Trifluoroacetone, photolysis

The photolysis of trifluoroacetone with light of wavelength 3130 A. has been studied by Sieger and Calvert.48 The products of decomposition were shown to be carbon monoxide, methane, ethane, 1,1,1-trifluoro-ethane, and hexafluoroethane. There was no indication of the presence of carbon tetrafluoride or methyl fluoride. The quantum yield of all products was low at low temperature and it is assumed that the excited molecule of trifluoroacetone has an appreciable lifetime and may be deactivated by collision before decomposition can occur. This contention is supported by the decrease in the quantum yields observed when foreign gases such as carbon dioxide are added, and by the fall in quantum yields with increase in trifluoroacetone concentration. 

In their original paper, Sieger and Calvert proposed the second of these routes as the predominant primary step. This is unlikely to be the case because no trace of products which could arise by the reactions of the trifluoroacetyl radical have been observed in the photolysis of either trifluoro- or hexafluoroacetone and it is assumed that this radical is unstable. In support of this is the evidence that the yield of ethane rises steadily with increase in temperature whereas that of hexafluoroethane remains approximately constant. Finally, in a recent re-investigation, Dawidowicz and Patrick49 have identified biacetyl in the products of the photolysis of trifluoroacetone. The most probable primary step is, therefore, the former, which provided the acetyl radical and a trifluoro-methyl radical. 

There are two other compounds which appear in the complex mixture of photolysis products and whose presence have yet to be explained. The first of these is the dimer of trifluoroacetone and the second is 2,2-difluoropropane. The latter may be formed by the addition of methyl radicals to CF2 which may be formed by the molecular decomposition of the ketone or by the disproportionation of trifluoromethyl radicals. 

Sieger and Calvert reported the photolysis products of 1,1,1-trifluoroacetone at A 3130 A to be carbon monoxide, methane, ethane, 1,1,1-trifluoroethane, and hexafluoroethane. A low quantum yield for decomposition near room temperature may be explained in terms of the excited trifluoroacetone having an appreciable lifetime and therefore suffering possible collisional deactivation before decomposition can occur. Two possible primary stepts of Type 1 have been proposed... 

Sieger and Calvert originally favoured the latter of these as the predominant primary step, but the more recent study of the photolysis by Dawidowicz and Patrick indicates the former as the predominant step, in view of the identification of biacetyl among the products. Some complications in the mechanism are evident from the presence also of acetone, 2,2-difluoropropane, and a dimer of trifluoroacetone in the reaction products. The possibility of a chain reaction at high temperature has been discussed ... 

The primary act for the photochemical decomposition of trifluoroacetophenone (ref. 589) at A 3130 A is believed to be predominantly of Type 1, with the formation of benzoyl and trifluoromethyl radicals. On the other hand, a primary act of Type 2 has been proposed by Pritchard and Thommarson for the photolysis of hepta-fluoropropyl ethyl ketone. Studies on the photolysis of these and other fluorinated ketones showed many features in common with that of trifluoroacetone, but have also led to some interesting observations on radical-radical interactions. For example, Pritchard et identified the following reactions (leading to the formation of vinyl fluoride) during the photolysis of 1,3-difluoroacetone... 

Sieger, R.A. (1954), The vapor phase photolysis of trifluoroacetone at wavelength 3130 A, Master of Science Thesis, The Ohio State University, Columbus, Ohio, 43210. 

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