Preparation and Properties of Organomagnesium Reagents

The reaction of magnesium metal with an alkyl or aryl halide in diethyl ether is the standard method for synthesis of Grignard reagents. The order of reactivity of the halides is RI RBr RC1. 

The formation of Grignard reagents takes place at the metal surface. Reaction commences with an electron transfer to the halide and decomposition of the radical ion, followed by rapid combination of the organic group with a magnesium ion.1 It 

One of the fundamental questions about the mechanism is whether the radical is really free in the sense of diffusing from the metal surface.7 For alkyl halides, there is considerable evidence that the radicals behave similarly to alkyl free radicals.8 One test for the involvement of radical intermediates is to determine whether cyclization occurs in the 6-hexenyl system, where radical cyclization is rapid (see Part A, Section 12.2.2). 

Small amounts of cyclized products are obtained after the preparation of Grignard reagents from 5-hexenyl bromide.9 This indicates that cyclization of the intermediate radical competes to a small extent with combination of the radical with the metal. Quantitative kinetic models that compare competing processes are consistent with diffusion of the radicals from the surface.10 Alkyl radicals can be trapped with high efficiency by the nitroxide radical TMPO.11 Nevertheless, there remains disagreement about the extent to which the radicals diffuse away from the metal surface.12 

It seems likely that aryl, vinyl, and cyclopropyl halides react by an alternative mechanism, since the corresponding radicals are less stable than alkyl radicals. It has been suggested that these halides may react through a dianion.13 

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