Kinetics of the thermal decomposition of 1,1-difluoroethane in shock waves. A consecutive first-order reaction

The kinetics of the thermal decomposition of 1,1-difluoroethane diluted with argon has been studied in a single-pulse shock tube in the temperature range 1040-1320°K at total reflected shock pressures of about 2300-4000 Torr. It is shown that 1,1-difluoroethane decomposes to vinyl fluoride by molecular elimination of hydrogen fluoride, CH₃CHF₂ →^k1 CH₂CHF + HF, with a first-order rate constant given by, log (k₁, sec^-1) = 13.9 ± 0.3 - (61.9 ± 1.8 kcal)/2.303RT, in reasonable agreement with a previous study in a conventional flow system. At temperatures above 1200°K a consecutive first-order reaction involving the decomposition of vinyl fluoride to acetylene plus hydrogen fluoride becomes important and must be taken into account. The observed activation energy, E_exp, for the initial HF-elimination process is compared with activation energies derived previously from chemical activation studies using the RRK and RRKM theories of unimolecular reactions, and the predicted theoretical values based on (a) the intimate ion-pair transition state model and (b) the electrostatic semiion pair model for four-center elimination reactions. Satisfactory agreement is found only in the case of the RRKM treatment of chemical activation systems.