Hydrogen abstraction from organosilicon compounds. The reactions of fluoromethyl radicals with tetramethylsilane. Polar effects in gas phase reactions

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Abstract
The photolysis of 1,1,3,3‐tetrafluoroacetone has been reinvestigated as a source of CHF2 radicals at temperatures up to 578°K, and the following rate constant ratio was determined for the reactions magnified image θ= 2.303 RT in kcal/mole. 1,1‐Difluoro‐ and 1,1,3,3‐tetrafluoroacetone were photolyzed in the presence of tetramethylsilane, and Arrhenius parameters were measured for the hydrogen abstraction reactions: R + Me4Si → RH + Me3SiCH2 R T(°K) E (kcal/mole) log A (mole−1cc sec−1) log k (500°K) (mole−1cc sec−1) CH2F 473–586 12.00 ± 0.30 11.68 ± 0.12 6.44 CHF2 416–526 10.18 ± 0.33 11.65 ± 0.15 7.21 By comparing with previous data on the CH3 and CF3 reactions, the activation energies were interpreted in terms of the enthalpy changes for the reactions and a polar effect operative between the attacking fluoromethyl radicals and the substrate which tends to reduce the activation energy progressively as the fluorine content of the radical increases. This polar effect, which decreases the activation energy for hydrogen abstraction from SiMe4 along the series CH3, CH2F, CHF2, and CF3, is in marked contrast to the polar repulsion between the fluorinated radicals and SiHCl3 which was tentatively proposed to explain the increase in activation energy observed in the same series of reactions with SiHCl3.

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