KINETIC EVIDENCE FOR RATE DETERMINATION DURING THE NUCLEOPHILIC STEP OF OLEFIN BROMINATION - THE CASE OF 5H-DIBENZ[B,F]AZEPINE-5-CARBONYL CHLORIDE

The bromination of 5H-dibenz[b,f]azepine-5-carbonyl chloride has been investigated in 1,2-dichloroethane at 5, 25, and 50-degrees-C. Working at reagent concentrations where the bromination was very slow, the fo rmation of a 1:1 charge-transfer complex (CTC) was shown spectrophotom etrically by the presence of a large difference absorption with respec t to the olefin and to Br2 alone. Although the formation constant of t his CTC was too low (K(f) less-than-or-equal-to 0.1 M-1) to be determi ned, the products (K(fepsilonCT) at the three temperatures were obtain ed from the slopes of the linear plots of the difference absorbances a gainst the olefin concentrations, and a value of the formation enthalp y, DELTAH = -0.9 (0.05) kcal mol-1, was obtained from a plot of In (K( fepsilonCT) against 1/T. The kinetics of bromination were measured at variable temperature and reagent concentrations. At 50-degrees-C and [ Br2] ranging between 2 x 10(-2) and 5 x 10(-1) M the usual third-order kinetics were observed. The same rate law was obeyed at 25-degrees-C and [Br2] less-than-or-equal-to 5 x 10(-2) M, whereas neither a third- order nor a fourth-order rate law was followed at 5 x 10(-1) M Br2 and olefin. At 5-degrees-C and [Br2] less-than-or-equal-to 5 x 10(-2) M t he third-order rate law was again observed, but at 5 x 10(-1) M Br2 an d olefin an overall fourth-order (third-order in Br2) rate law was cle anly obeyed. A very small apparent activation energy, E(a(obsd)) = 3.4 5(0.1) kcal mol-1, was found for the third-order process. A significan t conductivity, which was highest at the lowest temperature, was found during the course of the reactions. omo-10,11-dihydro-5H-dibenz[b,fla zepine-5-carbonyl chloride was the only reaction product. It was shown by D NMR measurements to exist in 1,2-dichloroethane solution in two forms, both having anti-oriented bromine atoms and being nonequivalent because of different bond angles and bond lengths at C(10) and C(11). They are interconverted through a seven-membered ring inversion by to rsion about the C(4a)-N(5)-C(5a) bonds, with a free activation energy DETAG(double dagger) = 16.6(0.2) kcal mol-1. Introducing the values of E(a(obsd)) for the third-order bromination and of DELTAH for the CTC formation in the equation E(a(obsd)) = E(a) + DELTAH[1/ (1 + K(f)[Ol]) ] gives a true activation energy, E(a), too small for a reaction as sl ow as the investigated one. This shows that the rate-determining step of this reaction cannot be the CTC ionization, but is rather the colla pse of bromonium-tribromide intermediates having a large and negative formation enthalpy. Low temperatures and high Br2 concentrations favor the transformation of the tribromide counteranion into pentabromide, and the third-order dependence of the rate on Br2 results from the fac t that the rate-determining step involves a species containing three B r2 and one olefin molecules. This shift from an overall third-order to a fourth-order rate law with decreasing temperature and increasing [B r2] could provide a mechanistic criterium for rate determination durin g the nucleophilic step of olefin bromination.