Kinetics and thermochemistry of the C₃H₅+HBr^⇌C₃H₆+Br equilibrium

Abstract

The kinetics of the reaction of the allyl radical with HBr has been investigated in a heatable tubular reactor coupled to a photoionization mass spectrometer. The allyl radical, R, was produced homogeneously in the reactor by a pulsed 248 nm exciplex laser photolysis of allyl bromide. 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 reaction was studied at temperatures from 400 to 523 K and in this temperature range the rate constants determined were fitted to an Arrhenius expression (error limits stated as 1σ+Student's t values, units in cm³ molecule^-1 s^-1): k=(4.6±3.2)×10^-13 exp[-(12.6±4.1) kJ mol^-1/RT]. The kinetic information was combined with the kinetics of the Br+propene reaction taken from the literature to calculate thermodynamic values for the allyl radical at 298 K using a second-law procedure. The entropy value is 248±15 J K^-1 mol^-1 and the enthalpy of formation is 166.1±4.3 kJ mol^-1 in good agreement with previous measurements. The allylic C–H bond energy of propene derived from the enthalpy of reaction value is 363.9±4.3 kJ mol^-1, which leads to a value of 59.4±4.9 kJ mol^-1 for the resonance stabilization energy of the allyl radical. The resonance stabilization energy calculated by abinitio methods is 59.9 kJ mol^-1.

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