On the basis of extensive quantum chemical calculations for 1,2 elimination of H X from CzHsX (X = F, C1, Br, and I) we have obtained a strongly asymmetric transition state that differs greatly from previous models for this reaction. The effect of a-chloro/fluoro substitution was also investigated, specifically for HF and H C l elimination. W e confirm the electron donor/acceptor property of these substituents but find the conventional explanation of their effect on the activation energy to be unsatisfactory.

The photolysis of pentafluoroacetone has been investigated in the 3130 A region, from room temperature to 360OC. The Qc0 varies from 0.7 to 0.9 over this range, and the decomposition is represented by CF,HCOCF, 4 CF,H + CO + CF,. The disproportionation/combination ratio for CF, and CF,H ( + CF,H + CF,) radicals is found to be 0.09. Arrhenius parameters for hydrogen atom abstraction from the ketone are log,, A = 12.7 (units are mole-l cc sec-1) and E = 14.3 kcal mole-' for CF,H, and log,, A = 12.1 and E =

In the photolysis of 1,3-difluoroacetone, CFH2 radicals recombine to form C2F2H4 and disproportionate to give vinyl fluoride and hydrogen fluoride. The ratio of the rates of these two reactions is a function of both the pressure and the temperature. The activation energy for H abstraction by CFH2 radicals from the ketone is 8.0 kcal. mole-1.

The data on ZIÍczFíHz/RczFiHi with added Nz at 391°K may well be better represented by a small positive intercept at ~0.05, but it is apparent that a large number of runs would have to be performed (see for instance ref 22) to permit a clear-cut determination as to the existence of an intercept, or not.(32) Similarly, so did plots of ñcia/ñczFsHs and Rtram/RciFtSi, although any difference in behavior of these two functions would certainly not be anticipated.(33) The data in Figure 7 only show the ratio of the products, but the absolute amount of elimination products is, of course, reduced with increasing pressure.

The chemical activation data for three-and four-centered hydrogen fluoride elimination from CHtFCDFZ have been analyzed to assign the energy released to the olefin fragment in the three-centered process and to estimate the threshold energies for elimination channels. Based upon the cis-trans isomerization rates of CHF=CHF, 78% of the total available energy was released to the olefin fragment for the cra channel. The analysis suggests the existence of an appreciable barrier (-10 kcal/mole) for the reverse reaction, addition of the CHZFCF carbene to DF. The threshold energies for aa, a@, and @a elimination from 1, I ,2-trifluoroethane-1 -dl were assigned as 71, 68, and 68 kcal/mole, respectively. Analysis of the chemical activation data for 1,1,2,2,-tetrafluoroethane, without distinguishing between the three-and four-centered elimination channels, suggests a threshold energy of -75 kcai/mole.