganic Applications", Elsevier, NewYork, 12, 12 (1966).6 Feigel, F. : Z. anal. Chem., 60, 28 (1921). 7 Feigel, F.: "Spot Test in Inorganic Applications", Elsevier, NewYork, 60, 189 (1966). 8 Gunjan singh : PhD thesis ,central library, A.P.S. University , REWA (M.P.) india (2004)

60


Table: IV-3

Identification of oxidation products by the compared observed melting points and reported melting points

Substrate Main oxidationproduct

Melting points of2,4-dinitrophenylhydrazone

derivatives of oxidation productsObservedmelting

point (0C)

reported melting point (0C)

Glycolic acid Formaldehyde 1550 0C 155.2 0C

3. Test for selenium, reduction product of SeO2

Free selenium- The red precipitated selenium formed by the reduction of

selenium dioxide in each reaction was tested as a small quantity of red

precipitated Se was dissolved in the CS2 . The surface of a piece of silver foil was

roughened with fine emery paper. The metal foil was the thoroughly cleaned. A

drop of test solution was placed the foil and solvent(CS2) was allowed to

evaporate. A gray fleck (of silver selenide) appeared, indicating that the red

precipitate obtained was of free selenium9.

9 F. Feigel; “Spot test” Vol. I Inorganic Applications, Elsevier Publishing Co. London, p-341 (1954)

DISCUSSION AND INTERPRETATION OF RESULTS

Preliminary Remarks

The problem entitled “Kinetic study on reactions of Glycolic acid

with selenium dioxide in water acetic acid medium” deals with the

mechanism of oxidation of Glycolic acid suggested that these reactions

occur at measurable rate within a range of temperature (35- 50C). The

detailed study of rate was switched on at measurable temperature i.e.

400C for respectively. Since there is a little difference between the activity

of these compounds and therefore this point will be discussed in

subsequent pages.

It is found that under the condition [SeO2] << [substrate], is reaction

follows first order kinetics in [SeO2], all these reactions are homogeneous

and characterized by induction period. The induction period can be

accounted in terms of slow approach of steady state.

In these subsequent pages, we will give a comparative account of all

the reaction studied. Chemical kinetics play very important role and adds

valuable and precious wealth of information’s towards its literature as is

obvious.

The study of mechanism of organic compounds is a subject of major

importance to all chemists for not only does it require consideration of the

properties and reaction of both organic and inorganic compounds, but

above all, it has vast implications in connective with the understanding of

the nature of life.

“No mechanism is perfect itself until unless is it’s supported by same

kinetic data” ...Franklin

62


GENERAL FEATURES FOR THE OXIDATION OF GLYCOLIC ACID WITH SeO2

Before elaborating the actual mechanism of the reaction path, it would be

better at this stage to revisualize the results recorded in chapter III, which leads

the following conclusion:

(1) The kinetics data have been collected for variant in concentration of

oxidant (SeO2) at fixed concentration of other reactants and temperature. The

linear plots of log (a-x) vs. Time, suggested that the first order rate dependency

with respect to oxidant.

(2) Perusal Tables: IIIB (2-2a) that pseudo first-order rate increases with

increase in the concentration of substrate. The plot of k1 versus [substrate] are

obtained linear passing through origin (Fig.IIIB-2). The double reciprocal plot

between 1/k1 and 1/ [substrate] is obtained linear with passing through origin

( Fig.IIIB-2). Based on above results it is concluded that-

(a) The plot of k1 vs. [substrate] is initially linear passing through origin and

reaction rate increases with increase in the concentration of substrate; hence,

the reaction follows first-order behavior with respect to the substrate.

(b) This evidence is the ruled out the formation of a complex during the

reaction.

(3) Reactions are fully acid catalyzed but velocity of the reaction slightly

increases with increase the concentration of H2SO4 acid. The plot of k1 vs. [H2SO4]

and plot of log k1 vs. log [H2SO4] is obtained linear with the positive unit slope,

confirming that the reactions are fully acid catalyzed.

(4) The first-order rate constant increases with increase composition of acetic

acid i.e. rate accelerated with increase in dielectric constant of the medium. The

plot of log k1 versus 103/D were obtained linear with positive slope.

(6) The presence of free radicals in the system understudy was tested

qualitatively by addition of 1-2 ml of acrylonitrile (monomer) in about 5-6 ml of

the reaction mixture employing trapping method. The non-occurrence of

turbidity and white precipitate clearly indicates the absence of free radicals in

the system.

(7) Acetaldehyde was formed as the end-product of oxidation of Glycolic

acid, which were identified by the determination of melting points of 2,4-

dinitrophenylhydrazone derivatives of oxidation products and existing

conventional methods.

(8) The stoichiometric determinations have been suggested that 2:1 mole

ratio for substrate and oxidant (SeO2).

(9) Various activation parameters namely temperature coefficient, energy of

activation (Ea), frequency factor (A), enthalpy of activation ( HΔ #), free energy of

activation ( GΔ #), and entropy of activation ( SΔ #) for each reaction are calculated

for Glycolic acid– SeO2 system and according to the reaction mechanism, rate

equation and order of reaction have been discussed. The iso-kinetic and Exner’s

have been explained.

Based on these results a probable reaction path for this oxidation might

be proposed in the following subsequent pages.

MECHANISM OF THE OXIDATION

The kinetic data as summarized in the beginning of the various section of

Chapter III reveal that the reaction velocity follows first-order kinetics. In the

oxidation of Glycolic acid with SeO2 it was found that the respective kinetic

findings in their finality are similar for substrate. It can, therefore, be concluded

64


that for the oxidation of Glycolic acid with SeO2 the mechanism could be

proposed as per following scheme:

Rate Expression

Taking into the consideration of various steps involved in the proposed mechanism, the rate equation could be derived as follows-

Rate Expression

Taking into the consideration of various steps involved in the proposed

mechanism, the rate equation could be derived as follows-

−dcdt

=[ H 3SeO3 ]=k1 [Substrate ]¿

The rate of formation of the main product cited by

+dcdt

[Product ]¿k2 [ Acid selenite ]……………….(7)

On the execution of steady – state approximation

−dcdt

=¿

The net rate of formation of acid selenite is given as

+dcdt

[ Acid selenite ]=k1 [Substrate ]¿

At stationary state

+dcdt

[ Acid selenite ]=0

Therefore, k1 [Substrate ]¿

…………………………………………….(10)

Since,

[ Acid selenite ]=k 1 [Substrate ] ¿¿

Since then,

¿

On inserting the value of acid selenite from equation (11) to(6),

The Reaction rate of take the form as

+dcdt

[Product ]=k2 k1 [Substrate ] [ H2 SeO3 ] ¿¿

When,

k 2≫k−1

66


The rate of reaction becomes

k obs.=k1 [Substrate ] [ H2 Se O3 ]¿

The derived rate equation (13) explains all the experimental facts, which

are in good agreement with our experimental kinetic data i.e. the observed first

order kinetic in [substrate], [oxidant] and [H+] ion etc.

CONCLUSIONS

1. Kinetic studies utilizing selenium dioxide as an oxidant in series of

reaction lead us to conclude that the activity of it is much limited and

needs to be explored in a Broadway. It possesses vital potentiality with

two-electron system and displays interesting behaviors at moderate

condition of temperature.

2. SeO2 is mild, economically cheap oxidant, easily available in market and it

possesses vital potentiality with two electron systems and displays

interesting behaviors at moderate condition of temperature to reduce the

cost of the oxidation reactions involved especially in drugs and

pharmaceutical industries and also in soft drink or cold drink industries.

3. This study will act as a milestone and will pave the way for future

researcher to enlighten the mechanism utilizing SeO2 as an oxidant for

some other organic compounds like disulphide, Anthranyl Styryl Ketone,

chalcones, aliphatic ketones, amines and amino acids in the similar

manners and also can be catalyzed by phosphotungstic acid and micelles

like CTAB etc. The contribution and information through kinetic study

will enrich chemical literature to a great extent in journals.

4. Its applied aspects may be judged in lather industries10, analytical,

chemical separation, and identification of organic compounds and paper

and pulp industries11.

10 V. Priya, M. Balasubramaniyan and N. Mathiyalagan: J. Chem. Pharm. Res., 2011, 3(1):522-528 11S.B. Patwari, S.V. Khansole and Y.B. Vibhute : J. Iran. Chem. Soc., Vol. 6, No. 2, 2009, pp. 399-404.

68


5. This work can better and suitably be utilized some branches of science to

which kinetics is relevant are –

Branch : Application of kinetics

Biology : Physiological process (e.g. digestion and

metabolism), bacterial growth, tissues

growth of malignancy.

Chemical engineering : Reactor design

Electrochemistry : Electrode processes

Geology : Flow processes

Inorganic chemistry : Reaction mechanism

Mechanical Engineering : Physical metallurgy, Crystal dislocation

mobility.

Organic chemistry : Reaction mechanism

Pharmacology : Drug action, pharmacodynamic

Physics : Viscosity, diffusion, nuclear processes

Psychology : Subjective time, memory.

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