Mechanism of Formation of Grignard Reagents. The Rate of Reaction of Cyclopentyl Bromide with Magnesium Is Transport Limited in Diethyl Ether

Article abstract of DOI:10.1021/ja00521a035

Three observations indicate that the rate of reaction of cyclopentyl bromide with metallic magnesium in several ethereal solvents is limited by the rate of its encounter with the metal.First, the rate of disappearance of cyclopentyl bromide is dependent on the first power of the organic halide concentration, the magnesium surface area, and rate at which the solution is stirred, and is inversely dependent on the solution viscosity (-d/dt infinite ¹, A¹, ω¹, η^-1).Second, the activation energy for the reaction in diethyl ether (E_a ca. 2.3 kcal/mol) is consistent with mixing and diffusion as rate limiting.Third, a number of representative alkyl halides react with magnesium in diethyl ether at very similar rates; no organic halides examined react faster than this rate.The dependence of the rate of reaction of neopentyl bromide on solution dielectric constant illustrates transition from transport-limited to non-transport-limited rates, and suggests appreciable charge separation in the chemical step which breaks the carbo-bromine bond.