https://doi.org/10.1351/goldbook.C01191
A reduction in the
rate
of certain reactions of a
substrate
RX in solution [by a path that involves a
pre-equilibrium
with formation of R+ (or R ) ions as reaction intermediates] caused by the addition to the reaction mixture of an electrolyte solute containing the 'common ion' X (or X+). For example, the rate of
solvolysis
of diphenylmethyl chloride in acetone-water is reduced by the addition of salts of the common ion Cl- which causes a decrease in the
quasi-equilibrium
concentration of the diphenylmethyl
cation
in the scheme:
This phenomenon is a direct consequence of the
mass-law effect
on
ionization
equilibria in electrolytic solution. More generally, the common-ion effect is the influence of the 'common ion' on the reactivity due to the shift of the
dissociation
equilibrium. It may also lead to an enhancement of the
rate
.
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1098 [Terms] [Paper]