The Kinetics of the Vapor Phase Exchange of Radioactive Bromine between Trichlorobromomethane and Bromine

The rate of exchange of radioactive bromine between trichlorobromomethane and bromine in the vapor phase in the temperature range 420−455°K has been determined. The rate is equal to k[Br2]½[CCl3Br], where log10k(moles/liter)−12(sec.)−1=(−33,100(±400))/(4.574T)+12.75(±0.20). The energy of activation for the elementary reaction between bromine atoms and trichlorobromomethane molecules is 10.3 kcal., and the frequency factor is 7.9×1010. It is proposed that the reaction proceeds via the sequence: Br*+BrCCl3→BrBr*+—CCl3, and Cl3C—+Br2*→Cl3CBr*+Br*, rather than by a Walden inversion mechanism. The observation that chloroform is formed when hydrogen bromide, bromine, and trichlorobromomethane are heated is cited as evidence for the presence of —CCl3 radicals and for the reaction Cl3C—+HBr→Cl3CH+Br. The —CCl3 radical is then more stable than the —CH3 radical by 10 kcal. or more. The vapor pressures of liquid trichlorobromomethane are given by the equation: log10p(mm)=(−8240/4.574T)+7.64. The exchange reaction Br2(g)+AgBr*→Br2*(g)+AgBr readily goes to completion and is the basis for a convenient semi-micro method for the preparation of radioactive elementary bromine.