Term: steric effect
https://doi.org/10.1351/goldbook.S05997

Definition:
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 s and Charton's ν scales. In a reactant molecule RY and an appropriate reference molecule RoY, the 'primary steric effect' of R is the direct result of differences in compressions which occur because R differs from Ro in the vicinity of the reaction centre 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.

Related Terms:
1) equilibrium constant (http://doi.org/10.1351/goldbook.E02177).
2) acceleration (http://doi.org/10.1351/goldbook.A00051).
3) angle strain (http://doi.org/10.1351/goldbook.A00349).
4) delocalization (http://doi.org/10.1351/goldbook.D01583).
5) strain (http://doi.org/10.1351/goldbook.S06037).
6) substituents (http://doi.org/10.1351/goldbook.S06076).
7) transition state (http://doi.org/10.1351/goldbook.T06468).
8) correlation analysis (http://doi.org/10.1351/goldbook.C01346).
9) linear free-energy relations (http://doi.org/10.1351/goldbook.L03551).
10) a values (http://doi.org/10.1351/goldbook.A00012).
11) taft equation (http://doi.org/10.1351/goldbook.T06247).
12) van der waals forces (http://doi.org/10.1351/goldbook.V06597).

Source: PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1168 (https://doi.org/10.1351/pac199466051077)

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. https://doi.org/10.1351/goldbook.S05997

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