Abstract
The thermal decompositions of 1,1,1-trifluoroethane (CH3CF3) and pentafluoroethane (CHF2CF3) 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(CH3CF3) = 1012.9±1.2 exp[−(276 ± 29) kJ [sdot] mol−1/RT] and k(CHF2CF3) = 1013.9±1.4 exp [−(324 ± 36) kJ [sdot] 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, CH3CF3 → CH2CF2 + HFand CH2CF2 → CHCF + HF, were favorable for the CH3CF3 pyrolysis. On the other hand, four initial steps: CHF2CF3 → CF3CF + HF, CHF2CF3 → CF2CF2 + HF, CHF2CF3 → CHF3 + CF2 and CHF2CF3 → CHF2 + CF3, were possible for the CHF2CF3 pyrolysis. Such a difference in the reaction paths between the CH3CF3, and CHF2CF3 pyrolyses can be explained by fluorine hyperconjugation and by repulsion between the fluorine atoms on the 1- and 2-carbons of each hydrofluoroethane.
The authors wish to express our gratitude to Professor Frank Scott Howell S. J. for his valuable comments. This work was supported in part by Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (C) No. 16560194.