KINETICS AND THERMOCHEMISTRY OF THE SEC-C4H9 + HBR-REVERSIBLE-N-C4H10 + BR EQUILIBRIUM

The kinetics of the sec-C4H9 + HBr reaction was studied in a tubular reactor coupled to a photoionization mass spectrometer. The SeC-C4H9 radicals were produced homogeneously in the reactor by pulsed UV laser photolysis. Radical decays in the presence of different HBr concentrations were monitored in time-resolved experiments. Rate constants were obtained as a function of temperature (299-533 K) and were fit to an Arrhenius expression: 1.7 (±0.6) × 10-12 exp{4.8 (±1.3) kJ mol-1/RT} cm3 molecule-1 s-1. This kinetic information was combined with knowledge of the rate constant of the reverse reaction to obtain both the entropy {343 (±9) J mol-1 K-1} and heat of formation {66.7 (±3.0) kJ mol-1} of the sec-C4H9 radical at 298 K in a second law determination. The secondary C-H bond energy in n-C4H10 derived from this heat of formation is 411 (±3) kJ mol-1. This value is in excellent agreement with the secondary C-H bond energy in C3H8 determined previously both in an investigation of a comparable equilibrium involving HBr {410 (±5) kJ mol-1} and in investigations of dissociation-recombination equilibria involving i-C3H7 {410 (±3) kJ mol-1 }. This study also provides a reconciliation of the formerly disparate sec-C4H9 heats of formation that were derived from studies of different kinds of chemical equilibria. © 1990 American Chemical Society.