Free-energy relationship

In physical organic chemistry, a free-energy relationship or Gibbs energy relation relates the logarithm of a reaction rate constant or equilibrium constant for one series of chemical reactions with the logarithm of the rate or equilibrium constant for a related series of reactions.[1] Free energy relationships establish the extent at which bond formation and breakage happen in the transition state of a reaction, and in combination with kinetic isotope experiments a reaction mechanism can be determined. Free energy relationships are often used to calculate equilibrium constants since they are experimentally difficult to determine.[2]

The most common form of free-energy relationships are linear free-energy relationships (LFER). The Brønsted catalysis equation describes the relationship between the ionization constant of a series of catalysts and the reaction rate constant for a reaction on which the catalyst operates. The Hammett equation predicts the equilibrium constant or reaction rate of a reaction from a substituent constant and a reaction type constant. The Edwards equation relates the nucleophilic power to polarisability and basicity. The Marcus equation is an example of a quadratic free-energy relationship (QFER).[citation needed]

IUPAC has suggested that this name should be replaced by linear Gibbs energy relation, but at present there is little sign of acceptance of this change.[1] The area of physical organic chemistry which deals with such relations is commonly referred to as 'linear free-energy relationships'.

  1. ^ a b IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "linear free-energy relation". doi:10.1351/goldbook.L03551
  2. ^ Lassila JK, Zalatan JG, Herschlag D (2011-06-15). "Biological phosphoryl-transfer reactions: understanding mechanism and catalysis". Annual Review of Biochemistry. 80 (1): 669–702. doi:10.1146/annurev-biochem-060409-092741. PMC 3418923. PMID 21513457.