Structure-reactivity correlations in the reaction of substituted N-methyl- 2-styrylpyridinium iodides with alkaline hydrogen peroxide

Abstract

The kinetics of reaction between N-methyl-2-styrylpyridinium iodides and alkaline hydrogen peroxide have been studied spectrophotometrically in 1:1 (v/v) water-methanol mixture. The reaction is first order in [substrate], [H₂O₂] and [OH^-]. Structure-reactivity study with various 2'- and 4'-substituted N-methyl-2-styrylpyridinium iodides shows that the reaction is accelerated by electron-withdrawing substituents and retarded by electron-releasing substituents. A Hammett &rho;-value of +0.916 &plusmn; 0.064 (<em>r </em>= 0.988;<em> s </em>= 0.063) is obtained. A suitable mechanism involving the attack of hydroperoxide ion on the &beta;-carbon atom of the substrate to form epoxide in a slow step followed by cleavage of epoxide is proposed. Analysis of <em>ortha-effects </em>using multiparameter extensions of the Hammett equation in the reaction of 2'-substituted-N-methyl-2-styrylpyridinium iodides with alkaline hydrogen peroxide shows that both localised and delocalised effects are important and the steric effect is insignificant.