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

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

1 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.