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; wherelog10kf (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 CHCl3is weaker than that in CH4by 6+E2kcal and the Cl3C&sngbnd;Br bond is weaker than the H3C&sngbnd;Br bond by 11+E2kcal, whereE2, 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/k2k4; log10Ke=(900/4.575T)−0.15. From these data, for CCl3Br, &Dgr;H°f,298°=−9.4 kcal,S°298°=80 euS° was calculated as 80 eu from electron diffraction and spectroscopic data.The equation fork4is identical with that obtained in a previous study of the exchange of radioactive bromine between Br2and CBrCl3, thus proving this free radical mechanism for the exchange reaction.
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