## Wikipedia - Hammett-yhtälö (fi) σ-constant

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.