Oxygenation of Substituted (Phenylthio)acetic Acids by Acidic Bromate: A Kinetic and Mechanistic Study and Validity of Linear Free-Energy Relationships

Abstract

Kinetics and mechanistic investigations on the oxygenation of (phenylthio)acetic acid and its substituted compounds using bromate in acid medium have been carried out. The reaction exhibited first-order in [bromate], 1.6 order in [H2SO4], less than one order in [substrate] and displayed solvent isotopic effect of 1.75 (kD2O/kH2O). The reaction rate is not affected by ionic strength variation, however, enhanced by lowering dielectric constant of the medium. Structural modifications in the aryl moiety of the substrate resulted in a change of reactivity, where electron-seeking substituents decreased the reaction rate. The order of reactivity among the studied substrates is paramethoxy > para-methyl > -H > para-chloro ≈ para-bromo > para-nitro(phenylthio)acetic acid and showed an excellent correlation between rate constant and the Hammett substituent constant (σ value). The reaction constant (ρ) value is negative (-1.37 at 303 K) and decreased by increase in the reaction temperature. Proposed mechanism involves decomposition of the complex formed between acid bromate and the substrate. Based on the proposed mechanism, an appropriate rate law has been derived and tested for its validity. Activation parameters and isokinetic temperature have been evaluated and discussed. The validity of linear free-energy relationships have been discussed thoroughly.