A measure of the susceptibility to the influence of
substituent groups on the
rate constant or
equilibrium constant of a particular organic reaction involving a family of related
substrates.
Defined by Hammett for the effect of ring substituents in
meta- and
para-positions of aromatic side-chain reactions by the
empirical '
-equation' of the general form:
in which
is a constant characteristic of the substituent X and
of its position in the reactant molecule.
More generally (and not only for aromatic series),
-values
(modified with appropriate subscripts and superscripts) are used to designate the
susceptibility of reaction series for families of various organic compounds to any
substituent effects, as given by the modified set of
-constants
in an empirical
-correlation.
Reactions with a positive
-value are accelerated (or the equilibrium constants of analogous equilibria are increased)
by substituents with
positive
-constants. Since the sign of
was defined so that
substituents with a positive
increase the
acidity of benzoic acid, such substituents are generally described as attracting electrons
away from the aromatic ring. It follows that reactions with a positive
-value are considered
to involve a
transition state (or reaction product) so that the difference in energy between this state and the
reactants is decreased by a reduction in
electron density at the reactive site of the substrate.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1161
