THERMAL DECOMPOSITONS OF 1,1,1-TRIFLUOROETHANE AND PENTAFLUOROETHANE IN A TURBULENT FLOW REACTOR

The thermal decompositions of 1,1,1-trifluoroethane (CH 3 CF 3 ) and pentafluoroethane (CHF 2 CF 3 ) were studied by using a turbulent flow reactor at atmospheric pressure over the temperature ranges of 1213-1333 K and 1273-1373 K. The rate coefficients for these thermal decompositions were determined from the first-order decays of the reactants to be k(CH 3 CF 3 ) = 10 12.9±1.2 exp[−(276 ± 29) kJ ⋅ mol −1 /RT] and k(CHF 2 CF 3 ) = 10 13.9±1.4 exp [−(324 ± 36) kJ ⋅ mol −1 /RT]s −1 . To examine the reaction paths, we identified the decomposition products using gas chromatography-mass spectrometry (GC-MS) and performed ab initio MO calculations. These results showed that the sequential HF elimination reactions, CH 3 CF 3 → CH 2 CF 2 + HFand CH 2 CF 2 → CHCF + HF, were favorable for the CH 3 CF 3 pyrolysis. On the other hand, four initial steps: CHF 2 CF 3 → CF 3 CF + HF, CHF 2 CF 3 → CF 2 CF 2 + HF, CHF 2 CF 3 → CHF 3 + CF 2 and CHF 2 CF 3 → CHF 2 + CF 3 , were possible for the CHF 2 CF 3 pyrolysis. Such a difference in the reaction paths between the CH 3 CF 3 , and CHF 2 CF 3 pyrolyses can be explained by fluorine hyperconjugation and by repulsion between the fluorine atoms on the 1- and 2-carbons of each hydrofluoroethane.