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Hammett equation (Hammett relation)

The equation in the form:
log 10 ( k k 0 ) = ρ σ
or
log 10 ( K K 0 ) = ρ σ
applied to the influence of meta- or para-substituents X on the reactivity of the functional group Y in the benzene derivative m- or p-XC6H4Y. k or K is the rate or equilibrium constant, respectively, for the given reaction of m- or p-XC6H4Y; k 0 or K 0 refers to the reaction of C6H5Y, i.e. X = H; is the substituent constant characteristic of m- or p-X: is the reaction constant characteristic of the given reaction of Y. The equation is often encountered in a form with log 10 k 0 or log 10 K 0 written as a separate term on the right hand side, e.g.
log 10 k = ρ σ + log 10 k 0
or
log 10 K = ρ σ + log 10 K 0
It then signifies the intercept corresponding to X = H in a regression of log 10 k or log 10 K on σ.
Source:
PAC, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1119
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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: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook.
Last update: 2014-02-24; version: 2.3.3.
DOI of this term: doi:10.1351/goldbook.H02732.
Original PDF version: http://www.iupac.org/goldbook/H02732.pdf. The PDF version is out of date and is provided for reference purposes only. For some entries, the PDF version may be unavailable.
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