Effects of Alkyl Groups in Electrophilic Additions and Substitutions

Abstract

KHARASCH has shown that the addition of hydrogen iodide, and, homolytic processes being suppressed, of other hydrogen halides to propylene and other monoalkylethylenes, involves a practically exclusive terminal uptake of the hydrogen atom¹. Holleman showed that the nitration of toluene involves a 96–97 per cent ortho-para-orientation². It has been found quite generally that alkyl-oriented additions and substitutions are accompanied by large increases of reaction-rate³, for example, by a factor of 24.5 for the nitration of toluene as compared with that of benzene. It is a question for discussion whether these strong kinetic effects of alkyl groups are to be credited mainly to hyperconjugation between CH-bonds and the olefinic or aromatic π-system, or to the inductive effect of alkyl groups arising in these cases from an electrical dissymmetry in a σ-CC-bond formed by differently hybridized carbon-atomic orbitals⁴. Evidence on this matter can be obtained by rate-comparisons between the relevant reactions of methyland tert.-butyl-substituted ethylenes and benzenes. Of these substituents only methyl can provide CH-hyperconjugation. Both groups should have similar positive inductive effects, that of the tert.-butyl group being the greater.