1441
PHOTOCHEMISTRY OF 1,1,3,3-TETRAFLUOROACETONE
Photochemistry of the Fluoro Ketones.
1,1,3,3-Tetrafluoroacetone~
by G. 0. Pritchard and J. T. Bryant Department of Chemistry, University of California, Santa Barbara, California (Received September $8,1966)
In the photolysis of the title ketone, CFzHradicals recombine to give C2F4Hz,and disproportionate to give CFzHz and CFZ, but the elimination of H F is negligible under the conditions used. The activation energy for hydrogen atom abstraction by CF2H radicals from the ketone is 6.9 kcal mole-'. The &O is unity over a 15-fold pressure variation at room temperature and remains close to unity up to 300". This behavior is compared with acetone, trifluoroacetone, and hexafluoroacetone.
Introduction After the primary photochemical step CFzHCOCF2H
+ hv +CFzH + COCFzH
the following reactions are of interest CFzHCO +CFzH CFzH
+ CF2HCOCFzH + CF2Hz
+ CO
+ CHzCOCFzH
+ CFzH +CFzHCFzH CFzH + CFzH +CFzHz + CF2 CFzH + CF2H +CF2=CFH + H F CF2H
(1) (2) (3)
(4) In a preliminary account2 it was shown that reaction 4 was very minor, indicating that the elimination of H F from the vibrationally excited CF2HCF2H*was not significant under the conditions used. In the previous investigation of the photolysis of CFH2COCFH2, the rapid elimination of H F from CFHzCFH2* was o b ~ e r v e d . ~The possible disproportionation reaction between two CFHz radicals appeared to be ~ n i m p o r t a n t . ~However, Bellas, Strausz, and Gunning (BSG)I have observed a disproportionation reaction between two CF2H radicals in the Hg-photosensitized decomposition of CF2HC1, so that the occurrence of reaction 3 in our system must be considered. Experimental Section The apparatus and procedure have been described previ~usly.~A pure sample of the ketone was supplied by E. I. du Pont de Nemours and C O . , and ~ its mass spectrum is recorded in Table I. Ketone pressures
used varied between 1 and 10 cm in the kinetic experiments. After photolysis, CO was separated at -195" and measured and checked on the mass spectrometer. Mass spectrometer analysis of a cut taken at -145" indicated pure CFzH2with a trace of SiF4,for which a correction was made. CzF4Hzwas collected at - 100" ; its mass spectrum is recorded in Table I, and it shows a very large transference peak at m/e 33 (CFH2+). The -145 and -100" fractions were also separated and characterized by vpc on a 0.5-.m 3% squalane on 60-80 mesh alumina column, to ensure that the observed transference peak was not due to CF2Hz in the ethane sample. In neither the -145" nor the -100" fractions was there any mass spectrometric evidence for CFH=CFZ, which has a large parent ion peak at m/e 82, and a base ion peak at m/e 63 (C2F2H+). It was also proved to be absent by vpc characterization. When the system was dosed with C2F3Hprior to photolysis of the ketone, it was identified unambiguously in the product analysis in the -145" fraction, both by mass spectrometry and vpc. In the quantum yield experiments the incident intensity varied between 1.9 and 2.8 X 1013 quanta/cc ~~
~~
(1) This work was supported by a grant from the National Science Foundation. (2) G. 0. Pritchard and J. T. Bryant, J. P h y s . Chem., 69, 1085 (1965). (3) G. 0.Pritchard, M. Venugopalan, and T . F. Graham, (bid., 68, 1786 (1964). (4) M. G. Bellas, 0.P. Strauss, and H. E. Gunning, Can. J. Chem., 43, 1022 (1965). ( 5 ) We are greatly indebted t o Mr. Roy J. Plunkett for arranging thls gift.
Volume 70, Number 6
May 1966
G. 0. PRITCHARD AND J. T. BRYANT
1442
Table I: Mass Spectrum of CzF~H2 and (CFzH)zCO"
m/e
12 13 15 25 26 29 31 33 43 50 51 60 61 63 69 79 82 83 100 101 102 130
--Relative CzFiHz
abundanc(CFzH)zCO
11 14 24 10 12 122 464 25 13 1000 33 34 45 259 53 11
15
Probable positive ion
C+ CH CHa + CzH + CzHz CHO CF + CFHz+ CzF CFz + CF,H + CzFHO + CzFHzO CzFzH CFa + CzFzHO C2FaH + C2F3Hz CzF4+ CzF4H + CzF4Hz+ CaFaHzO +
+
95 90 170
+
+
31 1000 103 22
+
f
15 132 128 14 36 59
+
+
+
a Peaks a t m/e 50. No quantitative kinetic data are presently available on the pyrolysis of alkyl fluorides,21 although the
elimination of H F from CF3CH3* has been observed in the flow pyrolysis of CF3N=NCH3 at 560°.26
Acknowledgments. We are indebted to a referee for some particularly helpful comments, to Drs. 0. P. Strausz and G. Haugen for sending prepublication copies of their manuscripts, and to Dr. J. Heicklen for a helpful discussion. (25) A. H. Dinwoodie and (1965).
R. N. Hasseldine, J . Chem. soc., 2266
Arsenic(1V) as an Intermediate in the Photochemical Oxidation of Ferrous Sulfate in the Presence of Arsenic Acid
by R. Woods Chemistry Department, University of Melbourne, Parkville N . B, Vidorh, Amtralia
(Recdved Odober 4, 1966)
The photochemical oxidation of ferrous sulfate in the presence of arsenic acid yields arsenic(111) in addition to iron(II1). A complex of iron(I1) and arsenic acid is postulated to be the photoactive species leading to arsenic(V) reduction. The stability constant and quantum yields of iron(II1) and arsenic(II1) of this complex were found to be 1.9, 1.8, and 0.9, respectively. The effect of oxygen on the quantum yields gives evidence to show that arsenic(1V) is formed as an intermediate in the photolysis.
The intermediate formation of the 4+ oxidation state of arsenic, produced by the oxidation of arsenic(111) by hydroxyl- or sulfate-free radicals has been postulated for a number of chemically and photochemically induced reactions.'-' No studies are found in the literature on the reduction of arsenic(V) to arsenic(1V).
Experimental Section Materials. Ferrous sulfate was prepared by adding an excess of iron wire (99.9% Fe) to sulfuric acid solution under an atmosphere of nitrogen. The solution was filtered, acidified with sulfuric acid, and stored under nitrogen, The iron(II1) in the ferrous sulfate The Journal of Physieal Chemistry
of the total iron conwas always less than 5 X centration. Arsenic acid was prepared by the prolonged boiling (1) M. Daniels and J. Weiss, J . Chem. soc., 2467 (1958). ( 2 ) M. Daniels, J . Phys. Chem., 66, 1473, 1475 (1962). (3) L. J. Csanyi, Discussions Faraday soc., 2 9 , 146 (1960). (4) R. Woods, I. M. Kolthoff, and E. J. Meehan, J. Am. Chem. soc., 85, 2385 (1963). (5) R. Woods, I. M. Kolthoff, and E. J. Meehan, ibid., 8 5 , 3334 (1963). (6) R. Woods, I. M. Kolthoff, and E. J. Meehan, ibid., 8 6 , 1698 (1964). (7) R. Woods, I. M. Kolthoff, and E. J. Meehan, Inorg. Chem., 4, 697 (1965).
