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

Abstract

The photolysis of 1,1,3,3-tetrafluoroacetone has been reinvestigated as a source of CHFz radicals at temperatures up to 578'K, and the following rate constant ratio was determined for the reactions (8) (7)2CHF2 -+ CHFzCHFz* CHFz + CHFzCOCHFz -+ CHzFz + CF~COCHFZ log (k~/k~1'2)(mole-1'zcc1'2sec-1'2) = (4.73 f 0.18) -(11.26 f 0.40)/8* 1,l-Difluoro-and 1,1,3,3-tetrafluoroacetone were photolyzed in the presence of tetramethylsilane, and Arrhenius parameters were measured for the hydrogen abstraction re- By comparing with previous data on the CH3 and CF, 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 SiMed along the series CH3, CHzF, CHFs, and CF3, is in marked contrast to the polar repulsion between the fluorinated radicals and SiHCls which was tentatively proposed to explain the increase in activation energy observed in the same series of reactions with SiHC13.* 8 = 2.303 RT in kcal/rnole. 69

Nucleophilic character of alkyl radicals. 18. Absolute rate constants for the addition of primary alkyl radicals to conjugated olefins and 1,4-benzoquinone

“…The oxidation products 25 and 26 are the lactones 25a and 26a for acrylic acid, 25b and 26b for methacrylic acid, the cyano esters 25c and 26c for acrylonitrile, a mixture of 25a and 26a and the acetic esters 25d and 26d for methyl acrylate, and the 7-hydroxy ketones 25e and 26e for methyl vinyl ketone. In all cases compounds 5 and 6…”

“…Internal consistency of the data was checked by eq 22. The results are summarized in Table I at 25 and 69 °C with the Arrhenius parameters deduced for acrylic acid, methyl acrylate, and acrylonitrile (Figure 6). These values differ somewhat in some cases from the ones previously reported1 and, in our opinion, are more exact because the kinetic model has less experimental error.…”

Correlation of activation energies of radical–molecule metathesis reactions with enthalpic, polar, and steric parameters

“…Under such conditions a linear relation between reaction enthalpy and activation energy is not expected. This sort of problem most likely manifests itself in H atom abstraction from silanes by fluoromethyl radicals which was studied by Kerr and Timlin [17]. These authors concluded that the E H values derived reflect contributions from polar and thermochemical factors.…”