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Hydrogen abstraction reactions from organosilicon compounds. The reactions of fluoromethyl radicals with trichlorosilane. The photolyses of di‐ and tetrafluoroacetone
Author(s) -
Kerr J. A.,
Timlin D. M.
Publication year - 1971
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.550030102
Subject(s) - chemistry , radical , hydrogen atom abstraction , arrhenius equation , trichlorosilane , activation energy , hydrogen , reaction rate constant , photochemistry , computational chemistry , medicinal chemistry , kinetics , organic chemistry , physics , quantum mechanics , silicon
The 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 determinedθ= 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 CH 2 F and CHF 2 radicals, and the following Arrhenius parameters were obtained for the hydrogen abstraction reactions R + SiHCl 3 → RH + SiCl 3 :TextR T (°K) E (kcal/mole) log A (mole −1 cc sec −1 ) log k (400°K) (mole −1 cc sec −1 )CH 2 F 335–443 6.06 ± 0.15 10.79 ± 0.08 7.48 CHF 2CHF 2 334–442 6.82 ± 0.09 11.32 ± 0.05 7.59The rates of reactions of CH 2 F and CHF 2 radicals toward hydrogen abstraction from SiHCl 3 are an order of magnitude lower than the corresponding rates for the CH 3 , C 2 H 5 , and CF 3 radicals as a result of increased activation energies for the CH 2 F and CHF 2 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 SiHCl 3 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 (CH 2 FH) ∝ D (CHF 2 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 SiHCl 3 , 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.