Journal of Organic Chemistry p. 4224 - 4232 (1986)
Update date:2022-08-04
Topics:
Bellucci, Giuseppe
Bianchini, Roberto
Vecchiani, Sandra
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.
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