| Popis: |
In order to assess the environmental impact of possible candidates such as hydrofluoroolefins to replace the CFCs in the industry, it is necessary to perform kinetic and product studies of their degradation in the air initiated by the main tropospheric oxidants. In this sense, the relative-rate technique has been used to determine rate coefficients for the reactions of 2-fluoropropene (CH3CF CH2); 3,3,3-trifluoro-2-(trifluoromethyl)propene ((CF3)2C CH2) and (E/Z)-1,2,3,3,3-pentafluoropropene ((E/Z)-CF3CF CHF) with OH radicals and Cl atoms at (298 ± 3) K and (760 ± 10) Torr total pressure of synthetic air using different reference compounds. The experiments were performed in an environmental chamber with “in situ” FTIR spectroscopy to monitor the concentration–time profiles of the reactants. The following room temperature rate coefficients (in cm3 molecule−1 s−1) were obtained: k1(OH + CH3CF CH2) = (1.55 ± 0.39) × 10−11, k2 (OH + (CF3)2C CH2) = (6.58 ± 2.25) × 10−13, k3 (OH + ((E/Z)-CF3CF CHF); 93% E isomer) = (2.62 ± 0.76) × 10−12, k4 (Cl + CH3CF CH2) = (1.64 ± 0.26) × 10−10, k5 (Cl + (CF3)2C CH2) = (3.50 ± 0.85) × 10−11 and k6 (Cl + ((E/Z)-CF3CF CHF); 93% E isomer) = (4.52 ± 0.98) × 10−11. To the best of our knowledge this work provides the first kinetic study for the reactions of CH3CF CH2 and (CF3)2C CH2 with OH radicals and Cl atoms. Free-energy relationships are presented, and the effect of F atom and CF3 group substitution in the hydrofluoroolefins is discussed in terms of reactivity toward both OH radicals and Cl atoms. On the basis of the kinetic measurements, tropospheric lifetimes of CH3CF CH2, (CF3)2C CH2 and (E/Z)-CF3CF CHF will primarily be determined by their reaction with hydroxyl radicals with estimated lifetimes of approximately 1, 18 and 4 days, respectively. For the reactions of the fluoroalkenes with chlorine atoms the corresponding values of the lifetimes are 14, 66 and 51, respectively. |
Full Text from ScienceDirect