https://doi.org/10.1351/goldbook.S05997
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 Es 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.
See:
Taft equation
,
van der Waals forces
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1168 [Terms] [Paper]