Kinetics and Mechanism of the Reaction of Cyclohexene with Bromine in the Presence of Pyridine: Competition between Different Electrophiles

Article abstract of DOI:10.1021/ja00545a014

The kinetics of the title reaction was investigated in 1,2-dichloroethane by the stopped-flow technique.Both free bromine and the pyridine-bromine charge-transfer complex (py*Br2) act as electrophiles toward the double bond, a third electrophile, the tribromide ion, being formed during the course of the reaction through incorporation of pyridine to give N-(trans-2-bromocyclohexyl)pyridinium bromide, which captures bromine to yield the corresponding tribromide; this remains as the only electrophile in the later stages of the reaction when all excess bromine and py*Br2 have been used up.The Br3(-) ion has a formation constant higher than 1E7 M^-1 in dichloroethane.The reaction of free bromine with cyclohexene is second order in the halogen (k= 2.39E5 M^-2s^-1 at 25 deg C); those of py*Br2 and of the above tribromide are first order in the halogenating species (k= 12.8 and 1.2 M^-1s^-1 at 25 deg C). py*Br2 produces mostly the pyridinium salt, Br3(-) trans-1,2-dibromocyclohexane.On the basis of the results, a mechanistic scheme is proposed, which assumes that the reaction with bromine involves a bromonium intermediate and those with py*Br2 and with Br3(-) involve a nucleophilic attack on an alkene-bromine charge-transfer complex.The present data may explain why different product compositions have been observed in reactions of cyclohexenes and conjugated dienes involving bromine, on one hand, and py*Br2 or Br3(-), on the other.