Kinetics of the R + HBr ⇄ RH + Br (CH 3CHBr, CHBr 2 or CDBr 2) equilibrium. Thermochemistry of the CH 3CHBr and CHBr 2 radicals

Abstract

The kinetics of the reaction of the CH 3CHBr, CHBr 2 or CDBr 2 radicals, R, with HBr have been investigated in a temperature-controlled tubular reactor coupled to a photoionization mass spectrometer. The CH 3CHBr (or CHBr 2 or CDBr 2) radical was produced homogeneously in the reactor by a pulsed 248 nm exciplex laser photolysis of CH 3CHBr 2 (or CHBr 3 or CDBr 3). The decay of R was monitored as a function of HBr concentration under pseudo-first-order conditions to determine the rate constants as a function of temperature. The reactions were studied separately from 253 to 344 K (CH 3CHBr + HBr) and from 288 to 477 K (CHBr 2 + HBr) and in these temperature ranges the rate constants determined were fitted to an Arrhenius expression (error limits stated are 1σ + Student’s t values, units in cm 3 molecule −1 s −1, no error limits for the third reaction): k(CH 3CHBr + HBr) = (1.7 ± 1.2) × 10 −13 exp[+ (5.1 ± 1.9) kJ mol −1/ RT], k(CHBr 2 + HBr) = (2.5 ± 1.2) × 10 −13 exp[−(4.04 ± 1.14) kJ mol −1/ RT] and k(CDBr 2 + HBr) = 1.6 × 10 −13 exp(−2.1 kJ mol −1/ RT). The energy barriers of the reverse reactions were taken from the literature. The enthalpy of formation values of the CH 3CHBr and CHBr 2 radicals and an experimental entropy value at 298 K for the CH 3CHBr radical were obtained using a second-law method. The result for the entropy value for the CH 3CHBr radical is 305 ± 9 J K −1 mol −1. The results for the enthalpy of formation values at 298 K are (in kJ mol −1): 133.4 ± 3.4 (CH 3CHBr) and 199.1 ± 2.7 (CHBr 2), and for α-C–H bond dissociation energies of analogous compounds are (in kJ mol −1): 415.0 ± 2.7 (CH 3CH 2Br) and 412.6 ± 2.7 (CH 2Br 2), respectively.