Influence of Mg surface layer for induction period of Grignard reagent formation

Factors that affect the induction period of Grignard reagent formation, which involves heterogeneous reaction between magnesium metal (Mg) and an alkyl halide in ether solvent, has been clarified to achieve safer and more efficient operation in chemical processes. The influence of the Mg surface, especially the effects of carbonate, hydroxide, and oxide layers on the induction period were investigated by measuring the exothermic behavior of Grignard reagent formation by a differential reaction calorimeter. Mg powder was kept in water bubbled with CO₂ or N₂ gas to form a coating on the Mg surface. The calorimetry results for the reaction indicated that both treatments increased the induction period. Thermogravimetric analysis-mass spectrometry was conducted to identify the chemical species and quantify the amount of surface material on the Mg particles. It was found that basic magnesium carbonate and magnesium hydroxide were formed on Mg exposed to CO₂ and N₂, respectively. Subsequent heating the carbonate- or hydroxide-coated Mg at 500 °C caused a MgO layer to form on the surface, which was found to dramatically reduce the induction period.