Term: σ-constant
https://doi.org/10.1351/goldbook.C05438

Definition:
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. log 10 ( K a K a 0 ), where Ka is the ionization constant of a m- or p-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.

Related Terms:
1) hammett equation (http://doi.org/10.1351/goldbook.H02732).
2) <i>ρ</i>-value (http://doi.org/10.1351/goldbook.V05019).
3) taft equation (http://doi.org/10.1351/goldbook.T06247).
4) ionization (http://doi.org/10.1351/goldbook.I03183).
5) power (http://doi.org/10.1351/goldbook.P04792).
6) resonance effect (http://doi.org/10.1351/goldbook.M03844).

Source: PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1171 (https://doi.org/10.1351/pac199466051077)

Citation: 'σ-constant' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.C05438

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