A DYNAMIC NMR INVESTIGATION OF THE ADAMANTYLIDENEADAMANTANE/BR2 SYSTEM - KINETIC AND THERMODYNAMIC EVIDENCE FOR REVERSIBLE FORMATION OF THE BROMONIUM ION/BR(N)(-) PAIRS

A dynamic H-1 NMR study of the adamantylideneadamantane (Ad=Ad)/Br2 system has been undertaken in d4-1,2-dichloroethane as a function of excess [Br2] and temperature. Bromine site exchange rate constants (k(NMR)) were determined by line shape analysis of the spectral region between delta 2.1 and 2.6. The temperature range between the fast and slow exchange limits for a given [Br2] is unusually narrow, being approximately 15-degrees-C. Increasing [Br2] at a given temperature dramatically reduces the k(NMR) due to the equilibrium formation of Br3-, Br5- and Br7-/bromonium ion pairs. It is shown that the dynamic phenomena proceed via the rate-limiting dissociation of a 1:1 charge-transfer complex to free Ad=Ad + Br2. General implications for the bromination of more reactive olefins are discussed.