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