Abstract
Relative rate experiments using UV photolysis of F~2~ or Cl~2~ have been used to determine rate constant ratios for several hydrofluorocarbon (HFC) reactions with Cl or F atoms and for HFC alkyl radicals with molecular halogens. For mixtures with F~2~ present, dark reactions are, also, observed which are attributed to thermal dissociation of the F~2~ to form F atoms. At 296 K, the rate of reaction (1a) [CF~2~HCH~3~ + F โ CF~2~CH~3~ + HF] relative to (1b) [CF~2~HCH~3~ + F โ CF~2~HCH~2~ + HF] is k~1a~/k~1b~ = 0.73 (ยฑ0.13) and is independent of T (= 262โ348 K). At 296 K, the ratio of reaction (2a) [CF~2~HCH~2~F + F โ products] to that of (k~1a~ + k~1b~) is (k~1a~ + k~1b~)/k~2a~ = 2.7 (ยฑ0.4), and for reaction (2b) [CF~3~CH~3~ + F โ products] (k~1a~ + k~1b~)/k~2b~ = 22 ยฑ 12. The temperature dependence (263โ365 K) of the rate constant of reaction (3) [CF~3~CFH~2~ + Cl โ products] relative to reaction (4) [CF~3~CFClH + Cl โ products] is k~3~/k~4~(ยฑ10%) = 1.55 exp(โ300 K/T). For the alkyl radicals formed from HFC 152a (CF~2~HCH~2~ and CF~2~CH~3~) and from HFC 134a (CF~3~CFH), rate constants for the reactions with F~2~ and Cl~2~ were measured relative to their reactions with O~2~. The rate constant of reaction (5cl) [CF~2~CH~3~ + Cl~2~ โ CF~2~ClCH~3~ + Cl] relative to (5o) [CF~2~CH~3~ + O~2~ โ CF~2~(O~2~)CH~3~] is k~5cl~/k~5o~(ยฑ15%) = 0.3 exp(200 K/T). For reaction (5f) [CF~2~CH~3~ + F~2~ โ CF~3~CH~3~ + F], k~5f~/k~5o~(ยฑ35%) = 0.23. The ratio for reaction (6f) [CF~2~HCH~2~ + F~2~ โ CF~2~HCH~2~F + F] relative to (6o) [CF~2~HCH~2~ + O~2~ โ CF~2~HCH~2~O~2~] is k~6f~/k~6o~(ยฑ40%) = 1.23 exp(โ730 K/T). The rate constant ratio for reaction (8cl) [CF~3~CFH + Cl~2~ โ CF~3~CFClH + Cl] relative to reaction (8o) [CF~3~CFH + O~2~ โ CF~3~CFHO~2~] is k~8cl~/k~8o~(ยฑ18%) = 0.16 exp(โ940 K/T). For reaction (8f) [CF~3~CFH + F~2~ โ CF~3~CF~2~H + F], k~8f~/k~8o~(ยฑ35%) = 0.6 exp(โ860 K/T). ยฉ 1993 John Wiley & Sons, Inc.