Hydrogen abstraction reactions from organosilicon compounds. The reactions of fluoromethyl radicals with trichlorosilane. The photolyses of di‐ and tetrafluoroacetone

Abstract

AbstractThe photolyses of 1,3‐difluoro‐ and 1,1,3,3‐tetrafluoroacetone have been reinvestigated as sources of fluoromethyl radicals, and the following rate constant ratios were determined magnified imageθ= 2.303 RT in kcal/mole. The results are in substantial agreement with the original investigations. The photolyses of the fluoroacetones were used as sources of CH2F and CHF2 radicals, and the following Arrhenius parameters were obtained for the hydrogen abstraction reactions R + SiHCl3 → RH + SiCl3: R T(°K) E (kcal/mole) log A (mole−1 cc sec−1) log k (400°K) (mole−1 cc sec−1) CH2F 335–443 6.06 ± 0.15 10.79 ± 0.08 7.48 CHF2 CHF2 334–442 6.82 ± 0.09 11.32 ± 0.05 7.59 The rates of reactions of CH2F and CHF2 radicals toward hydrogen abstraction from SiHCl3 are an order of magnitude lower than the corresponding rates for the CH3, C2H5, and CF3 radicals as a result of increased activation energies for the CH2F and CHF2 reactions. The interpretation of the results is hindered by a lack of accurate thermochemical data on the radicals. The activation energies, as they stand, can be rationalized in terms of a polar repulsion between SiHCl3 and the radicals, increasing regularly and leading to a progressive increase in activation energy with increasing fluorine substitution in the radical. This interpretation is consistent with the estimates D(CH2FH) ∝ D(CHF2H) ∝ 97 kcal/mole. On the other hand, there is some indication, from the spread of A‐factors in the series of fluoromethyl radicals reacting with SiHCl3, that internal compensation of Arrhenius parameters is occurring in some of the reactions; and when this is taken into account, the interpretation of activation energies is more difficult.