Reactivity of Magnesium Surfaces during the Formation of Grignard Reagents

A reaction cell which allows for photomicrographic observation of magnesium surfaces during formation of Grignard reagents is described. As the reaction proceeds, a finite number of discrete sites initiate relatively rapidly. Following this stage, the reactive area increases by the growth of sites rather than by the initiation of new sites. Sites are randomly located across the magnesium surface (neither clustered nor dispersed) consistent with their formation at crystalline defects or metallic impurities. Various methods of magnesium surface activation are tested. For example, iodine and ferric chloride increase reactivity by increasing the density of reactive sites and by increasing the rate at which individual sites react. Scratching the magnesium surface increases reactivity by decreasing the time required for initiation of reactive sites. A mercury/magnesium amalgam provides a uniformly reactive surface, but the reaction rate is very slow.