Rate constants and C[sbnd]C bond scission ratios for hydrolysis of 2,2,3-trifluoro-3-(trifluoromethyl)oxirane determined by means of a closed-circulation reactor

Abstract

The hydrolysis rate constant of 2,2,2-trifluoro-3-(trifluoromethyl)oxirane (hexafluoropropene oxide; HFPO), a versatile precursor of fluorinated chemicals, was determined at 279–307 K, and the rate of hydrolysis was used to estimate the tropospheric lifetime of HFPO with respect to hydrolysis in clouds or uptake by the ocean. The low solubility of HFPO in water made it difficult to determine the hydrolysis rate constant because of mass-transfer limitation between the gas and liquid. A closed-circulation reactor was used to measure the rate of decrease of the partial pressure of HFPO while an HFPO-air mixture flowed over a stirred test solution under various experimental conditions. The rate of hydrolysis increased as the OH− concentration increased in an aqueous NaOH solution but was almost independent of the H2SO4 concentration in aqueous H2SO4 solutions. Much scissioning of CC bonds in HFPO produced carbon monoxide and trifluoroacetate in aqueous NaOH, but similar scissioning did not in water or aqueous H2SO4. The first-order rate constant for the pH-independent hydrolysis (kwater in s−1), the bimolecular rate constant for the hydroxide-catalyzed hydrolysis, and the temperature dependence of these parameters were estimated by simultaneously fitting equations based on a two-film model to the time series of HFPO partial pressures under different experimental conditions. The equations included the rate constants as common parameters. The product of kwater and the Henry’s law constant, KH (M Pa−1), at a temperature of T (K) was determined to be kwater × KH = 3.7 × 10−11 exp[−3300 × (T−1 − 1/298.2)]. The tropospheric lifetime of HFPO estimated using this equation indicates that removal of HFPO via hydrolysis in clouds is probably not a substantial sink of HFPO and suggests that, in the absence of other atmospheric sinks of HFPO, hydrolysis of HFPO in the ocean would be the major sink of HFPO.