Grignard reagents, bonding from alkyl halides

This process is similar to the formation of Grignard reagents 4 from alkyl halides and Mg(0). In the preparation of Grignard reagents, Mg(0) is oxidized to Mg(II) by the oxidative addition of alkyl halides to form two covalent bonds. 

The halide radical anion (6) dissociates to give a carbon radical (R, where a radical is defined as a reactive intermediate with one electron in a p-orbital see Chapter 7, Section 7.4.3) and a halide anion, X. If X and 5 react with each other, a Mg-X bond is formed and the magnesium complex becomes a radical (6) rather than a radical cation. Note that the radical cation can accept electron density from the electron-rich X. If 6 subsequently reacts with the carbon radical (R ), Grignard reagent (7) is generated. This sequence is taken to be the mechanism of Grignard reagent formation from alkyl halides 1. The ether... 

Reduction of carbon-halogen bond by metal yields carbanion. Reaction of alkyl halide with Mg in the presence of anhydrous ether as solvent generates Grignard reagent. The Grignard reagent behaves like a carbanion. Alkyllithiums are also obtained from alkyl halides and behave as carbanions. 

Grignard reagents replace halides from the [NiX(L)]+ derivatives by o-bonded alkyl and aryl groups (equation 174), giving a series of diamagnetic organometallic complexes of nickel(H) (189) (Table 74).1444... 

Although the tin hydride + alkyl halide method is probably the most important way of making alkyl radicals, we should mention some other methods that are useful. We said at the beginning of the chapter that carbon-metal bonds, particularly carbon-transition metal bonds, are weak and can homolyse to form radicals. Alkyl mercuries are useful sources of alkyl radicals for this reason. They can be made by a number of routes, for example, from Grignard reagents by transmetailation. 

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