Preferred Label : steric effect;
IUPAC 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
R sup o /sup Y, the 'primary steric effect' of R is the direct result of differences
in compressions which occur because R differs from R sup o /sup 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.;
Origin ID : S05997;
See also
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
R sup o /sup Y, the 'primary steric effect' of R is the direct result of differences
in compressions which occur because R differs from R sup o /sup 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.
