Kinetics and thermochemistry of the R + HBr ? RH + Br (R = CH2Cl, CHCl2, CH3CHCl or CH3CCl2) equilibrium

Abstract

The kinetics of the reactions of CH2Cl, CHCl2, CH3CHCl and CH3CCl2 with HBr have been investigated in a heatable tubular reactor coupled to a photoionization mass spectrometer. The radicals, R, were produced homogeneously in the reactor by pulsed 248 nm exciplex laser photolysis. 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 and pressure range. The reactions were studied separately over a wide temperature range and at these temperature ranges the rate constants determined were fitted to an Arrhenius expression (error limits stated are 1σ+ Student's t values, units in cm3 molecule–1 s–1): k(CH2Cl)=(6.6 ± 1.7)× 10–13 exp[–(4.1 ± 0.2) kJ mol–1/RT], k(CHCl2)=(4.1 ± 1.0)× 10–13 exp[–(9.8 ± 1.0) kJ mol–1/RT], k(CH3CHCl)=(3.0 ± 0.9)× 10–13 exp[+(3.0 ± 0.2) kJ mol–1/RT] and k(CH3CCl2)=(4.4 ± 0.9)× 10–13 exp[–(5.9 ± 0.7) kJ mol–1/RT]. The kinetic information obtained was combined with the what is known of the recently measured rate constants of the reverse reactions to calculate the entropy and the heat of formation values of the radicals studied. The thermodynamic values were obtained at 298 K using a second law procedure. The results for entropy values are as follows (units in J K–1 mol–1): 271 ± 7 (CH2Cl), 280 ± 7 (CHCl2), 279 ± 6 (CH3CHCl) and 288 ± 5 (CH3CCl2). The results for ΔfH°298 are as follows (units in kJ mol–1): 117.3 ± 3.1 (CH2Cl), 89.0 ± 3.0 (CHCl2), 76.5 ± 1.6 (CH3CHCl) and 42.5 ± 1.7 (CH3CCl2). The C—H bond energy of analogous chlorinated hydrocarbons derived from the enthalpy of reaction values are as follows (units in kJ mol–1): 419.0 ± 2.3 (CH3Cl), 402.5 ± 2.7 (CH2Cl2), 406.6 ± 1.5 (α-C—H bond in CH3CH2Cl) and 390.6 ± 1.5 (α-C—H bond in CH3CHCl2). Improved heats of formation for the CH2ClO2 radical, ΔfH°298(CH2ClO2)=–(4 ± 11) kJ mol–1, and for the CHCl2O2 radical, ΔfH°298(CHCl2O2)=–(17 ± 7) kJ mol–1 are also calculated from the previously measured R′+ O2⇌ R′O2(R′= CH2Cl or CHCl2) equilibriums.