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 Estimating and Minimizing the Rate of a Radioactive Exchange Reaction

Year: 1950 DOI: 10.7907/nysr-5c25
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.