Reaction of tertiary aliphatic amines with halogens. Kinetics and products of the reaction of tribenzylamine with molecular bromine and tribromide ion

Abstract

A reinvestigation of the reaction of tribenzylamine with bromine has proved that the crystalline product previously reported to be an amine·Br₂ 1 : 1 adduct of type (C₆H₅CH₂)₃NBr⁺Br^– is actually a mixture of tribenzylammonium and NN-dibenzylbenzylidenaminium tribromides. A kinetic study carried out in 1,2-dichloroethane with the stopped-flow technique has shown the transient formation of a 1 : 1 tribenzylamine·Br₂ charge-transfer complex having a formation constant K_f of 9.7(1.0) I mol^–1 at 25 °C. This is followed by a fast oxidation-reduction reaction leading to NN-dibenzylbenylidenaminium bromide and HBr, which is captured by the amine to give tribenzylammonium bromide. Both bromide salts are instantaneously transformed, by association with the remaining Br₂, into the corresponding tribromides, which are then consumed at a much slower rate by the oxidative cleavage of the amine. Tetrabutylammonium tribromide has been independently shown to react slowly with tribenzylamine through two processes, with respective rate constants at 25 °C k_2Br3^–== 3.1(0.4)× 10^–4 and k_2Br2≥ 8.9 × 10² l mol^–1 s^–1, the former interpretable as a bromide-assisted and the latter as an unassisted elimination of HBr from the 1 : 1 amine·Br₂ charge-transfer complex.