Rates of H+CH3X reactions

Abstract

Rate constants for the reaction H + CH₃X → CH₃ + HX (X=Br, Cl, F) have been measured over a wide temperature range using a discharge flow reactor, with pseudo‐first‐order decays of CH₃X measured mass spectrometrically and excess H concentrations measured by ESR. Both absolute data (∼±20%) and rate constant ratios (∼±10%) were obtained. k (CH₃Br) was measured over 298–996 °K, k (CH₃Cl) over 510–998 °K,,and k (CH₃F) over 605–871 °K. Arrhenius expressions (k in cm³ mole⁻¹⋅sec⁻¹, E in cal/mole) fitted k (CH₃Br) = 2.8×10¹³ exp(−5300/RT) over 300–800 °K, k (CH₃Cl) = 3.7×10¹³ exp(−9300/RT) over 500–800 °K, and k (CH₃F) = 1.8×10¹³ exp(−9400/RT) over 600–900 °K. The ratio k (CH₃Br)/k (CH₃Cl) obeyed 0.73 exp(4000/RT) over 600–1000 °K, the data overlapping recent point source flame data of Hart et al. with excellent agreement. The combined results from this work and from Hart et al. on k (CH₃Cl)/k (CH₃F) fit 1.7 exp(300/RT) over 600–1100 °K.