Oxidation of Vanillin and Related Compounds by Sodium N-Chloro-p-toluenesulfonamide in Acid Medium: A Kinetic and Mechanistic Approach

Abstract

The kinetics of oxidation of vanillin, vanillyl alcohol, vanillylamine hydrochloride and vanillyl mandelic acid (hereafter abbreviated as substrate) by N-chloro-p-toluenesulfonamide or chloramine–T (CAT) in HClO4 medium has been investigated at 30±0.1°C. The oxidation reaction follows identical kinetics with first-order in [CAT]o, fractional order in [substrate]o, and inverse fractional-order in [H+]. Variation of ionic strength and addition of the reaction product, p-toluenesulfonamide, or halide ions had no significant effect on reaction rate. Decrease in dielectric constant of the medium decreases the rate. The solvent isotope effect was studied in D2O medium. The reaction does not induce polymerization of acrylonitrile. Michaelis-Menten type of kinetics has been proposed and activation parameters for the rate-limiting step as well as overall reaction have been computed. The decomposition constants of substrate-oxidant complexes have been evaluated. Under comparable experimental conditions, the rate of oxidation increases in the following order vanillin>vanillylamine hydrochloride>vanillyl alcohol>vanillylmandelic acid. An isokinetic relationship is observed with β=339 K, indicating enthalpy as a controlling factor. Vanillic acid was identified as the oxidation product of each substrate and was confirmed by IR and GC-MS data. A general mechanism involving the formation of a complex between substrate and the conjugate acid (CH3C6H4SO2NHCl) of the oxidant has been proposed. The derived rate law is in agreement with the experimental results.