The effect of methylation of ethene and ethyne on the nucleophilicity of their π-bonds

Dissociation energies D e of complexes B⋯HX (X = F, Cl, Br or I) are calculated ab initio. Lewis bases B were based ethene or ethyne and progressively methylated. In each complex, hydrogen bonds were to the π bond . The established equation D e = c N B . E HX ( c = 1 kJ mol −1 ) was used with the known electrophilicities E HX of HX to determine the effect of methylation on the nucleophilicity of the π bond of B. It is found that graphs of D e versus E HX have an intercept n that is directly proportional to the number of methyl groups, but the value of N B changes only slowly.