Title: Hammond principle
Long Title: IUPAC Gold Book - Hammond principle
DOI: 10.1351/goldbook.H02734
Status: current

Definition
The hypothesis that, when a transition state leading to an unstable reaction intermediate (or product) has nearly the same energy as that intermediate, the two are interconverted with only a small reorganization of molecular structure. Essentially the same idea is sometimes referred to as 'Leffler's assumption', namely, that the transition state bears the greater resemblance to the less stable species (reactant or reaction intermediate/product). Many text books and physical organic chemists, however, express the idea in Leffler's form, but attribute it to Hammond. As a corollary, it follows that a factor stabilizing a reaction intermediate will also stabilize the transition state leading to that intermediate. The acronym 'Bemahapothle' (Bell, Marcus, Hammond, Polanyi, Thornton, Leffler) is sometimes used in recognition of the principal contributors towards expansion of the original idea of the Hammond postulate.

Related Terms
- More O'Ferrall–Jencks diagram: https://goldbook.iupac.org//terms/view/M04030
- reaction intermediate: https://goldbook.iupac.org//terms/view/R05171
- stable: https://goldbook.iupac.org//terms/view/S05900
- transition state: https://goldbook.iupac.org//terms/view/T06468
- unstable: https://goldbook.iupac.org//terms/view/U06569

Source
- PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1119 (https://doi.org/10.1351/pac199466051077)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/H02734/html
- json: https://goldbook.iupac.org/terms/view/H02734/json
- xml: https://goldbook.iupac.org/terms/view/H02734/xml

Citation: Citation: 'Hammond principle' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.H02734
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Accessed: 2026-01-28T06:28:09+00:00