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DOI: 10.1039/A608224F (Paper) Reference Section for: J. Chem. Soc., Faraday Trans., 1997, 93, 1709-1719

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

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Jorma A. Seetula and Irene R. Slagle

Abstract

The kinetics of the reactions of n-C3H7, isoC3H7, n-C4H9, isoC4H9, sec-C4H9 and tert-C4H9 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 cm3 molecule-1 s-1): k(n-C3H7)=(1.6±0.2) ×10-12 exp[+(5.4±0.2) kJ mol-1/RT], k(isoC3H7)=(1.4±0.2)×10 -12 exp[+(6.9±0.2) kJ mol-1/RT], k(n-C4H9)=(1.3±0.2) ×10-12 exp[+(6.4±0.4) kJ mol-1/RT], k(isoC4H9)=(1.4±0.2) ×10-12 exp[+(6.1±0.2) kJ mol-1/RT], k(sec-C4H9)=(1.4±0.3) ×10-12 exp[+(7.5±0.3) kJ mol-1/RT] and k(tert-C4H9)=(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-C3H7); 281±5, 86.6±2.0 (isoC3H7); 329±5, 80.9±2.2 (n-C4H9); 316±5, 72.7±2.2 (isoC4H9); 330±5, 66.7±2.1 (sec-C4H9) and 315±4, 51.8±1.3 (tert-C4H9). 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 ΔfH298° values of six pentyl and four hexyl free radical isomers.

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