https://doi.org/10.1351/goldbook.C05438

Specifically the substituent constant for ,

*meta*- and for*para*-substituents in benzene derivatives as defined by Hammett on the basis of the @[email protected] constant of a substituted benzoic acid in water at \(25\ ^{\,\unicode{x26ac}}\text{C}\), i.e. \(\log _{10}(\frac{K_{\text{a}}}{K_{\text{a}}^{0}})\), where \(K_{\text{a}}\) is the @[email protected] constant of a*m*- or*p*-substituted benzoic acid and \(K_{\text{a}}^{0}\) 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, \(\sigma ^{+}\), \(\sigma ^{-}\) and \(\sigma ^{0}\) are typical; also constants which represent dissected electronic effects such as \(\sigma _{I}\) and \(\sigma _{R}\). For this purpose it might be better always to spell out the term in full, i.e. as 'Hammett sigma constant', and restrict \(\sigma \)-constants to the scale of substituent constants which is based on benzoic acid. A large positive \(\sigma \)-value implies high electron-withdrawing @[email protected] by inductive and/or @[email protected], relative to H; a large negative \(\sigma \)-value implies high electron-releasing @[email protected] relative to H.

*See also:*Hammett equation

, *ρ*-value

Taft equation