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Kinetic and competitive bromination studies on simple alkenes in carbon tetrachloride support the hypothesis that complexation is an essential step in alkene bromination. The kinetic data were reproducible and showed that the reaction was first order in alkene and second order in bromine. These reaction rates were not retarded by a free-radical inhibitor. The observed reactivity order was (fastest to slowest) (£)-3-hexene: (Z)-3-hexene: (S)-4-octene. While the individual reaction rates were reproducible, the apparent third-order rate constants varied systematically with the alkene-bromine composition and were higher at higher bromine fractions. These results are consistent with predictions based on a kinetic model involving the formation of a bromine-alkene complex as the first step. Subsequent steps could involve the direct attack of a second bromine molecule on this complex or the formation of bromonium ions. Under competitive conditions, the relative reactivity of (Z)-S-hexene compared to (5)-4-octene was much greater than predicted from the kinetic studies. This enhanced reactivity is consistent with complex formation assuming that the (Z)-3-hexene complexes form more readily than the (2?)-4-octene. This assumption is in agreement with the observed iodine-alkene complexation data.

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