Effect of cyano-group substitution of rates of metathetical reactions. Part 2.—Abstraction of chlorine atoms by cyclohexyl radicals from CHCl2CN and CH2ClCN

Abstract

Temperature dependence of the liquid phase radiation induced abstraction of chlorine atoms by cyclohexyl radicals from CHCl2CN and CH2ClCN was studied competitively. Reactions involved are c-C6H11+ CHCl2CN→c-C6H11Cl + CHClCN (1), c-C6H11+ CH2ClCN→c-C6H11Cl + CH2CN (1′), c-C6H11+ CHBr3→c-C6H11Br + CHBr2(3), c-C6H11+ CH3Br→c-C6H11Br + CH3(3′) where c- denotes cyclo, the present results, combined with previously measured activation parameters for bromine atom abstraction from bromo-methanes by cyclohexyl radicals, relative to chlorine atom abstraction from tetrachloromethane, yield the following rate expressions: log10(k1/k6)=–0.60 ± 0.49–[(2.8 ± 3.5) kJ mol–1/2.3 RT], log10(k1′/k6)=–0.87 ± 0.36–[(17.2 ± 3.1) kJ mol–1/2.3 RT], where k6 is the rate constant for the reaction of cyclohexyl radicals with CCl4 and the error limits are the standard deviations from least mean square Arrhenius plots.

The effect of chlorine atoms substitution by CN on ECl values was derived by combining E1–E6 with E6–ECl(c-C6H11+ CHCl3) and E1′–E6 with E6–ECl(c-C6H11+ CH2Cl2). Activation energies for chlorine atom abstraction from chloromethanes and chloroacetonitriles by cyclohexyl radicals are compared. The decrease in ECl by 15–16 kJ mol–1 upon substitution of Cl by CN in the chloromethane series is attributed mainly to a cyano-group stabilization effect on the incipient radicals.