Substitution and Elimination at C sp3 —X a Bonds, Part II

the resonance structure for the radical anion [R X]- in which there is a three-electron, two-center bond between C and X is most useful for drawing the SrnI mechanism. Of course, the LUMO need not be this particular orbital, and one may often draw better resonance structures in which the odd electron is localized elsewhere. 

The best nucleophiles for the SrnI mechanism can make a relatively stable radical in the initiation part, either by resonance (enolates) or by placing the radical on a heavy element (second-row or heavier nucleophiles). The best electrophiles for the SrnI mechanism are able to delocalize the odd electron in the radical anion (aromatic leaving groups, carbonyl compounds), can make a stable radical (3° alkyl halides), and have a weak R-X (Br, I) bond. Tosylates and other pseudohalides are very poor SrnI electrophiles. If light is required for substitution to occur, the mechanism is almost certainly SrnI. 

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