Comparison of Molecular Bromine and Tribromide Ion as Brominating Reagents. 1. Kinetic Evidence for Different Mechanisms of Addition to Cyclohexene.

Article abstract of DOI:10.1021/jo00218a013

The kinetics of bromination of cyclohexane with molecular bromine and with tetrabutylammonium tribromide in a series of chlorinated hydrocarbons of dielectric constant ranging between 2.2 and 10.7 have been investigated with the stopped-flow technique.Third-order (second order in Br2) and second order (first-order in Br3^-) rate laws were respectively followed by the bromine and the tribromide reactions.Only in the least polar solvent, carbon tetrachloride, the reaction of molecular bromine gave erratic results even when fitted to a two-term rate equation.The rates of the bromine and of the tribromide reactions exhibited opposite temperature coefficients, negative for the former and positive for the latter.Added tetrabutylammonium bromide had only a modest negative salt effect on the rate of the Br3^- reaction, excluding its dissociation to free Br2 and reaction of this electrophile.The rate of the free Br2 reaction was substantially determined by the solvent polarity.A linear plot of ln k₃ against the Kirkwood function of the dielectric constant, indicating a highly polarized transition state, was obtained for reactions carried out in mixtures of 1,2-dichloroethane and chloroform.An inverted trend was observed for the rates of the tribromide reaction in the same binary mixture of solvents, but no linear plot was obtained, and the rates measured in the series of solvents correlated with the stability constants of Br3^- and with the ability of solvents to hydrogen bond bromide ions.The reaction of tribromide, but not that of molecular bromine, was subjected to a kinetic solvent isotope effect (k_H/k_D=1.175 (0.031)) in CHCl3/CDCl3.The results are consistent with an ionic mechanism involving the rate-determining formation of a bromonium-tribromide ion pair intermediate for the free Br2 addition.For the Br3_- reaction they point to the formation of an olefin-Br2 1:1 charge-transfer complex in equilibrium with the olefin and Br3_-, followed by a rate- and product-determining nucleophilic attack by bromide on the transient complex through a transition state more charge delocalized than the initial state.