Definition of isodesmic reaction

A reaction (actual or hypothetical) in which the types of bonds that are made in forming the products are the same as those which are broken in the reactants, e.g.

PhCOOH + p-ClC6H4COO- PhCOO- + p-ClC6H4COOH

ClCH=CH2 + ClCH2CH2Cl CH2=CH2 + Cl2CHCH2Cl

Such processes have advantages for theoretical treatment. The Hammett equation as applied to equilibria (cf. (a)) essentially deals with isodesmic processes.

For the use of isodesmic processes in quantum chemistry, see HEHRE et al. (1970).