SUBSTITUTED AROMATIC ANILS - A KINETIC STUDY USING PYRIDINIUM CHLOROCHROMATE ( PCC )

Abstract

The kinetics of co - oxidation study of anils and oxalic acid by PCC in aqueous acetic acid had been studied under different experimental conditions. The order is one with respect to oxalic acid , anil , and PPC. The order is 1.5 with respect to [ H+ ]. The oxidation was found to increase with increase of perchloric oxalic acids. The rate of the reaction was increased by the addition of NaClo4 . The protonated species of the oxidant is involved in the rate - determining step. The effect of various meta and para - substituted anils on the reaction rate had been studied at four different temperatures viz., 35 , 40 , 45 and 50 0 c and the thermodynamic parameters are also calculated. The analysis of rate constants of ortho substituents reveas that the localized and delocalized effects are major components while steric effect place only the minor role.

Keywords

delocalized, anils , PCC , rate , parameters , localized and delocalized

INTRODUCTION

The kinetics of oxidation reactions and the elucidation of the reaction mechanisms from the kinetic data have always been the most fascinating subjects in chemistry. In any kinetic investigation one may be interested to arrive at [i] the relationship between the rate and various factors like concentration of the reactants , temperature , reaction medium etc., and [ii] the interpretation of the empirical rate laws in the light of mechanism proposed.

II. GENERAL CHARACTERISTICS OF PYRIDINIUM CHLOROCHROMATE ( PCC )

The kinetics and mechanism of oxidation reactions using Chromium (VI) oxidants are fairly wellstudied.1-5 In recent ears a significant number of reactions has been reported with PCC . PCC also known as Corey’s reagent , was first prepared by Corey et al.6,7 It is a complex of Chromium trioxide , Pyridine and hydrochloric acid. The pure sample of PCC is quite stable for more than a month . Its solution in water and in acetic acid are also fairly stable. Its dilute solution either in water or in acetic acid shows maximum absorption in the UV region at ^ = 350 nm . It liberates iodine quantitatively from acidified potassium iodine solution.

III.PREPARATION OF PYRIDINIUM CHLOROCHROMATE (PCC)

PCC was prepared by the method of Corey and Suggs.6,7 To 184 ml of 6 ml of HCl ( 1: 1 mol. ) was added 100 g of CrO3 rapidly with stirring. After five minutes the homogeneous solution was cooled to O 0 c and 79.1 g ( 1ml) Pyridine was carefully added over 10 minutes. Recooling to O 0 c gave an yellow or orange solid ( PCC )which was collected on a sintered glass funnel and dried for 1 h in vacuum ( yield 180.8 g- 84% ).

IV.PURIFICATION OF ACETIC ACID

Acetic acid of AR grade was first refluxed with Chromium trioxide ( 20 g per litre ) for 4 h . and then distilled. It was refluxed again with Chromium trioxide ( 2g per litre ) for 2h. then it was fractionated through a Dufton column and the fraction at 117 - 118 0 was collected. Purification of water De -ionised water was distilled twice over Potassium permanganate in an all glass corning vessel. All the aqueous solutions were prepared using this double - distilled water.

V.STOICHIOMETRY

The stoichiometry of the reaction was determined by carrying out several sets of experiments with varying amounts of PCC largely in excess over anil. The estimation of unreacted PCC showed that one mole of anil reacts with two moles of PCC.

Ph - CH = N - Ph + 2 PyHOCrO2Cl + ( COOH ) 2→PhCHO + PhNO + 2 CO2 + PyHCl + H2O + Cr2O3

VI.PRODUCT ANALYSIS

Under kinetic condition the reaction was carried out and the products were separated and analysed. The products obtained were analysed by Co -TLC and IR spectrum as previously reported. It showed that benzaldehyde and nitrobenzene were products formed in the reaction. The liberation of CO2 during the oxidation was detected as described earlier.

VII.KINETIC MEASUREMENTS

The reactions were carried out in 60% (v/v) acetic acid - water under pseudo - first order conditions taking always the substrate in excess. The solutions of the described concentrations were prepared.

1. PCC in water

2. A nil in acetic acid

3. Oxa lic acid in water

4. Perchloric acid in water

5. Aluminium nitrate in water

6. Sodium perchlorate in water.

All the solutions were kept in a thermostat at constant temperature. The measurements were made between 300 c to 450c . the required volumes of these solutions for each run were mixed and 2 ml aliquots of the reaction mixture were pipette out at convenient time intervals and quenched in 10 ml 3 % KI solution acidified with 3 M H2SO4 (10ml). the liberated iodine was titrated with standard sodium thiosulphate using starch. The pseudo - first order rate constants were evaluated from log titre values verses time plots.

VIII..RESULTS AND DISCUSSION

Experimental results indicate that the rate of oxidation of anils by Cr(VI) in the presence of oxalic acid is several times greater than the independent oxidation rates of anils and oxali acid by Cr(VI) In order to understand the nature of oxidized products of anils and to study the structure - reactivity relationships , we have undertaken the kinetic study of the PCC oxidation in acetic acid medium in the presence of oxalic acid.

8.1.Effect of varying PCC concentration

The reaction was found to be first order with respect to PCC 8.2.Effect of varying the concentration of Anil

The reaction was found to be first order with respect to Anil. 8.3.Effect of varying solvent Composition

It was found that the rate deccreases with increase of percentage of acetic acid. 8.4.Effect of varying Oxalic acid

It was found that the rate increases with increase of concentration of oxalic acid. 8.5.Effect of varying Perchloric acid

It was found that the rate increases with increase of concentration of Perchloric acid. 8.6.Effect of temperature The rate of oxidation of some meta and para substituted anils had been studied at four different temperatures viz., 350 c , 400 c, 450 c and 50 0 c . It was observed , as we expected ,that the rate increases very much with increase of temperature .

Tables at a glance

Table 1 Table 2 Table 3 Table 4 Table 5 Table 6

References

F.H.Westheimer , G.T.E.Graham and F.H.Westheimer , J.Am. Chem. Soc., 1958 , 80 : 3030   J.O.Edwards , Chem. Rev., 1952 , 50 : 455   K.B.wiberg and T.Mill , J. Am. Chem. Soc., 1958 , 80 : 3032 M.Kodama -Bulletin of Chemical Society of Japan ,1969