Kinetics and Thermodynamics of Oxidation of Some meta-substituted Anilines by Tetrabutylammonium Bromochromate in Aqueous Acetic Acid Medium

The tetrabutylammonium bromochromate (TBABC) oxidation of anilines, in an aqueous acetic acid medium in the presence of perchloric acid is described. The reaction is first order with respect to aniline, TBABC and acid. The reaction rate has been determined at different temperatures and activation parameters calculated. The TBABC oxidation of meta-substituted anilines obeys Hammett relationships.


INTRODUCTION
Aniline is known to be carcinogenic and also reacts easily in the blood to convert haemoglobin into methaemoglobin, preventing oxygen uptake.Aniline and its derivatives are used as intermediates for the manufacturing of various organic compounds such as colorants, agrochemicals and pharmaceutical agents.Therefore, a serious effect on human health over a long period of time is possible, even if aniline is in low concentrations 1 .
Aniline can enter the body either by inhalation of air containing aniline, ingestion of food or water containing aniline, or by dermal contact with aniline.Inhalation of air containing aniline can cause respiratory tract irritation.Exposure to high levels can cause a number of adverse health effects including breathing difficulties, dizziness, headache, irregular heart-beat, methaemoglobinaemia (blood disorder), blue colour to the skin, convulsions and in extreme cases coma and death.Ingestion of aniline can lead to gastrointestinal irritation, nausea, vomiting and diarrhea.Exposure to high levels may lead to effects similar to those for inhalation.Dermal contact with aniline can cause skin irritation and sensitization.Absorption of large quantities of aniline through the skin may cause effects similar to those for inhalation.There is an increased risk in young infants of developing methaemoglobinaemia due to aniline exposure 2 .
Oxidation is an essential reaction for different organic synthesis.Chromium compounds have been used in aqueous and non-aqueous medium for the oxidation of a variety of organic compounds.Chromium compounds especially Cr(VI) reagents have been versatile reagents and capable of oxidizing almost all the oxidisible organic functional groups.The development of newer chromium (VI) reagents for the oxidation of organic substrates continues to be of interest 3 .In recent years, many such reagents have been developed with some success.Some new Cr(VI) based reagents like tetraheptylammonium bromochromate 4 , tripropyl ammonium fluorochromate 5 , isoquinolinium b r o m o c h r o m a t e 6 , t e t r a h e x y l a m m o n i u m bromochromate 7 , benzimidazolium fluorochromate 8 and tetrabutylammonium bromochromate 9 have been used to study the oxidation of various organic compounds.
The oxidation kinetics of anilines by various oxidizing agents have been extensively studied [10][11][12][13][14] .Since anilines are very harmful to human health and the environment, removal of aniline from the    environment is the ultimate goal of research today.For this a deep understanding of the mechanism of the oxidation process of aniline is needed.Furthermore, one of the important tools in deciding the mechanism of reactions is the study of substituent effects and thermodynamic parameters.The Hammett equation, also known as Linear Free Energy Relationships (LFER) have been found useful for correlating reaction rates and equilibrium constants for metaand para-substituted derivatives of benzene.The isokinetic relationship is also an important tool for deciding the nature of a mechanism 14 .Keeping this in view, we report the kinetic features of the oxidation of a series of meta-substituted anilines by TBABC in an aqueous acetic acid medium to propose a suitable mechanism.

Materials and reagents
All the employed chemicals and solvents were of analytical grade.Anilines were used with substituents H, m-OCH 3 , m-OC 2 H 5 , m-CH 3 , m-F, m-Cl and m-NO 2 .Tetrabutylammonium bromochromate (TBABC) was prepared by a reported method 9 and its purity was checked by the iodometric method.Doubly distilled water was used for all purposes.The solid anilines were used as such and the liquid anilines were used after vacuum distillation.Acetic acid was purified by standard method and the fraction distilling at 118 o C was collected.

Kinetic measurements
The pseudo -first-order conditions were attained by maintaining a large excess ( x 15 or more) of aniline over TBABC.The solvent was 50% acetic acid -50% water (v/v), unless specified otherwise.The reactions were followed, at constant temperatures (± 0.01 K), by monitoring the decrease in [TBABC] spectrophotometrically at 362 nm using UV-Vis spectrophotometer, Shimadzu UV-1800 model.

Data analysis
Correlation analysis were carried out using Microcal Origin (Version 6.1) computer software.The goodness of the fit is discussed using the correlation coefficients and standard deviations.

Stoichiometry and Product analysis
The stoichiomety of the reaction was determined by carr ying out several sets of experiments with varying amounts of TBABC largely in excess over aniline.The estimation of unreacted

RESULTS AND DISCUSSION
The oxidation of anilines by TBABC has been conducted in 50% acetic acid and 50% water medium at 303 K, under pseudo first order conditions and the result obtained were discussed in the following paragraphs.
The values of k 1 were independent of the initial concentration of TBABC (Table 1) suggesting the reactions were of first order with respect to TBABC.The reaction was catalysed by hydrogen ions and the order with respect to [H + ] was one.Linear plots of log k 1 versus log [Substrate] with unit slope demonstrate the first-order dependence of the rate on [Aniline].
The oxidation of aniline in a nitrogen atmosphere failed to induce the polymerization of acrylonitrile.Furthermore, the rate of oxidation decreased with the addition of Mn(II), indicating the involvement of a two-electron reduction of Cr(VI) to Cr(IV) (Table 1).Therefore, a one-electron oxidation, giving rise to free radicals, is unlikely.
The oxidation of aniline has been studied in the binary mixture of acetic acid and water as the solvent medium.The concentration of acetic acid was varied from 30% to 70% and the rate were measured.The reaction rate increased remarkably with the increase in the proportion of acetic acid in the solvent medium (Table -2).Positive slope of log k 1 versus 1/D plot indicates that the reaction involves a cation-dipole type of interaction in the rate determining step 15 (Fig. 1).
Solvent variations may affect the kinetics and the energy of the electron transfer processes in a complex manner, particularly in mixed solvent media as the physico-chemical properties of mixed solvent media are often quite different from those of the pure solvents or of their ideal mixtures 16 .The dependence of the kinetic parameters for reactions on the composition of mixed aqueous solvents often affords complicated patterns.
Rates of oxidation of anilines were determined at different temperatures between 298 and 313 K in 50% -50% (v/v) acetic acid -water medium in presence of perchloric acid and the pseudo-first order rate constants were determined (Table -3).Various activation parameters were calculated and the values were presented in Table -4.The entropy of activation is negative for anilines.
The effect of structure on reactivity of metasubstituted anilines were studied.It is interesting to note that the reactivity decreases in the order m-CH spectrophotometrically following the disappearance of Cr(VI) at 362 nm; b Estimated from pseudo first order plots c In the presence of 0.001 mol dm -3 acrylonitrile; d In the presence of 0.003 mol dm -3 Mn (II); Solvent composition = 50% AcOH-50% H 2 O (v/v) Table 2: Effect of varying solvent polarity on the rate of reaction of aniline m-OCH 3 , m-OC 2 H 5 , m-CH 3 , m-F, m-Cl and m-NO 2 anilines by TBABC at 303 K 10 4 k 1

10 2 [ 3 Fig. 1 :
Fig. 1: Showing plot of 1/D against log k 1 showing effect of solvent polarity at various temperatures in the presence of oxalic acid

10 2 [Fig. 2 :
Fig. 2: Showing Hammett plot of log k 2 versus substituent constant σ p for the oxidation of aniline, m-OCH 3 , m-OC 2 H 5 , m-CH 3 , m-F, m-Cl and m-NO 2 anilines by TBABC in aqueous acetic acid medium at different temperatures

Scheme 1 :
Scheme 1: Mechanism of oxidation of aniline by TBABC