Preferred Label : ρ-value;
Detailed label : ρ-value (rho-value);
IUPAC definition : 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
i meta /i - and i para /i -positions of aromatic side-chain reactions by the empirical
'ρσ-equation' of the general form: \[\log _{10}(\frac{k_{{X}}}{k_{{H}}}) \rho \
σ _{{X}}\] in which σX is a constant characteristic of the substituent X and of its
position in the reactant molecule. More generally (and not only for aromatic series),
ρ-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 σ-constants in an empirical
ρσ-correlation. Reactions with a positive ρ-value are accelerated (or the equilibrium
constants of analogous equilibria are increased) by substituents with positive σ-constants.
Since the sign of σ was defined so that substituents with a positive σ 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 ρ-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.;
Origin ID : V05019;
False automatic mappings
See also
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
i meta /i - and i para /i -positions of aromatic side-chain reactions by the empirical
'ρσ-equation' of the general form: \[\log _{10}(\frac{k_{{X}}}{k_{{H}}}) \rho \
σ _{{X}}\] in which σX is a constant characteristic of the substituent X and of its
position in the reactant molecule. More generally (and not only for aromatic series),
ρ-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 σ-constants in an empirical
ρσ-correlation. Reactions with a positive ρ-value are accelerated (or the equilibrium
constants of analogous equilibria are increased) by substituents with positive σ-constants.
Since the sign of σ was defined so that substituents with a positive σ 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 ρ-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.