T H E
J O U R N A L
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PHYSICAL CHEMISTRY Registered in U.S . Patent Ofice @ Copyright, 1971, by the American Chemical Society ~
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VOLUME 76, NUMBER 5 MARCH 2, 1972
The Vacuum Ultraviolet Photolysis of Cyclohexanone] by Alfred A. Scala* and Daniel G. Ballan Department of Chemistrg, Worcester Polgtechnic Institute, Worcester, Massachusetts 01609
(Received April 82, 1971)
Publicatton costs assisted by the U.8.Atomic Energy Commission
In the vacuum ultraviolet photolysis of cyclohexanone, the major modes of decomposition of the electronically (CHZ)~, C; CO f excited ketone are as follows: CO + CSH10, A; CH2 CO 2CzH4, B; CH2 CO CzH4 + CaHe,D; H CeHgO, E; Hz C6Hs0, F. The quantum yield for reaction A is 0.68 at 147.0 nm and it becomes less important as the energy of the incident light is increased. Ethylene is the most abundant hydrocarbon product at each wavelength, being produced primarily from reaction B and/or reaction C followed by decomposition of the tetramethylene diradical of two molecules of ethylene (4 = 0.2). Reaction D is unimportant and occurs with a maximum quantum yield of 0.03. Experiments with cyclohexanone-a-d4 have shown that the C5HL0 produced in reaction A undergoes secondary decomposition t o CgH4 and C&b. The remainder of the decomposition of cyclohexanone is accounted for by reactions E and F which become more significant as the energy of the incident light is increased. The mechanisms for reactions A-D are best interpreted in terms of diradicals (CH2). where n = 1, 3, 4,and 5. That nonacyl u cleavage is unimportant at 147.0 nm indicates that energy absorption occurs primarily at the carbonyl group. Intramolecular hydrogen atom transfer to yield 5-hexenal was not observed.
+
+
Introduction The near-ultraviolet photochemistry of cyclohexanone has been thoroughly investigated and is characterized by reactions 1-4.2 The quantum yield for reaction 1 is close to 0.8 while the quantum yield
u (3)
* CO
+
CZH4
+ CSH6
(4)
for reactions 3 and 4 combined is about 0.04. The formation of 5-hexenal occurs with a quantum yield of about 0.2. Reaction 2 is an intramolecular hydrogen atom transfer and is less important in cyclohexanone photolysis than the formation of 4-pentenal is in cyclopentanone photolysis. Shortridge and Lee3 have shown that in the benzene sensitization of cyclohexanone reaction 1occurs through singlet sensitization while
+
+
+
+
reaction 2 occurs through triplet sensitization. Freeman4 has also demonstrated that the occurrence of reaction 2 in the liquid phase y-radiolysis of cyclohexanone involves the lowest triplet excited state of cyclohexanone. I n view of the scarcity of information concerning the interaction of complex organic molecules with vacuum ultraviolet radiation and our continuing interest in this subject, we have photolyzed cyclohexanone in the vacuum ultraviolet. The object of this study was to determine the primary processes which occur and to compare the photolytic mechanism in the vacuum ultraviolet with the near-ultraviolet photochemistry of cyclohexanone. (1) This research was supported by the U. S. Atomic Energy Commission [AT (30-1)-39451. (2) For a review see (a) R. Srinivasan, Advan. Photochem., 1, 83 (1963). (b) J. Calvert and J. N. Pitts, Jr., “Photochemistry,” Wiley, New York, N. Y.,1966,p 406. (3) R.G.Shortridge, Jr., and E. K. C. Lee, J . Amer. Chem. Soc., 92, 2228 (1970). (4) A. Singh and G. R. Freeman, J . Phys. Chem., 69,666 (1965).
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616 Experimental Section Photolyses. Microwave powered xenon, krypton, and argon resonance lamps were attached to a 1-1. reaction vessel through a ground glass joint. The lamps which had LiF windows contained 0,5 Torr of gas and were gettered with titanium. No cooling was required in order to maintain a constant output. The outputs of the lamps used in this study as measured on a McPherson l m vacuum monochromator were: Xe, 147.0 nm 99%, 129.5 nm 1%; Kr, 123,6 nm 89%, 116.5 nm 11%; Ar, 106.7 nm 78%) 104.8 nm 11%) 121,5 nm (Lyman a) 11%. The intensity of the lamps was approximately: Xe, 2 X 10l6 quanta/sec; Kr, 5 X l O I 4 quanta/sec; Ar, 5 X l O l a quanta/sec. Conversions were usually kept below 0.1%. Since photolysis of cyclohexanone, I.P. 9.14 eV, at 123.6 nm (10.0 eV) and 106.7-104.8 nm (-11.7 eV) causes ionination of cyclohexanone, saturation current measurements were made using a 500 cm3 cylindrical cell containing two parallel plate stainless steel electrodes (5-cm diameter) separated by a distance of 5 cm. A potential difference was applied using a John Fluke Model 415 B regulated high voltage power supply and the current was measured on a Victoreen Model 1001 micromicroammeter, The current in the empty cell was always less than 1% of that observed during the photolysis of cyclohexanone. Saturation current measurements were usually determined with 2 Torr of cyclohexanone in the cell. At this pressure and at intensities of ca. 5 X 10" quanta/sec, good saturation currents were obtained which were independent of small changes in pressure. Analyses. After irradiation, noncondensables were distilled off at liquid or solid nitrogen temperature and measured in a gas burette. The relative analysis of the noncondensable gases was performed on A.E.I. MS-10 mass spectrometer. The condensables were then injected into an B and 11 Model 810 gas chromatograph equipped with a flame ionization detector and containing a 30 ft squalane on 60-80 mesh Chromosorb P column operated at 25' and a helium flow of 60 cm3/min. The CO:CzH4 ratios for the argon photolysis experiments, where the conversions were too low for accurate analysis using the MS-10, were determined by distilling everything noncondensable at 195°K and obtaining the analysis on a Bell & Howell 21-491 high resolution mass spectrometer. Calculation of the contribution of products other than ethylene to the m / e 28 attributed to ethylene indicate that their contribution would be less than 5%. In special high conversion, high pressure experiments which were performed to determine if 5-hexenal was a product, the photolysis mixture was immediately injected onto either a 9 ft, 10% diisodecyl phthalate column operated at 50" and a helium flow of 30 ema/ min, or a 3 ft, Poropak Q-S column operated at 160" and a helium flow of 40 cm3/min. Each of these colThe Journal of Physical Chemistry, Vol. 76, No. 6 , 1978
A L P R ~A. D SCALAAND DANIEL G. BALLAN umns has the capability of separating 5-hexenal from cyclohexanone. The isotopic composition of the ethylene was determined by injecting the entire photolysis mixture onto a 6-ft silica gel column operated a t 45' and a helium flow of 20 crng/min and condensing the ethylene at the exit of the chromatograph, The composition of the ethylene was then determined using the MSlO mass spectrometer. The cracking patterns used to calculate these results were obtained by running authentic samples of the various deuterated ethylenes on the MS-10. The quantum yield measurements a t 123.6 and 106.7104.8 nm are based upon the determination of the ionization efficiency of cyclohexanone, the saturation current during photolysis and the ethylene yield.5 Nitric oxide was used as a standard in these experiments. Quantum yields a t 147.0 nm are based upon COz actinometry.6 Materials. ltatheson, chromato-quality, cyclohexanone was purified by distillation on a thirty theoretical plate, spinning band column. Only a middle fraction which contained no detectable impurities, as determined by analyses on both a 12 ft, 1501, (wt/wt) squalane on Chromosorb P column, operated at ambient temperature and a helium flow of 60 cm3/min and on a 10 ft, 10% (wt/wt) diisodecyl phthalate on Chromosorb P column operated at 60" and a helium flow of 30 cm3/min, was used. This sample was thoroughly degassed by several trap-to-trap distillations and stored in a darkened vessel on the vacuum line. Cyclohexanone-a-da was prepared by the method of Seibl and G a ~ m a n by , ~ six exchanges of the a-hydrogens in a well stirred mixture of 10% DC1-D3P04 in DzO and cyclohexanone. After several trap-to-trap distillations, the nuclear magnetic resonance spectrum of the ketone showed no absorption due to a-hydrogens, The isotopic purity of the cyclohexanone-a-dr as determined by mass spectrometry indicated that 97-98% exchange had occurred in the a-positions and the composition of the ketone was 91% dh, 7% d3, and 2% dz. Nitric oxide (Matheson) , hydrogen sulfide (Matheson) and ethylene (Thomas A. Edison) were all used without further purification other than trap-to-trap distillation. The small amount of ethane, propane and propylene contained in the ethylene was insignificant in our experiments.
Results The product distributions obtained in the photolysis of cyclohexanone a t 147.0, 123.6 and 106.7-104.8 nm both in the presence and absence of nitric oxide are presented in Table I. The conversion in most experi(6) P. Ausloos and S. G. Lias, Radiat. Res. Rev., 1, 75 (1998). (6) J. Y . Yang and F. M. Servedio, Can. J. Chem., 46, 338 (1968). (7) J. Seibl and T. Gauman, Helv. Chim. Acta, 46, 2857 (1963).
VACUUM ULTRAVIOLET PHOTOLYSIS OF CYCLOHEXANONE Table I : Photolysis of Cyclohexanone" 147.0 nm Additive
7
F
None
100 (0 I91 Ib 29.9 2.3 1,2 52.6(0.48)* 2.9 9-65 2.9 0.25 5.2 5.5 5.4 0.64 21.7 32.4
PreBsure, 2.0 Torr.
---.
c
10% N O
100 (0. 89)b 22.9 1.5 1.1 4 7 . 4 (0.42)' 0.1 6.6 0.25 0.26 5.2 1.2
