Hammett equation (Hammett relation)

https://doi.org/10.1351/goldbook.H02732
The equation in the form: log10⁡(kk0)=ρ σ or log10⁡(KK0)=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; k0 or K0 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 log10⁡k0 or log10⁡K0 written as a separate term on the right hand side, e.g. log10⁡k=ρ σ+log10⁡k0 or log10⁡K=ρ σ+log10⁡K0 It then signifies the intercept corresponding to X = H in a regression of log10⁡k or log10⁡K on σ.
See also: ρ-value, σ-constant, Taft equation, Yukawa–Tsuno equation
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1119 [Terms] [Paper]
Citation: 'Hammett equation' 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.H02732 RIS BibTex EndNote
Div. IIIPDFTextXMLJSON