Effect of Substituent on the Kinetics of Chlorination of Imidazole by Chlorine in Aqueous Medium

The kinetics of chlorination of imidazole and its regioisomers, 1-methylimidazole, 2-methylimidazole and 4-methylimidazole by molecular chlorine has been carried out in aqueous medium to study the effect of the methyl substituent at various positions in the imidazole ring on the reactivity of the substrate. The reactions follow second order kinetics and since the reactions are rapid, a special technique- hydrodynamic voltammetry was used. The specific reaction rate has been studied at various temperatures to determine the thermodynamic parameters, energy of activation, entropy change, enthalpy of activation and free energy of activation. The specific reaction rate values have been found in the following order imidazole less than 1-methylimidazole less than 2-methylimidazole less than 4-methylimidazole for their chlorination using molecular chlorine. Stereochemical principles invoked, justified the observed reactivity order for the regioisomers under study. Thus the present work provides the quantitative verification of the relative nucleophilicity of substrates using kinetics as an investigational tool.