The effect on a chemical or physical property (structure, rate or @[email protected]) upon introduction of @[email protected] 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 @[email protected] (or products). A steric effect on a rate process may result in a rate increase ('steric @[email protected]') or a decrease ('steric retardation'). (The adjective 'steric' is not to be confused with stereochemical.) Steric effects arise from contributions ascribed to @[email protected] as the sum of (1) non-bonded repulsions, (2) bond @[email protected] and (3) bond stretches or compressions. For the purpose of @[email protected] or @[email protected] various scales of steric parameters have been proposed, notably @[email protected], Taft's \(E_{\text{s}}\) and Charton's \(\nu \) 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 @[email protected] by non-bonded compressions. Some authors make a distinction between 'steric' effects attributed to van der Waals repulsions alone, and '@[email protected]' effects, attributed to deviations of bond angles from 'ideal' values.
Taft equation
van der Waals forces
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1168 [Terms] [Paper]