Reactions

The Grignard reagent formation proceeds via a single-electron transfer (SET) process occurring at the surface of the magnesium metal. The reaction starts with the transfer of a single electron from the magnesium surface to the R-X bond, generating a radical anion [R-X]•^- and a magnesium radical cation. This radical anion is unstable and rapidly fragments, cleaving the R-X bond to release a halide anion and an alkyl or aryl radical. This latter then combines with the magnesium radical cation to generate an organomagnesium cation [R-Mg]⁺, which finally reacts with the halide anion to afford the desired product R-Mg-X.

In practice, the preparation of a Grignard reagent must be carried out under strictly anhydrous conditions using dried solvents and inert atmosphere. Also, the addition of an activating agent such as a small amount of iodine is necessary, because magnesium metal is naturally coated with an oxide layer. This layer blocks contact with the organic halide and prevents SET, required to initiate the reaction. This passive oxide layer can be disrupted by adding iodine.