Electrochemical reactions. Part XIV. Carbon–halogen bond fragmentation in the radical anions from 4-(chlorostyryl)pyridines. Factors which determine the rate of such fragmentations

Abstract

In dimethylformamide at 296 K the radical anion from 4-(3-chlorostyryl)pyridine is slowly destroyed by protonation. The radical anion from 4-(4-chlorostyryl)pyridine reacts at 296 K (k= 1·5 ± 0·4 s^–1) to form 4-styrylpyridine but at 252 K fragmentation is not detected before the radical anion is destroyed by protonation. Other halogenated radical anions show different rates of fragmentation of the carbon–halogen bond depending on the position of substitution and these observations are explained by taking account of the free electron density distribution in the first antibonding molecular orbital. Reduction of 4-(4-chlorostyryl)pyridine in aqueous methanol parallels reduction of 4-styrylpyridine, and the chloro-substituent is not lost. The half-wave potentials for the two reduction waves of substituted 4-styrylpyridines have been correlated with the Hammett σ constant.