Definition of Constant Sigma Constant

Specifically the substituent constant for meta- and for para-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. lg(Ka/Kao), where Ka is the ionization constant of a m- or p-substituted benzoic acid and Kao 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, +, - and o are typical; also constants which represent dissected electronic effects such as I and R. For this purpose it might be better always to spell out the term in full, i.e. as "Hammett sigma constant", and restrict -constants to the scale of substituent constants which is based on benzoic acid.

A large positive -value implies high electron-withdrawing power by inductive and/or resonance effect, relative to H; a large negative -value implies high electron-releasing power relative to H. CHAPMAN and SHORTER (1972, 1978); JOHNSON (1973); SHORTER (1973). See also Hammett equation, -value, Taft equation.