The kinetics of bromination of cyclohexene 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 Brz) 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 Brz and reaction of this electrophile. The rate of the free Brz reaction was substantially determined by the solvent polarity. A linear plot of In 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 a 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 (kH/kD = 1.175 (0.031)) in CHC13/CDC13. The results are consistent with an ionic mechanism involving the rate-determining formation of a bromonium-tribromide ion pair intermediate for the free Brz addition. For the Big reaction they point to the formation of an olefin-Br2 1:l charge-transfer complex in equilibrium with the olefin and Br;, 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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.