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 Δ_fH^°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, Δ_fH^°₂₉₈(CH₂ClO₂)=–(4 ± 11) kJ mol^–1, and for the CHCl₂O₂ radical, Δ_fH^°₂₉₈(CHCl₂O₂)=–(17 ± 7) kJ mol^–1 are also calculated from the previously measured R′+ O₂⇌ R′O₂(R′= CH₂Cl or CHCl₂) equilibriums.