On the nature of the radical pairs involved in the Grignard reaction

During the simultaneous reaction of alkyl bromides and of p-halosubstituted benzyl bromides with magnesium in THF/C 6 D 6 (1:4), CIDNP net effects (emission) are observed in the 1 H nmr spectrum. The effects are opposite to those occurring during the reaction of preformed ethylmagnesium bromide and the benzyl bromides. These results constitute firm evidence that only alkyl radical pairs are responsible for the CIDNP effects, and that the radicals R* are generated in the Grignard reaction proper, rather than in unrelated side reactions such as Wurtz coupling or metal halogen exchange.