REVERSIBLE FORMATION OF BROMONIUM IONS - PREFERENTIAL REACTION OF BR- ON THE BR+ OF THE BROMONIUM ION PRODUCED FROM THE SOLVOLYSIS OF THE TRANS-2-BROMOTRIFLATE OF CYCLOHEXANE IN MEOH OR HOAC CONTAINING ADDED BROMIDE

The trans-2-bromotriflate of cyclohexane (4) has been solvolyzed at ambient temperatures in HOAc and MeOH containing varying [Br-] and in the presence of 0.5 M of a scavenger olefin, cyclopentene. The kinetics of solvolysis of 4 have been determined by observing the rate of change in color of an acid/base indicator that instantly responds to the production of HOTf or HBr. The kinetics show that the rate of solvolysis of 4 at constant ionic strength is independent of [Br-]. The products of solvolysis of 4 under the various conditions have been determined by quantitative GLPC analysis. In the presence of Br-, the products consisted of the trans 1,2-dibromides and bromosolvates of cyclohexane and cyclopentane. The cyclopentyl products are shown to be formed from the electrophilic addition of Br2/Br3- to cyclopentene while trans-1,2-dibromocyclohexane (2) was obtained from Br- capture of the bromonium ion of cyclohexene on carbon. The Br2 arises from capture by Br- of the bromonium ion on Br+. On the basis of the ratio of the cyclopentyl products/2, Br- capture of the solvolytically produced bromonium ion by attack on Br+ is 4-5 times more prevalent than attack on carbon in HOAc and -25 times more prevalent than attack on carbon in MeOH. The results have strong implications on the reversal of bromonium ions formed during the electrophilic bromination of olefins in hydroxylic solvents.