The Kinetics of the Forward and Reverse Reactions for the Vapor Phase Thermal Bromination of Chloroform

Abstract

The rates of the forward and reverse reactions for the homogeneous vapor phase thermal bromination of chloroform, CHCl3+Br2=CBrCl3+HBr(A), have been measured in the range 420–455°K and are given by −d(Br2)/dt=kf(CHCl3)(Br2)12[1+k2(HBr)/k3(Br2)]−1−kr(CBrCl3)(Br2)12[1+k3(Br2)/k2(HBr)]−1; where log10kf (liter/mole)12 sec−1=(−32,030/4.575T)+11.15, log10kr=(−32,930/4.575T)+12.70. The mechanism of the reaction is, M+Br2=2Br+MCl3CH+Br=Cl3C−+HBr→(1)←(2)Cl3C−+Br2=CCl3Br+Br→(3)←(4)where log10k1=(−9300/4.575T)+9.36, log10k4=(−10,200/4.575T)+10.91, (k2/k3)=0.040. The activation energies imply that the CH bond in CHCl3 is weaker than that in CH4 by 6+E2 kcal and the Cl3C–Br bond is weaker than the H3C–Br bond by 11+E2 kcal, where E2, the activation energy of reactions (2) and (3), is estimated to be ≤7 kcal. The directly determined equilibrium constant for (A) agrees well with the kinetic value, Ke=k1k3/k2k 4; log10Ke=(900/4.575T)−0.15. From these data, for CCl3Br, ΔH°f,298°=−9.4 kcal, S°298°=80 eu S° was calculated as 80 eu from electron diffraction and spectroscopic data. The equation for k4 is identical with that obtained in a previous study of the exchange of radioactive bromine between Br2 and CBrCl3, thus proving this free radical mechanism for the exchange reaction.