Kinetics of thermal racemization of some 1,2-dihalides

Abstract

The first-order rate constants for the thermal racemization of trans-1,2-dibromocyclopentane, -cyclohexane, -cycloheptane, and -cyclo-octane, 1,2-dibromopentane, threo-2,3-dibromopentane, and trans-2-chloro-1-bromo-cyclohexane have been measured at different temperatures and the activation parameters have been determined. Values of the activation energies ranging between 29·2 and 33·2 kcal mol^–1 have been found for the dibromides, the activation entropies being very low; the chloro-bromide had the highest activation energy (35·9 kcal mol^–1). A parallelism has been observed between the racemization and the iodide-promoted debromination of trans-1,2-dibromocycloalkanes. All data are consistent with a racemization mechanism identical with that proposed for the diaxial–diequatorial rearrangement of steroidal and cyclohexyl trans-1,2-dibromides.