Activation and Deactivation of a Chemical Transformation by an Electromagnetic Field: Evidence for Specific Microwave Effects in the Formation of Grignard Reagents

It's field density, not temperature! Modifying the electric field strength in a microwave experiment can completely change the outcome of a reaction. Whereas a low field strength in Grignard reagent formation from Mg metal and aryl halide leads to acceleration of the initiation step, using a high field strength at the same temperature suppresses Mg insertion, favoring solvent decomposition and passivation of the Mg metal (see scheme).