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 E_Cl values was derived by combining E₁–E₆ with E₆–E_Cl(c-C₆H₁₁+ CHCl₃) and E_1′–E₆ with E₆–E_Cl(c-C₆H₁₁+ CH₂Cl₂). Activation energies for chlorine atom abstraction from chloromethanes and chloroacetonitriles by cyclohexyl radicals are compared. The decrease in E_Cl 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.