REACTIVITY OF PHENOL AND ANILINE TOWARDS QUINOLINIUM CHLORO CHROMATE: A COMPARATIVE OXIDATION KINETIC STUDY

. The kinetics of Oxidation of Phenol and aniline by quinolinium Chlorochromate (QCC) in aqueous acetic acid medium leads to the formation of quinone and azobenzene respectively. The reactions are first order with respect to both Phenol and aniline. The reaction is first order with respect to quinolinium chlorochromate(QCC) and is catalyzed by hydrogen ion. The hydrogen-ion dependence has the form: k obs = a + b [H + ]. The rate of oxidation decreases with increasing dielectric constant of solvent, indicating the presence of an ion-dipole interaction. The reaction does not induced the polymerization of acrylonitrile. The retardation of the rate by the addition of Mn 2+ ions confirms that a two electron transfer process is involved in the reaction. The reaction rates have been determined at different temperatures and the activation parameters have been calculated. From the above observations kinetic results a probable mechanism have been proposed.

The kinetic mechanistic aspects of oxidation reactions by QCC reports are available in the literature [21][22][23][24][25][26] The kinetics of oxidation of Phenols by various oxidizing reagents have been well studied 27-29 . Phenol is so inexpensive that it attracts many small-scale uses. It once was widely used as an antiseptic, especially as carbolic soap. It is a component of industrial paint strippers used in the aviation industry for the removal of epoxy, polyurethane and other chemically resistant coatings. Phenol derivatives are also used in the preparation of cosmetics including sunscreens, hair colorings, and skin lightening preparations.
The kinetics of oxidation of anilines by various oxidizing reagents have been well studied 30-32 . Anilines are the most widespread and principal contaminates of industrial waste waters. Better understanding of the mechanism of oxidation of such compounds to harmless products.

EXPERIMENTAL Materials and methods
All the chemicals used were analytical grade. Phenol and Aniline were used after vacuum distillation. Double distilled water was used for all purposes.

Preparation of Quinolinium Chlorochromate [QCC]
Chromium trioxide (7 g) was dissolved in 8 ml of water in a e beaker and 11 ml of 40% hydrochloric acid were added with stirring at room temperature. A clear orange red solution was formed and 9 ml of quinoline were added drop-wise with stirring. The mixture was heated on a water bath for about 15 min, then cooled to room temperature and allowed to stand for 1 h. The bright yellow crystalline quinolinium chloroochromate was isolated by filtration. It was recrystallized from water and dried in vacuo for about 2 h. The compound melted at (128 º C)[lit m.p 127-130 º C ] 33 and further analysed through spectral studies.

Acetic acid
The procedure followed for the purification of acetic acid was essentially similar to that of Weissberger 34 Glacial acetic acid (AR) 2 litre was partially frozen and about one litre of the liquid was removed. The residue was melted and refluxed with chromium trioxide (30 g) for 4 h and fractionally distilled. The portion distilling between 116-118 º C was collected, partially frozen and about half of the acid was discarded as liquid. The remaining residue was melted and fractionated again after treating with chromium trioxide (30 g). The fraction boiling at 116-118 º C was collected and kept in brown bottles.
The reactions were carried out under pseudo-first order conditions by maintaining the substrate concentration in excess of quinolinium chlorochromate. Known volumes of substrates, water and acetic acid were mixed to bring the percentage of acetic acid to the desired value and thermostated. The reaction were started by adding the oxidant to the mixture and aliquots were removed at definite time intervals and the decrease in [QCC] by digital photoelectric colorimeter at 470nm.The reactions were followed up to 70% completion. The rate constants were obtained from the slope of the plot of log absorbance versus time by the least square method. The results were reproducible within ±1.

Stoichiometry of QCC and Phenol
The kinetics of reaction was to establish the stoichiometry of the reaction and identify any side reactions. The stoichiometry of the reaction [QCC]: [Phenol] was determined by taking excess of [QCC] over [Phenol] and allowing the reaction to go for completion. After sufficient length of time, all the substrate has completely reacted to quinolinium chlorochromate leaving behind the unreacted quinolinium chlorochromate . The unreacted quinolinium chlorochromate was estimated iodometrically. The estimation of unreacted quinolinium chlorochromate showed that one mole of substrate consumed by one mole of oxidant. The stoichiometry between phenol and QCC was found to be 1:1.

Stoichiometry of QCC and Aniline
The kinetics of reaction was to establish the stoichiometry of the reaction and identify any side reactions. The stoichiometry of the reaction [QCC]:[aniline] was determined by taking excess of [QCC] over [aniline] and allowing the reaction to go for completion. After sufficient length of time, all the substrate has completely reacted to quinolinium chlorochromate leaving behind the unreacted quinolinium chlorochromate . The unreacted quinolinium chlorochromate was estimated iodometrically. The estimation of unreacted quinolinium chlorochromate showed that one mole of substrate consumed by one mole of oxidant. The stoichiometry between aniline and QCC was found to be 1:1.

Product analysis of Phenol
The reaction mixture containing Phenol (0.045 M) in acetic acid and QCC (0.001M) in acetic acid was added and the medium was maintained using sulphuric acid. Then the reaction mixture was slightly warmed and was kept aside for about 48 h for the completion of reaction. After 48 h, the reaction mixture was extracted with ether and dried over anhydrous sodium sulphate. The ethereal layer was washed with water several times and kept on a water bath for ether evaporation and cooled to get the product. Para benzoquinone was identified as product.
The product 1,4 benzoquinone was identified by its physical constant (m.p.113 º C) and was confirmed through IR-spectral data and further confirmed by mass spectrum.

Product analysis of Aniline
The reaction mixture containing Aniline (0.03 M) in acetic acid and QCC (0.001M) in acetic acid was added and the medium was maintained using sulphuric acid. Then the reaction mixture was slightly warmed and was kept aside for about 48 h for the completion of reaction. After 48 h, the reaction mixture was extracted with ether and dried over anhydrous sodium sulphate. The ethereal layer was washed with water several times and kept on a water bath for ether evaporation and cooled to get the product. Azobenzene was identified as product.
The product azobenzene was identified by its physical constant (m.p. 68 º C) and was detected by comparing with those of the authentic samples.
The product azobenzene was confirmed through IR-spectral data and further confirmed by mass spectrum

RESULTS AND DISCUSSION Effect of varying the Oxidant
The order with respect to oxidant is one in both the cases of study as evidenced by the linear plot of plot of log absorbance versus time (Fig.1) and also from the constancy of the first order rate constant in (Table 1) International Letters of Chemistry, Physics and Astronomy Vol. 59

EFFECT OF VARYING THE SUBSTRATE ( PHENOL AND ANILINE)
The reactions were carried out by varying the concentration of phenol and aniline keeping the other variables constant. The rate of the reactions increased with increase in the concentration of phenol and aniline. The plot of log k 1 versus log [Substrate] gave a straight line with a slope of unity (Fig. 2) showing a first order dependence on the substrate. (Table 2 ).

EFFECT OF VARYING THE IONIC STRENGTH
The reactions were studied with varying concentration of sodium sulphate and keeping the other variables constant. The results indicate that ionic strength has negligible effect on the reaction rate, which confirmed the participation of an ion and neutral molecule in the rate determining step 35 (Table -3

Effect of varying the hydrogen ion concentration
The reactions followed with different concentration of hydrogen ion keeping the concentration of quinolinium chlorochromate and substrate constant. The rate has been found to increase with increase in concentration of H + . A plot of log k 1 versus log [H + ] give a straight line (Fig. 3) . Since the plot of log k 1 versus log [H + ] did not give an ideal slope of unity. It is not possible to take the order with respect to [H + ] as one and it can be concluded that the reaction is simply an acid catalyzed one 36 . (Table 4).

Effect of varying the solvent composition
Both the reaction rate were measured at different acetic acid -water mixtures. These reactions are observed that an increase in the percentage of acetic acid considerably increased in the rate of the reaction. The plot of log k 1 versus D -1 gave a straight line with a positive slope (Fig. 4) suggests the involvement of an ion -dipole interaction in the rate determining step [37][38][39] (Table 5).

Effect of added acrylonitrile
The added acrylonitrile has no effect on the reaction mixture indicating the absence of free radical mechanism ,no turbidity was obtained.

Effect of varying the manganous sulphate
These reactions carried out with the varying concentrations of Mn 2+ ions keeping all the other factors constant The added Mn 2+ ions has decreased the rate of the reaction. It indicates that two electron process may be involved in the reaction [40][41][42] (Table 6 ).

EFFECT OF VARYING THE TEMPERATURE
Both reactions has been studied at four different temperatures keeping all other factors constant (Table 7). International Letters of Chemistry, Physics and Astronomy Vol. 59

Thermodynamic parameters of Phenol and aniline:
The thermodynamic parameters have been calculated from the least square procedure of a linear plot of ln(k 2 /T) versus 1/T (Fig. 5 )

Mechanism of QCC Oxidation of Phenol
From the above observation it is clear that the reaction is showing first order with respect to QCC and Phenol . The oxidation by Cr(VI) will vary with the nature of the Cr(VI) species used and the solvent will play an important role on the rate of the reaction. In aqueous solution and in the absence of other ions the following are existing, 45 Here the dimerisation equilibrium is of considerable importance. In water the dichromate ion will be predominating species only at the concentrations greater than about 0.05 mol dm -3 . In this case as the concentration of Cr(VI) is less than 0.05 mol dm -3 . The monomeric form predominates and the active oxidising species is HCrO 4 -. The reaction is acid catalysed one. The rate increased with decrease in the dielectric constant of the medium and increase in ionic strength has negligible effect on the rate. The reaction did not induce polymerization of acrylonitrile indicating the absence of free radical path way. The retardation of the rate by addition of Mn 2+ ions confirmed that a two electron transfer process is involved in the reaction. Based on the above facts the following mechanism was proposed (Scheme 1).

Rate law:
The above mechanism leads to the following rate law.

Rate = k 1 k 2 k 3 [Phenol] [QCC] [H + ]
Mechanism of QCC oxidation of aniline From the above observation it is clear that the reaction is showing first order with respect to QCC and aniline . The reaction is acid catalysed one. The rate increased with decrease in the dielectric constant of the medium and increase in ionic strength has negligible effect on the rate. The reaction did not induce polymerization of acrylonitrile indicating the absence of free radical path way. The retardation of the rate by addition of Mn 2+ ions confirmed that a two electron transfer process is involved in the reaction. Based on the above facts the following mechanism was proposed (Scheme 2).

Rate law:
The above mechanism leads to the following rate law.

Conclusion
In this paper we have reported the detail mechanisms of the aniline and phenol by Quinolinium chlorochromate. The reaction is first order with recpect to oxidant, phenol and aniline. The reaction is acid catalysed by hydrogen ion concentration. The oxidation of phenol yield by para benzoquinone. The oxidation of aniline yield by azobenzene. Since aniline is very harmful to the environment, its removal from the environment is the ultimate coal of basic research. For this purpose deep understanding of the mechanism of the process of phenol and aniline are needed. The negative value of ∆S # provided support for the formation of the activated complex in the slow step. Both of these reactions same rate law obtained.