σ-constantIUPAC Term

Specifically the substituent constant for i meta /i - and for i para /i -substituents in benzene derivatives as defined by Hammett on the basis of the ionization constant of a substituted benzoic acid in water at 25 C, i.e. log 10 ( K a K a 0 ), where Ka is the ionization constant of a i m /i - or i p /i -substituted benzoic acid and Ka0 that of benzoic acid itself. The term is also used as a collective description for related electronic substituent constants based on other standard reaction series, of which, σ , σ- and σ 0 are typical; also constants which represent dissected electronic effects such as σ I and σ R. For this purpose it might be better always to spell out the term in full, i.e. as 'Hammett sigma constant', and restrict σ-constants to the scale of substituent constants which is based on benzoic acid. A large positive σ-value implies high electron-withdrawing power by inductive and/or resonance effect, relative to H; a large negative σ-value implies high electron-releasing power relative to H.