The rate constants for the unimolecular elimination of HF from chemically-activated CH2FCH2F, produced in the photolysis of 1,3-difluoroacetone alone and in the presence of silanes, have been interpreted in terms of the RRKM theory of unimolecular reactions, and a critical energy of 63 kcal mol-'* obtained for the reaction. The rate constants are compared with previous data, and the dependence of the results on the thermochemistry of the system, and on the transition state model adopted are discussed.* 1 kcal mol-l = 4.184 kJ mol-'.

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The kinetics of the thermal elimination of H F from 1,2-difluoroethane CHzFCHzF -+ CHz=CHF + H F have been studied in a static system over the temperature range 734-820'K. was shown to be first order and homogeneous, with a rate constant of The reaction log kz(s-l) = 13.39 f 0.2 -(62.9 f 0.9)/0 where e = 2.303RT in kcal/mole. The A-factor falls within the normal range for such reactions and is in line with transition state theory; the activation energy is similarly consistent with an estimate based on data for the analogous reactions of ethyl fluoride and other alkyl halides. The above activation energy has beenc ompared with values of the critical energy calculated from data on the decomposition of chemically activated 1,2difluoroethane by the RRKM theory and the bond dissociation energy, D(CHZF-CH2F) = 88 f 2 kcal/mole, derived. It follows from thermochemistry that AHrO(CH2F) = -7.8 and D(CH2F-H) = 101 f 2 kcal/mole. Bond dissociation energies in fluoromethanes and fluoroethanes are discussed.

The photolyses of 1,3-difluoro-and 1,1,3,3-tetrafluoroacetone have been reinvestigated as sources of fluoromethyl radicals, and the following rate constant ratios were determined:(1) (2) 2CHzF + CHZFCH*F* CHzF + CHzFCOCHzF -+ CHsF + CHFCOCHzF log [kl/k21~2](mole-'~2cc1~2sec-'~2) = (3.6 f 0.1) -[(7.6 + 0.2)/0]$

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