Substitution Reactions

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 (S_N reactions; X is an anion or molecule with an unshared electron pair), electrophilic reactions (S_E reactions; X is a cation or other particle with an affinity for an electron pair), and radical reactions (S_R 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, S_N2), 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