Kinetics and thermochemistry of the R+HBr⇄RH+Br (R=n-C3H7, isoC3H7, n-C4H9, isoC4H9, sec-C4H9 or tert-C4H9) equilibrium

Abstract

The kinetics of the reactions of n-C ₃ H ₇ , isoC ₃ H ₇ , n-C ₄ H ₉ , isoC ₄ H ₉ , sec-C ₄ H ₉ and tert-C ₄ H ₉ radicals, R, with HBr have been investigated in a heatable tubular reactor coupled to a photoionization mass spectrometer. The reactions were studied by a time-resolved technique under pseudo-first-order conditions, where the rate constants of R+HBr reactions were obtained by monitoring the decay of the radical as a function of time. The radical was photogenerated in situ in the flow reactor by pulsed 248 nm exciplex laser radiation. All six reactions were studied separately over a wide range of temperatures and, in these temperature ranges, the rate constants determined were fitted to an Arrhenius expression (error limits stated are 1σ+Students t values, units cm ³ molecule ^-1 s ^-1 ): k(n-C ₃ H ₇ )=(1.6±0.2) ×10 ^-12 exp[+(5.4±0.2) kJ mol ^-1 /RT], k(isoC ₃ H ₇ )=(1.4±0.2)×10 ^-12 exp[+(6.9±0.2) kJ mol ^-1 /RT], k(n-C ₄ H ₉ )=(1.3±0.2) ×10 ^-12 exp[+(6.4±0.4) kJ mol ^-1 /RT], k(isoC ₄ H ₉ )=(1.4±0.2) ×10 ^-12 exp[+(6.1±0.2) kJ mol ^-1 /RT], k(sec-C ₄ H ₉ )=(1.4±0.3) ×10 ^-12 exp[+(7.5±0.3) kJ mol ^-1 /RT] and k(tert-C ₄ H ₉ )=(1.2±0.3 )×10 ^-12 exp[+(8.3±0.3) kJ mol ^-1 /RT]. The kinetic information was combined with the kinetics of the Br+RH reactions to calculate the entropy and the heat of formation values for the radicals studied. The thermodynamic values were obtained at 298 K using a second-law procedure. The entropy values and enthalpies of formation are (entropy in J K ^-1 mol ^-1 and enthalpy in kJ mol ^-1 ): 284±5, 100.8±2.1 (n-C ₃ H ₇ ); 281±5, 86.6±2.0 (isoC ₃ H ₇ ); 329±5, 80.9±2.2 (n-C ₄ H ₉ ); 316±5, 72.7±2.2 (isoC ₄ H ₉ ); 330±5, 66.7±2.1 (sec-C ₄ H ₉ ) and 315±4, 51.8±1.3 (tert-C ₄ H ₉ ). The C–H bond strength of analogous saturated hydrocarbons derived from the enthalpy of reaction values are (in kJ mol ^-1 ): 423.3±2.1 (primary C–H bond in propane), 409.1±2.0 (secondary C–H bond in propane), 425.4±2.1 (primary C–H bond in n-butane), 425.2±2.1 (primary C–H bond in isobutane), 411.2±2.0 (secondary C–H bond in n-butane) and 404.3±1.3 (tertiary C–H bond in isobutane). The enthalpy of formation values are used in group additivity calculations to estimate Δ _f H ₂₉₈ ^° values of six pentyl and four hexyl free radical isomers.