Mechanism of Formation of Grignard Reagents

Experiment shows that there is no carbon kinetic isotope effect in the formation of CHsMgl from CH3I (k12/k13 = 0.9992, σ = 0.0005). Calculations indicate that an observable isotope effect should accompany this reaction if it proceeds by an inner-sphere electron-transfer mechanism in which breakage of the carbon-halogen bond must be involved in the rate-determining step. Accordingly, it is proposed that the alternative mechanism must be followed, with the rate-determining step involving formation of a radical-ion pair by outer-sphere electron transfer from the magnesium to the organic halide.