Also found in: Dictionary.
chemical changes in which the “attacking” particle displaces an atom or group of atoms from the other reactant:
X + Y — S → X — S + Y
Depending on the nature of X, substitution reactions are subdivided into nucleophilic reactions (SN reactions; X is an anion or molecule with an unshared electron pair), electrophilic reactions (SE reactions; X is a cation or other particle with an affinity for an electron pair), and radical reactions (SR reactions; X is a particle with an unshared electron).
A distinction is made between unimolecular and bimolecular substitution reactions. For example, the hydrolysis of tert-butyl chloride involves unimolecular nucleophilic substitution; slow dissociation into carbonium and chloride ions occurs in the first stage, and a fast reaction between the carbonium ion and water, yielding tert-butyl alcohol, occurs in the second stage.
(CH3)3C—Cl → (CH3)3c+ + Cl−
(CH3)3C + H2O → (CH3)3C—OH + H+
One-stage hydrolysis, in which the breaking of the carbon-halogen bond and the formation of a C—OH bond proceed simultaneously (nucleophilic bimolecular substitution, SN2), is more characteristic of primary and secondary alkyl halides:
In the first case the kinetics of the process corresponds to a first-order reaction; in the second case, to a second-order reaction. Electrophilic substitution is most characteristic of aromatic compounds, for example:
An example of radical substitution, or metalepsis (radical-chain chlorination of paraffins), is
Cl2→Cl. + Cl. Cl. + CH4 → CH3. + HCl
CH3. + Cl2 → Ch3Cl + Cl. etc.
Double decomposition reactions are a special case of the substitution reaction:
X — Y + Z — W → X — W + Y—Z
as are displacement reactions:
Zn + CuSo4 → ZnSo4 + Cu
B. L. DIATKIN