Definition of linear free-energy relation

1. A linear correlation between the logarithm of a rate constant or equilibrium constant for one series of reactions and the logarithm of the rate constant or equilibrium constant for a related series of reactions. Typical examples of such relations (also known as linear Gibbs energy relations) are the Brønsted relation , and the Hammett equation (see also -value ).

   The name arises because the logarithm of an equilibrium constant (at constant temperature and pressure) is proportional to a standard free energy (Gibbs energy) change, and the logarithm of a rate constant is a linear function of the free energy (Gibbs energy) of activation.

   It has been suggested (IUPAC PHYSICAL ORGANIC GLOSSARY (1983)) that this name should be replaced by Linear Gibbs Energy Relation , but at present there is little sign of acceptance of this change.

   The area of physical organic chemistry which deals with such relations is commonly referred to as "Linear Free-Energy Relationships".