Kinetics of thermal racemization of some 1,2-dihalides

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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.

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