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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 K a is the ionization constant of a m- or p-substituted benzoic acid and K 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, 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.
PAC, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1171
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IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.
Last update: 2014-02-24; version: 2.3.3.
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