Term: Leffler’s relation
https://doi.org/10.1351/goldbook.08183

Definition:
In a series of elementary reactions, the changes in Gibbs activation energies are often found to be proportional to the changes in Gibbs energies for the overall reaction.\[\updelta \Delta^{\ddagger}G = \alpha \Delta_{\rm{r}}G^{\circ}\]
This relation was interpreted in terms of the simple assumption that a small change in any transition-state property \(P_{\ddagger}\) is a linear combination of changes in reactant- and product-state properties, \(P_{\ce{R}}\) and \(P_{\ce{P}}\). \[\updelta P_{\ddagger} = \alpha \updelta P_{\ce{P}} + (1-\alpha)\updelta P_{\ce{R}}\]
Within the limits of this assumption, the parameter \(\alpha\) is an approximate measure of the fractional displacement of the transition state along the minimum-energy reaction path from reactants to products.

Note: There are many exceptions to the validity of Leffler’s assumption that \(\alpha\) is a measure of the position of the transition state.

Source: PAC, 2022, 94, 353. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 2021)' on page 453 (https://doi.org/10.1515/pac-2018-1010)

Citation: 'Leffler’s relation' 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.08183

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