Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
The rate constant for the reaction of the hydroxyl radical with 1,1,1,3,3-pentafluorobutane (HFC-365mfc) has been determined over the temperature range 278-323K using a relative rate technique. The results provide a value of k(OH+CF3CH2CF2CH3)=2.0×10-12exp(-1750±400/T) cm3 molecule-1 s-1 based on k(OH+CH3CCl3)=1.8×10-12 exp (-1550±150/T) cm3 molecule-1 s-1 for the rate constant of the reference reaction. Assuming the major atmospheric removal process is via reaction with OH in the troposphere, the rate constant data from this work gives an estimate of 10.8 years for the tropospheric lifetime of HFC-365mfc. The overall atmospheric lifetime obtained by taking into account a minor contribution from degradation in the stratosphere, is estimated to be 10.2 years. The rate constant for the reaction of Cl atoms with 1,1,1,3,3-pentafluorobutane was also determined at 298±2 K using the relative rate method, k(Cl+CF3CH2CF2CH3)=(1.1±0.3)×10-15 cm3 molecule-1 s-1. The chlorine initiated photooxidation of CF3CH2CF2CH3 was investigated from 273-330 K and as a function of O2 pressure at 1 atmosphere total pressure using Fourier transform infrared spectroscopy. Under all conditions the major carbon-containing products were CF2O and CO2, with smaller amounts of CF3O3CF3. In order to ascertain the relative importance of hydrogen abstraction from the (SINGLE BOND)CH2(SINGLE BOND) and (SINGLE BOND)CH3 groups in CF3CH2CF2CH3, rate constants for the reaction of OH radicals and Cl atoms with the structurally similar compounds CF3CH2CCl2F and CF3CH2CF3 were also determined at 298 K k(OH+CF3CH2CCl2F)=(8±3)×10-16 cm3 molecule-1 s-1; k(OH+CF3CH2CF3)=(3.5±1.5)×10-16 cm3 molecule-1 s-1; k(Cl+CF3CH2CCl2F)=(3.5±1.5)×10-17 cm3 molecule-1 s-1; k(Cl+CF3CH2CF3)〈1×10-17 cm3 molecule-1 s-1. The results indicate that the most probable site for H-atom abstraction from CF3CH2CF2CH3 is the methyl group and that the formation of carbonyl compounds containing more than a single carbon atom will be negligible under atmospheric conditions, carbonyl difluoride and carbon dioxide being the main degradation products. Finally, accurate infrared absorption cross-sections have been measured for CF3CH2CF2CH3, and jointly used with the calculated overall atmospheric lifetime of 10.2 years, in the NCAR chemical-radiative model, to determine the radiative forcing of climate by this CFC alternative. The steady-state Halocarbon Global Warming Potential, relative to CFC-11, is 0.17. The Global Warming Potentials relative to CO2 are found to be 2210, 790, and 250, for integration time-horizons of 20, 100, and 500 years, respectively. © 1997 John Wiley & Sons, Inc.
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