The sign of the electron-releasing effect of the methyl group

Abstract

In 1926 the late professor Sir Robert Robinson, O.M., F.R.S., noting that a group, which acts as a source of electrons, should, according to the conventions of physics , be termed negative, introduced the symbol —I to denote the electron-releasing inductive effect of a group (such as the methyl group CH 3 = Me in toluene) (1). In 1926 the late Professor Sir Christopher Ingold, F.R.S., reversed this sign convention from —I to +1 (2). This reversal has only once briefly been discussed, as far as my knowledge goes (3). The writer is probably the only living person who knows the reason for this sign reversal, because in 1926 he asked Ingold the question. Ingold’s answer was simple, mathematical , and logical: ‘The electron-releasing effect of the methyl group in toluene increases the electron density in the aromatic nucleus, and the differential coefficient of an increasing quantity is positive.’The symbols +1 for inductive electron-release and —1 for inductive electron-attraction introduced by Ingold have been generally accepted and are now used world-wide.