The versatility of the Grignard reagent has made it one of the most valuable synthetic tools for the organic chemist. They are prepared by adding a solution of alkyl or aryl halide in diethyl ether or THF to magnesium turnings to form the Grignard (RMgX) (equation 1.6).
However the simplicity of this equation is misleading; the apparent insertion of the Mg from the metal surface to a carbon-halogen bond is unlikely. The actual mechanism is highly complex. Although there is considerable debate over the mechanism, , the essential steps are shown in Scheme 1.8.
Scheme 1.8 Mechanism for the formation of a Grignard reagent.
The etheric solvent plays a key role by dissolving MgX from the surface of the metal.
At higher concentrations, polymeric species also exist in which the Mg atoms are linked by halide bridges. The situation is further complicated by the existence of the Schlenk equilibrium. In solution, MgR2, RMgX, and MgX2 are known to exist (Scheme 1.9).
Scheme 1.9. Schlenk equilibrium of Grignard reagents in solution.
The Schlenk equilibrium can be driven to the right by removal of the MgX2 species from solution by complexation with dioxane, pyridine or diglyme (equation 1.7).
The utility of a Grignard reagent to the organic chemist is extensive (Scheme 1.10).
Scheme 1.10. Reactions of Grignard reagents