The detection of ionic intermediates during the bromination of 5H-dibenz[b,f]azepine-5-carboxamide in 1,2-dichloroethane

Abstract

The bromination of 5H-dibenz[b,f]azepine-5-carboxamide (carbamazepine)(1) in 1,2-dichloroethane gives only the trans-dibromide 2. The reaction, followed spectrophotometrically, obeys a third-order rate law (second-order in Br₂), but the value of k₃ changes considerably depending on the monitoring wavelength. It is shown that this is due to the presence of reversibly formed Br₃^– salt intermediates, that make a more important contribution to the absorbance in the lower wavelength range of the monitored 360–540 nm region. A significant conductivity is also measured during the course of the bromination. Both spectrophotometric and conductimetric measurements are consistent with the presence of Br₃^– salt intermediates at a maximum concentration amounting to ca. 2% of that of the initial reagents. The structure of dibromide 2, determined by X-ray diffraction, shows a considerable angle strain at carbons bearing bromine. This strain is responsible for an easy spontaneous debromination of 2, as well as for a relatively high barrier for the formation of 2 from the bromonium–tribromide intermediate, that makes possible the accumulation of the intermediate itself during the bromination of 1.