Kinetics and Thermochemistry of the CH3, C2H5, and i-C3H7. Study of the Equilibrium R + HBr R-H + Br

Article abstract of DOI:10.1021/ja00218a017

The kinetics of the reactions between CH3, C2H5, and i-C3H7 with HBr were studied in a tubular reactor coupled to a photoionization mass spectrometer.Rate constants were measured as a functio of temperature to determine Arrhenius parameters.The following rate constant expressions were obtained (units of the preexponential factors are cm³ molecule^-1 s^-1 and those of the activation energies are kJ mol^-1; the temperature range covered in each study is also indicated): CH3 + HBr <8.7E-13 exp(1.3/RT), 296-532 K>; C2H5 + HBr <1.0E-12 exp(3.4/RT), 295-532 K>; i-C3H7 + HBr <1.1E-12 exp(4.5/RT), 478-532 K>.These results were combined with determinations of the reverse rate constants to obtain equilibrium constants for the reactions R + HBr <=> R-H + Br.Second-law-based analyses yielded heats of formation and entropies of CH3, C2H5, and i-C3H7 that are in close agreement with recent determinations of heats of formation in prior investigations of dissociation-recombination equilibria and calculations of entropies.The observed negative activation energies for R + HBr reactions (and negative activation energies inferred for R + I2 reactions from the current results) provide the basis for a detailed explanation for the disparities that currently exist between heats of formation of alkyl radicals that have been obtained from studies of bromination and iodination kinetics and those that are derived from kinetic studies of other reactions.A complex mechanism for R + HBr reactions that is consistent with the observed kinetic behavior of these reactions is discussed.