Kinetics and mechanism of the reaction of benzyl halides with zinc in dimethylformamide

The reaction of metal zinc with various benzyl halides in dimethylformamide has been studied. The low‐temperature ESR‐spectroscopy is used to show that reaction of zinc with benzyl bromide and benzyl iodide occurs to form benzyl radicals. Benzyl chloride reacts with zinc at 77 K to form benzyl radicals and ion‐radical pairs RX· ^ - - Zn· ^ + . Nevertheless these pairs disappear at temperatures higher than 110 K. The investigation of the reaction stereochemistry and the use of a radical trap provide evidence in favor of a radical mechanism. The optical activity has been proved in the course of the formation of organozinc halide from (+)‐R‐1‐halogen‐1‐phenylethane at low temperature. The kinetic features of the reaction have been studied. Benzyl iodide reacts with zinc at transport‐controlled rate. The reaction of zinc with benzyl chloride and benzyl bromide in DMF occurs according to Langmuir–Hinshelwood scheme, the adsorption of reagents taking place on similar active centers of metal. The kinetic and thermodynamic parameters of the reaction have been clarified. The comparison of kinetic parameters with those reported in literature evidences that the limiting step of reaction is halogen atom transfer (inner sphere electron transfer).