Comparison of molecular bromine and tribromide ion as brominating reagents. 1. Kinetic evidence for different mechanisms of addition to cyclohexene

Abstract

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.

Spectroscopic and Theoretical Investigations of Electrophilic Bromination Reactions of Alkynes: The First Evidence for π Complexes as Reaction Intermediates

“…This, however, is not too surprising since the direct influence of the polarity of the chlorinated solvent on the reaction rate had already been noted and discussed in earlier studies. [25] In fact, the far less polar solvent chloroform probably decelerates the formation of the charged intermediate. CHCl 3 also leads to a significant change in the activation parameters.…”

Competing kinetic pathways in the bromine addition to allylic ethers in 1,2‐dichloroethane: Opposite temperature effects

“…The tribromide is known to react with alkenes through a concerted mechanism, and a charged intermediate like a bromiranium or the opened β-bromocarbenium cation does not occur in this reaction. Products derived from stereoselective anti addition are exclusively formed [18]. The sole formation of the vicinal trans dibromide excludes the occurrence of an opened intermediate in the reaction of 2 with molecular bromine.…”

“…Yield: 98%. (18). The same product was also obtained from the reaction of 2 with Br 2 (10% excess of tetrabutylammonium bromide); then the salt was eliminated by washing with water and the product isolated by distillation.…”

Synthesis, Characterization and Structure of DBU-hydrobromide-perbromide: A Novel Oxidizing Agent for Selective Oxidation of Alcohols to Carbonyl Compounds

“…1 Organic tribromide salts as mild reagents are good candidates for this selective conversion, because they are not only stable, crystalline solids, and relatively soluble in most organic solvents but also can be handled more conveniently than liquid bromine and can be used successfully for this purpose. Although they are used as brominating agents of aromatic rings, 2 α-bromination of ketones, [3][4][5][6] and ketals, 3,7 electrophilic addition, 3,4,[8][9][10][11][12][13][14] and substitution, 4,13,[15][16][17][18][19] they are also used as catalyst for the oxidation of aromatic aldehydes to carboxylic acids 20 or ω-bromoesters, 21 and dialkyl and alkyl aryl sulfides to sulfoxides, 22 protection of carbonyl, 23 and hydroxyl groups, 24 cleavage of ethers and dithioacetals 25 and conversion of thioamides into amides. 26 They can also be applied in the synthesis of heterocyclic ring systems such as aziridines 27 and benzothiazoles.…”