05997 IUPAC Gold Book - steric effect 10.1351/goldbook.S05997 S05997 current 1 The effect on a chemical or physical property (structure, rate or equilibrium constant) upon introduction of substituents having different steric requirements. The steric effect in a reaction is ascribed to the difference in steric energy between, on the one hand, reactants and, on the other hand, a transition state (or products). A steric effect on a rate process may result in a rate increase ('steric acceleration') or a decrease ('steric retardation'). (The adjective 'steric' is not to be confused with stereochemical.) Steric effects arise from contributions ascribed to strain as the sum of (1) non-bonded repulsions, (2) bond angle strain and (3) bond stretches or compressions. For the purpose of correlation analysis or linear free-energy relations various scales of steric parameters have been proposed, notably A values, Taft's \(E_{\rm{s}}\) and Charton's \(\nu \) scales. In a reactant molecule $\ce{RY}$ and an appropriate reference molecule $\ce{R^{\circ}Y}$, the 'primary steric effect' of $\ce{R}$ is the direct result of differences in compressions which occur because $\ce{R}$ differs from $\ce{R^{\circ}}$ in the vicinity of the reaction centre $\ce{Y}$. A 'secondary steric effect' involves the differential moderation of electron delocalization by non-bonded compressions. Some authors make a distinction between 'steric' effects attributed to van der Waals repulsions alone, and 'strain' effects, attributed to deviations of bond angles from 'ideal' values. A values https://goldbook.iupac.org/terms/view/A00012 Taft equation https://goldbook.iupac.org/terms/view/T06247 acceleration https://goldbook.iupac.org/terms/view/A00051 angle strain https://goldbook.iupac.org/terms/view/A00349 correlation analysis https://goldbook.iupac.org/terms/view/C01346 delocalization https://goldbook.iupac.org/terms/view/D01583 equilibrium constant https://goldbook.iupac.org/terms/view/E02177 linear free-energy relations https://goldbook.iupac.org/terms/view/L03551 strain https://goldbook.iupac.org/terms/view/S06037 substituents https://goldbook.iupac.org/terms/view/S06076 transition state https://goldbook.iupac.org/terms/view/T06468 van der Waals forces https://goldbook.iupac.org/terms/view/V06597 PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1168 (https://doi.org/10.1351/pac199466051077) https://goldbook.iupac.org/terms/view/S05997/html https://goldbook.iupac.org/terms/view/S05997/json https://goldbook.iupac.org/terms/view/S05997/plain Citation: 'steric effect' 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.S05997 The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms. If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org . The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using. 2026-05-10T18:57:08+00:00