Part I. Kinetics of the Vapor Phase Exchange of Radioactive Bromine Between Trichlorobromomethane and Bromine. Part II. Kinetics of the Forward and Reverse Reactions of the Vapor Phase Thermal Bromination of Chloroform. Part III. A Method of Estimati

The rate of the homogeneous vapor phase thermal radioactive exchange reaction CBrCl3 + Br2* → exchange and the rates of the forward and reverse reactions of the homogeneous vapor phase thermal bromination of chloroform CHCl3 + Br2 ⇄ CBrCl3 + HBr (A) have been measured in the temperature range 420-455° K. For the bromination reaction a mechanism which is consistent with all the experimental data is Br2 ⇄ 2Br (d) (5) Br + CHCl3 ⇄ -CCl3 + HBr (1) (2) -CCl3 + Br2 ⇄ CBrCl3 + Br. (3) (4) The rate determining steps are (1) and (4). The rate of the exchange reaction is given by k0(CBrCl3)(Br) and the equation for the rate determining constant k0 = k0(T) is identical to that for k4 as a function of temperature. The exchange reaction takes place, therefore, by the reaction sequence (d), (4), and (3). The activation energies of (1) and (4) imply that the CH bond in CHCl3 is weaker than that in CH4 by at least 6.6 kcal, and the CBr bond in CBrCl3 is weaker than that in CH3Br by at least 11 kcal. The equilibrium constant K = (CBrCl3)(HBr)/ (CHCl3)(Br2) is 1.96 at 442° K; the heat of reaction (A) is -0.9(±0.65) kcal. In a single exploratory experiment, it was observed that the exchange reaction between bromine and trifluorobromomethane is much slower than that between bromine and trichlorobromomethane. A method of estimating and minimizing the error of measurement of the rate of a radioactive exchange reaction is given.