Comparison of Molecular Bromine and Tribromine Ion as Brominating Reagents. 2. Kinetic and Product Investigation of the Bromination of 3-Substituted Cyclohexenes

Article abstract of DOI:10.1021/jo00372a023

The kinetics and products of bromination of several 3-substituted cyclohexenes with molecular Br2 and with tetrabutylammonium tribromide have been investigated in 1,2-dichloroethane and chloroform.In both solvents the reactions of the unsubstituted, 3-alkyl-substituted, and 3-halogen-substituted olefins followed a third-order rate law (second order in Br2).Changing the solvent from 1,2-dichloroethane to chloroform caused a 13- to 41-fold decrease in k₃.A tert-butyl group produced a 6- or 3-fold deceleration in k₃ in the two solvents, whereas a 3-bromo or 3-chloro substituent reduced the k₃ by five orders of magnitude.Tetrabutylammonium tribromide reacted with all investigated substrates following a second-order rate law in both solvents.Added bromide had no significant effect on the rates nor on the products.These reactions were 6- to 18-fold faster in chloroform than in 1,2-dichloroethane.A tert-butyl group caused a 35- to 36-fold decrease, a 3-bromo and a 3-chloro substituent an about three orders of magnitude decrease, and a benzoyloxy or para-substituted benzoyloxy group an about two orders of magnitude decrease in k₂.The products of the molecular Br2 reactions consisted of mixtures of diaxial and diequatorial dibromo adducts in ratios depending on the substituent. 3-Benzoyloxy-substituted substrates gave, in addition, cis 1,2- and cis-1,3-dibromides.Only diaxial and diequatorial dibromo adducts, with a large prevalence (70-90percent) of the former, were always obtained in all tribromide reactions.The kinetic and product results are consistent with an addition mechanism of molecular Br2 involving a rate-determining ionization of olefin Br2 charge-transfer complexes (CTC's) to bromonium-tribromide ion pairs followed by fast collapse to dibromo adducts.For the tribromide reactions they suggest instead a rate- and product-determining nucleophilic attack by bromide on olefin-Br2 CTC's in equilibrium with the olefin and tribromide ions.