Kinetics of the R+HBr ⇆ RH+Br (CH3CHBr, CHBr2 or CDBr2) equilibrium.: Thermochernistry of the CH3CHBr and CHBr2 radicals

The kinetics of the reaction of the CH(3)CHBr, CHBr(2) or CDBr(2) radicals, R, with HBr have been investigated in a tem perature-control led tubular reactor coupled to a photoionization mass spectrometer. The CH(3)CHBr (or CHBr(2) or CDBr(2)) radical was produced homogeneously in the reactor by a pulsed 248 nm exciplex laser photolysis of CH(3)CHBr(2) (or CHBr(3) or CDBrA 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(3)CHBr + 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](7 + Student's t values, units in cm(3) molecule(-1) s(-1), no error limits for the third reaction): k(CH(3)CHBr + HBr) = (1.7 1.2) x 10-(13) exp[+ (5.1 +/- 1.9) kj mol(-1)/ RT], k(CHBr(2) + HBr) = (2.5 +/- 1.2) x 10(-13) exp[-(4.04 +/- 1.14) kj mol(-1)/RT] and k(CDBr(2) + HBr) = 1.6 x 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(3)CHBr and CHBr(2) radicals and an experimental entropy value at 298 K for the CH(3)CHBr radical were obtained using a second-law method. The result for the entropy value for the CH(3)CHBr radical is 305 9 j l(-' mol-'. The results for the enthalpy of formation values at 298 K are (in kj mol(-1)): 133.4 +/- 3.4 (CH(3)CHBr) and 199.1 +/- 2.7 (CHBr(2)), and for ot-C-H bond dissociation energies of analogous compounds are (in kj mol-1): 415.0 +/- 2.7 (CH(3)CH(2)Br) and 412.6 +/- 2.7 (CH(2)Br(2)), respectively. (c) 2008 Elsevier B.V. All rights reserved.