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.

Extension of Structure–Reactivity Correlations for the Hydrogen Abstraction Reaction by Bromine Atom and Comparison to Chlorine Atom and Hydroxyl Radical

“…The database of experimental gas-phase rate constants k Br is less extensive and less internally consistent than those for k Cl or k OH , nor do we have the advantage of a compilation of evaluated and recommended values as we had for k Cl and k OH . In addition, because of the lower reactivity of Br•, Arrhenius parameters have typically been obtained at higher temperature and therefore require longer, and questionable, extrapolations to 298 K. A compilation is given in Table . − All formulas have the carbon from which hydrogen abstraction occurs at the beginning followed by the substituents attached, that is, CH 3 X, CH 2 XY, or CHXYZ. For compounds with multiple potential abstraction sites, the experimental rate constant is typically assumed to be dominantly that of the most reactive site, a relatively safe assumption (see below) given the known high selectivity of Br•, compared with, for example, Cl•.…”

Ideal gas thermodynamic properties of CH4−(a+b+c+d)FaClbBrcId Halomethanes

“…Other references on infrared and Raman work are [62-6lS]. Sullivan and Davidson [66] studied' the thermal bromination of chloroform (4) OHOls (g}+Br2 (g)~OCIsBr (g)+HBr (g) (4) and reported ilHr o (4,298)=-1.4 kcal mol-I. Benson [67] using experimental data of Sullivan and Davidson and more accurate values of O~ and So of species involved in reaction (4) obtained ilHr o (4,208)=~0.80±O.13 kenl mol-I.…”

The kinetics of the thermal bromination of CF3I. Determination of the bond dissociation energy D(CF3−I)

“…1 2 could also be formed from IBr via reactions (-1 5) and ( 16) , but we have already seen that, in the presence of Brz, IBr will yield negligible I2 (see Table I). Hence we conclude that in the early stages Brz and CF3I react only as represented in overall reaction (5) to give IBr and CF3Br.…”