1-chloro-1,1-difluoroethane

Phototransformation in air

Key value for chemical safety assessment

Half-life in air:

4 860 d

Additional information

The rate constant of the reaction with OH radicals and 1-chloro-1,1-difluoroethane has been determined by several authors. The preferred rate constant is 3.3 10^-15cm³/mol.s at 298 K (Atkinson, 1994), this value will be used in the assessment.

Summaries of supporting studies

A statistically relevant correlation between the reaction rate coefficient k_OH, for the OH radical reaction with 161 organic compounds in the gas phase at 300 K and the corresponding vertical ionization energiesEi,vreveals two classes of compounds: aromatics where –log(k_OH/cm.s) ˜3/2Ei,v(eV)-2 and aliphatics where–log(k_OH/cm.s) ˜4/5Ei,v(eV)+3 (Güsten et al. 1984). The prediction of the rate coefficient, k_OH, for the reaction of OH with organic molecules from the above equations has a probability of 90%. Assuming a global diurnal mean of the OH radical concentration of 5·10⁵cm3, the upper limit if the tropospheric half-life of organic compounds and their persistence can be estimated. The rate constant for 1,1,1-difluorochloroethane is 3.95·10^-15cm³/mol.sec based on the negative logarithm of the reaction rate constant: 14.403 and the lowest vertical ionisation energy of 12.50.

1,1,1-Difluorochloroethane was photolyzed at 147 nm in the pressure range of 3.6-20.6 torr. The effects of added NO, H₂S, and CF₄were investigated by Ichimura et al. (1977). The extinction coefficient at 147 nm and 296 °K was determined to be 64 ± 8 atm^-1.cm^-1. The molecule photodecomposes largely bya,ßelimination of HCl to give 1,1 -difluoroethylene (F0.74 ± 0.06). There is no observable elimination of HF, but there is strong evidence for the elimination of the elements of FCl though the relative importance of this process is minor, as are contributions from carbon-carbon and carbon-halogen bond fission. The 1,1-difluoroethylene formed is undoubtedly vibratonally excited and is the source of a pressure-dependent small yield of fluoroacetylene. Over the pressure range studied there is no evidence that the major primary process itself is affected by changes in total pressure as is the case in the 147-nm photolysis of ethyl chloride (Ichimura et al. 1977).

Using a two-dimensional chemical-radiative-transport model of the global atmosphere, the atmospheric lifetimes of 28 hydrohalocarbons (HCFCs and HFCs) have been determined (Naik et al. 2000). The rate constant of reaction with OH at 277K is 1.96E-15 cm³/mol.sec. The atmospheric lifetimes are 14.2 yrs (tOH trop), 20.3 yrs (tOH trop scaled), 160.0 yrs (tstrat) and 18.0 yrs (ttotal).

Tropospheric lifetimes are estimated for 53 one- and twocarbon hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) by Nimitz and Skaggs (1992). Algorithms are developed that are easy to apply and accurate enough for initial screening purposes. The formula presented predicts lifetimes for molecules with atmospheric lifetimes below 30 years with a root mean square error of a factor of 2.4. The calculate lifetime was 22.6 yrs and the estimated lifetime was 13.9 yrs.

Hydrochlorofluorocarbons (HCFCs) may be used as alternatives for the chlorofluorocarbons (CFCs). Lifetimes for the HCFCs are predicted by Prather and Spivakovsky (1990) in two ways: integrating their loss with a global model, and scaling to another compound with a better known lifetime. Both approaches are shown here to yield similar results. Three-dimensional fields of modeled tropospheric OH concentrations are used to calculate lifetimes against destruction by OH for the HCFCs and other hydrogenated halocarbons. The OH fields are taken from a three-dimensional chemical transport model that accurately simulated the global measurements of methyl chloroform (derived lifetime of 5.5 yr). The lifetimes of various hydro-halocarbons are shown to be insensitive to possible spatial variations and seasonal cylses. It is possible to scale the HCFC lifetimes to that of methyl chloroform or methane by using a ratio of the rate coefficients for reaction with OH at an appropriate temperature, about 277 K. The lifetimes are 18.8 yr (trop-OH), 17.8 yr (total) and 19.1 yr (scaled).

The rate constants for the reaction of OH with CH₃CFCl₂(HCFC-141b) over the temperature range 250-297 K were measured with improved detection sensitivity as a function of flash energy and concentration of OH radical precursor (H₂O). The rate constants were reduced at the lower OH concentrations employed, suggesting complications due to secondary reactions. The new values disagree with the low-temperature values reported earlier but agree with the values extrapolated from the higher temperature results (T> 298 K). In contrast, rate constants for CH₃CFCl₂and CH₂FCF₃(HFC-134a), re-measured at the lower flash energy at 270 K, were found to agree with those reported earlier at higher flash energy (Zhang et al. 1992). The rate constant at 270 K is 2.45 ± 0.31·10^-15cm³/mol.sec.

The Montreal Protocol will lead to the eventual phase-out of the production of chlorofluorocarbons (CFCs) and other halogenated organic compounds that are implicated in the depletion of stratospheric ozone (Haymann and Derwent 1997). The rate constant of atmospheric transformation was calculated. k(T) = 1.3·10^-12cm³/mol.sec; k(298K) = 3.1·10^-15cm³/mol.sec, the corresponding lifetime is 22.5 year.

Measurements of the reactivity of hydroxyl radicals with some simple halocarbons are

reported. together with some new measurements of the air concentration of halocarbons in the N- and S hemisphere. The OH reactivities are used to derive life-times of the halocarbons with respect to photooxidation in the troposphere. The rate constant of the atmospheric transformation of CF2Cl2 was =10^-16cm³/mol.sec, the corresponding lifetime is > 330 year.

OH radicals are generated from the photolysis of a precursor which can be H₂O, H₂O₂or HNO₃(Gierczak et al. 1991). The concentration of the substance (i.e. 142b) is put in excess and considered constant during th e experiment. The rate constant can be infered from the rate of disappearance of the OH radical. In this experiment (Gierczak et al) OH is produced by photolysis of H₂O by a Xenon flash lamp in the 165 -185 nm region or using 248 nm laser pulse photolysis of H₂O₂or HNO₃. The temporal observation of the OH concentration is done by induced fluorescence from a pulse laser and follow up of the fluorescence signal from OH with a photomultiplier. Rate constant of the reaction OH is 2.95 ± 0.25·10^-15cm³/mol.s at 298 K