ROBERTW. CARR, JR.AND G. B. KISTIAKOWSKY
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Reaction o is truly second order in nitric oxide, and its rate constant at 24' is 3.4 atm.-2 hr.+ or 0.56 MW2 sec. -l. Acknowledgment. The author wishes to thank
Professors Gerald Porter and H. S. Johnston for useful discussions concerning their work and for access to their data. He also wishes to thank Mrs. Barbara Peer for assistance with the manuscript.
The Photolysis of Ketene in the Presence of trans-2-Butene
by Robert W. Carr, Jr. and G. B. Kistiakowsky Gibbs ChemW Laboratory, Harvard Univm8ity, Cambr-idge, Masaachuaetts (Received June 68,196.5)
Experiments on the photolysis of ketene in the presence of trans-2-butene at 2800, 3130, 3340, and 3660 8. show that appreciable amounts of triplet methylene are formed in the primary process of ketene photolysis. Of the total methylenes 15% are triplet at 2800 and 3130 8.,the proportion increasing to 30% at 3340 A. and 40% at 3660 A. Triplet methylene is capable of either abstracting a hydrogen atom to form a methyl radical or adding to the double bond of trans-2-butene. Small amounts of added oxygen completely suppress the reactions of the triplet methylene and at 3340 and 3660 A. also reduce the yields of products formed through reactions of singlet methylene.
Introduction The photolysis of ketene has often been used as a method of producing methylene in order to study its reactions.' Although ketene itself has been the subject of Inany investigations2 and its dissociation to yield carbon monoxide and methylene in the primary process is generally agreed upon, not much is known about the excited states involved. It has usually been supposed that methylene is formed in a singlet electronic state, but recently evidence that triplet methylene also results has been r e p ~ r t e d . ~In~ ~the vapor phase photochemistry of many organic carbonyl compounds excited triplet state molecules are formed, as well as excited singlets, and, since ketene is a member of this class of compounds, it might exhibit similar behavior. If triplet ketene were formed and could dissociate, it would be expected to yield triplet methylene since formation of the first triplet state of carbon monoxide would require more energy than available from light absorption. The criterion of stereospecific addition of methylene to the double bond of l,%disubstituted olefins has been
used as evidence that the reactive state of methylene derived from diazomethane is a singlet.6?6 The addition of triplet methylene would presumably occur with spin conservation, forming a biradical. Rotation about the former carbon-carbon double bond could thus occur before any cyclization, and the cyclopropanes formed would be a mixture of cis and trans isomers, Experiments on the photolysis of ketene in the presence ~~
(1) For recent reviews on methylene see (a) J. A. Bell, Progr. Phye. &g. Chem., 2 , 1 (1964); (b) W. B. DeMore and 8. W. Benson, Advan. Photochem., 2 , 219 (1964); (c) E. M. Frey, Progr. Readdon Kinetics, 2 , 131 (1964). (2) For a short review see W. A. Noyes, Jr., and I. Unger, Pure Appl. Chem.,9,461 (1964). (3) (a) J. W. Simons and B. 8. Rabinovitch, J . Phys. Chem., 68, 1322 (1964); (b) F. H.Dorer and B. S. Rabinovitch, ibid., 69, 1964 (1966). (4) The communications of 8. KO,I. Unger, and W. A. Noyes, Jr. [ J . Ana. Chem. SOC., 87, 2297 (1965)l and of H. M. Frey [Chem. C m m u n . (London), 1, 260 (196511 appeared aa this article waa being written.
(6) P. 8. Skell and R. C. Woodworth, J . Am. Chem. Soc., 78, 4496 (1966). (6) D. W.Setser and B. S. Rabinovitch, Can. J. Chem., 40, 1426 (1962).
PHOTOLYSIS O F
KETENEIN THE PRESENCE OF t r a n s - 2 - B ~ ~ ~ ~ ~
of trans-2-butene a t sdciently high pressures that cis-trans isomerization of any initially formed singlet methylene adduct cannot occur should therefore yield information on the reactive state(s) of methylene.
Experimental Section Ketene was prepared through the pyrolysis of Mallinckrodt reagent grade acetic anhydride, using the method described by Jenkins? It was purified by first being pumped on for 5 min. a t - 130" followed by two trap to trap distillations from -78 to -196") the first and last 10% to distil being discarded each time. Gas chromatographic and mass spectrometric analyses showed only a trace (
