Kinetic evidence for complex formation in alkene bromination

Abstract

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.

Investigation of the Early Steps in Electrophilic Bromination through the Study of the Reaction with Sterically Encumbered Olefins

“…1,2 For alkenes that do not form a highly stabilized β-bromocarbocation, the reaction invariably proceeds with the initial instantaneous formation of an olefin-Br 2 charge transfer complex 3 (CTC) which has been shown to be on the reaction pathway. 4,5 Central to any rationalization of the stereochemical and structure/reactivity characteristics of this deceptively simple reaction is the postulate of the three-membered bromonium ion intermediate (1) first proposed by Roberts and Kimball in 1937 to account for the trans mode of addition. 6 Generally, these species are too short lived to be detected during the electrophilic addition reaction, and although widely accepted as intermediates, until the late 1960s, virtually no information about their structures or physicochemical properties was available.…”

Evidence for different addition mechanisms in the bromochlorination of 3-tert-butylcyclohexene with bromine chloride and with monopyridinebromine(I) chloride

“…With CsHsNBrCl in CHC13. A solution of CsHsNBrCl (1.75 g) in 17 ml of CHCI3 was added within 20 min to a stirred solution of 1 (1.0 g) in 20 ml of the same solvent at 0°. After 10 min the reaction mixture was treated as described under B.…”

13C and 2H Kinetic Isotope Effects and the Mechanism of Bromination of 1-Pentene under Synthetic Conditions

“…Interestingly, one of the least understood variations of the reaction is synthetically most commonthe bromination of alkenes in CCl 4 . Kinetic results in CCl 4 are conflictingrate laws which are first order in [Br 2 ], second order in [Br 2 ], a combination of the two, and 1.5 order in [Br 2 ] 7 have all been observed. Induction periods and irreproducible rates have been noted, and substantially different rates have been reported by different workers. , Brominations in moderately polar solvents such as 1,2-dichloroethane are much more well behaved kinetically, but even very careful studies in very nonpolar solvents have led to conflicting results …”

“…The deceptively simple, undergraduate-textbook mechanism of bromination of alkenes via a bromonium ion gives way to a host of intricacies in reality. Complications include the involvement of bromocarbenium ions and alkene/Br 2 π complexes, reversibility of bromonium ion formation, − the observation of second-order involvement of Br 2 or more complex kinetics, , catalysis by trace components, , and a variety of unique observations with hindered alkenes. , Scheme illustrates some of the mechanistic possibilities. Considerable progress has been made in delimiting the possible mechanistic entanglements and understanding the bromination mechanism under many conditions, particularly in polar solvents.…”