Unimolecular Radical Reactions

Unimolecular radical reactions result from the instability of the first formed radical. The radicals may completely decompose or they may rearrange before reaction with other molecules or radicals present. In decomposition reactions, the radical decomposes to give a stable molecule and a new radical. In rearrangement reactions, the breaking of an adjacent C-C bond in a cyclic system leads to the concomitant formation of a new bond, usually carbonyl, and a new isomeric radical. There could also be migration of an atom, via intramolecular abstraction by the radical center, thus creating a new, isomeric radical.

6.1.1.1 Radical-Molecule Interaction: Addition to Unsaturated

Systems

This involves the addition of a radical to an olefinic double bond to give a new saturated radical. Typical reaction is the radical-induced polymerization of olefins. The intermediate step involving the addition of a radical to an aromatic double bond is widespread in free-radical chemistry, e.g., in the radical substitution of aromatic compounds (homolytic aromatic substitution). The net overall reaction is displacement of an aromatic substituent by a radical: AR-X+Y^AR-Y+X^

6.1.1.2 Radical-Molecule Interaction: SH2 (Substitution Homolytic Bimolecular) Reactions

The Sh2 reactions are bimolecular reactions involving homolytic attack of radicals on a molecule. The radical attacks a univalent atom, usually a terminal halogen or hydrogen in an abstraction reaction, to give rise to a new radical, e.g.: Pfr+CBrCls^CCls+PhBr

Homolytic substitution at multivalent atoms also occurs, but neither normally occurs at saturated carbon centers. SH2 reactions can be synchronous, occurring via a transition state, or stepwise via an intermediate that may exist for only a finite time.

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