Substitution Reactions

Also found in: Dictionary.
Related to Substitution Reactions: addition reactions

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 (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


References in periodicals archive ?
Importantly, this type of regioselectivity had never been accomplished by the conventional electrophilic substitution reactions.
The first mechanistic proposal for the process of phase-transfer catalysis was formulated by Starks [1, 3] for a liquid-liquid system (LL-PTC) using a nucleophilic substitution reaction (Scheme 1).
In another study, TFR was exploited to carry out nucleophilic aromatic substitution reaction of nitrobenzene by a strong nucleophile like anilide salt of tetramethylammonium ion to give p-nitroso or nitro diphenylamine in a continuous manner.
The Amberlite XAD-4[TM] (PS, from Rohm and Haas) beads were modified by the aromatic electrophilic substitution reaction of benzenic rings of the copolymer using PC[l.
After TIP acquires two glycerol groups, further substitution reactions with water, TPG, or glycerol do not occur.
The major difference is that 3 utilized a chiral side group that contained 12 stereogenic centers that underwent multiple substitution reactions as it was being prepared.
Summaries of the present views of bonding (Mingos and May), of ligand substitution reactions (Darensbourg) and of polyhedral rearrangements (Johnson and Rodgers) update earlier reviews by these authors.
Summary: This study comprises the synthesis and characterization of twenty thiazole-derived carbamates (3a-e), N-substituted amides (8a-h) and carboxamide (10) from 2-aminothiazoles (1a, b) via nucleophilic substitution reactions with activated carbonyl compounds including, chloroformates (2a-d), acid chlorides (7a-e) and glutaric anhydride (9), respectively.
The aromatic substitution reactions are not limited to azobenzene and benzylidene but are even more facile with N,N-dimethylbenzylamine [13,14].
Updated and expanded to cover the literature as of the end of 2005, it covers basic kinetic and mechanistic terminology and methodology, ligand substitution reactions, stereochemical change, reaction mechanism of organometallic systems, oxidation-reduction reactions, inorganic photochemistry, bioinorganic systems, and experimental methods.
It was found that radical substitution reactions could become significant, and, in some cases, were the only reaction observed.
The formation, decomposition and substitution reactions of the unusual organometallic (alkyl)penta(aqua)chromium(III) complexes also are being investigated.