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as calculated from his Table, is, in fact, 4O7,, a figure SO high that it lends no confidence to the proposed structure. (3) Soklakov states that rhombohedral sulfur is obtained by “crystallization from hot solutions of sulfur in benzene and toluene,” perhaps in order to identify it with his own preparations xyhich were made at 80-100”. Rhombohedral sulfur is in fact obtained from ice-cold solutions.5~6 (4) Soklakov’s preparations contain ca. 20 atomic % of arsenic. These can scarcely be termed “sulfur.” (5) It is not clear how Soklakov was able to arrive a t values of Fmeasd for reflections with general values of (hkl), since on powder photographs, as is well known, these are composed of two or more sets of nonequal F’s, and it is therefore impossible to calculate F’s from the observed composite intensities. It is equally unclear why Soklakov gave only one value for Fcalcd for reflections of this type. (6) Soklakov’s observed powder pattern consists of 40 lines, 28 of which he attributed to rhombohedral sulfur on the basis of values of sin2 O ’A2 calculated with the then available lattice constants3; his agreement between (sin2 O/X2)meesd and (sin2 O/A2)calcd is not, however, outstanding, and, moreover, agreement just as good, if not better, is obtained when the observed values of Soklakov are compared with those observed for orthorhoiiibic sulfur,7 and in view of the preparative methods mentioned by Soklakov it would not be surprising if his samples contained a large proportion of orthorhombic sulfur. (This seems as good a place as any to point out that while Soklakov attributed some of the lines in his pattern to monoclinic sulfur, the reference he gave to the powder pattern of that substance* actually referred to orthorhombic sulfur, as will be seen from careful reading of the text of that reference, as opposed to merely reading its Table 11.) (7) Soklakov indexed his powder pattern by the use of the lattice constants obtained from single crystal^,^ which are essentially the same as the lattice constants obtained in the later single crystal study.2 The values of (hkl) assigned by Soklakov to his observed lines bear no reseniblance t o the indices of the reflections observed on the single crystal Weissenberg photographs (in particular, the space group extinction that F = 0 unless h - k I = 3n, a condition suggested by the face development and observed in the single crystal study.2 is consistently violated by the indices assigned by Soklakov). It must be concluded that the sin2 O h 2 agreement which Soklakov obtained is purely fortuitous. ( 8 ) Soklakov’s powder pattern bears no resemblance to one previously reported for rhombohedral sulfur.lo
+
T h e Journal 0.f Physical Chemistry
Soklakov dismisses that pattern and offers as an explanation for the difference between it and his own the possibility that the structure had broken down, and that the scattering of the breakdown products had been recorded by the authors of ref. 10. This “explanation” ignores the fact that the powder pattern of ref. 10 agrees in every respect nith what is expected on the baks of other work on rhonibchedral We find it difficult to escape the conclusion that the evidence presented by Soklakov that rhombohedral sulfur consists of infinite chains should be disregarded. Acknowledgment. This work was supported in part by a grant from the U. S. Army Research Office (Durham). ( 6 ) A. Aten, 2 . physik. Chem., 8 8 , 321 (1914). (6) E. Engel, Compt. rend., 1 1 2 , 866 (1891). (7) H. E. Swanson, 11.I Cook, T. Isaacs, and E. H. Evans, National Bureau Standards (U. S.),Circular 539, T’ol. 9, U. S . Government Printing Office, Washington, D . C., 1960, p. 54.
(8) A. Pinkus, J. Kim, J. McAtee, and C. Concilio, J . Am. Chem. Soc., 81, 2652 (1959). (9) J. D. H. Donnay, Acta Cryst., 8, 245 (1955). (10) A. Caron and J. Donohue, J . P h y s . Chem., 64, 1767 (1960).
The Photochemistry of a-Keto Acids and @-KetoEsters. 111. Photolysis of Pyruvic Acid in the Vapor Phase by George F. Yesley and Peter A. Leermakers Hall Laboratory of Chemistry, Wesleyan Uniaersity, Middletown, Connecticut (Received February 18,196.4)
Recent observations in these laboratories1z2 and an observation of an earlier worker3 concerning the photochemical behavior of pyruvic acid in solution have prompted us to investigate the vapor phase photochemistry of this simplest of the a-keto acids. I n solution, the reaction path is highly dependent on the medium. In hydrogen-donating organic solvents photoreduction to dimethyltartaric acid predominates whereas in aqueous solution pyruvic acid is photodecarboxylated to yield acetoin (the hcad-to-head dimer of acetaldehyde) without any significant fo matioii of acetaldehyde. . Although being activJy studied ~
(1) P. A. Leermakers and G. F. Vesley, J (1963).
Org. Chem., 28, 1160
(2) P. A. Leermakers and G. F. Vesley, J Am. Chem. Soc., 8 5 , 3776 (1963). (3) W , Discherl, 2. physiol. Chem., 219, 177 (1933).
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in these laboratories, the mechanism of the aqueous photodecarbox:ylation is not clear. I n the vapor phase, irradiation of pyruvic acid a t 5 mm. pJessure a t a temperature of 80-85" with light of 3660 A. leads to extremely efficient photodecomposition, quantum yield equal to 1.02 f 0.06 and probably unity. Carbon dioxide is produced in quantitative yield, the other major product being acetaldehyde formed in 60-65% net yield. About 1-2% each or methane and carbon nionoxide are also formed. Figure 1 shows the yields of carbon dioxide and acetaldehyde as a function of time from the photolysis of 0.723 mrnole of pyruvic acid in the absence and presence of 50 min. of nitrogen and oxygen. (Parallel experiments demonstrated that no dark reaction takes place after 2 hr. a t the reaction temperature of 8 5 O . )
0.7
0.6
0.5 c
9 B b 0.4 I
a 4
0.3
0.2
0.1
( 0
20
40
60
Time, mln.
80
100
120
Figure 1. Variation of the yields of carbon dioxide and acetaldehyde from 0.723 inmole of pyruvic acid with time: 0, no added gas; X , 50 mm. of K2added; E, 50 mm. of 0 added.
2
From the data one sees that; the ratio of acetaldehyde to carbon dioxide does not change with time, nor with the addition of added nitrogen and oxygen, arid that the absolute yields (and hence quantum yields) are unaffected by the added gases. Fifty millimeters of ethylene is also ineffective a t quenching the reaction. From the high yield of acetaldehyde and virtually insignificant yields of carbon monoxide and met,hane, and from the lack of an effect of 50 mni. of added oxygen,
we conclude that the primary photochemical process is a concerted decarboxylation from a cyclic transition state, and that acetaldehyde is nct formed via a free radical path. We prop-se the following two possible mechani sins
ii il
CH~-CTC
-col --t
CHaCHO
&Iechaiiism 1 is favored for the following reasons: (a) The configuration of the transition state is probably that of the ground state (for maximum hydrogen bonding), thus little rearrangement need take place in the excited state to get to the transition state. (h) The transition state is a five-membered rather than a four-membered ring. (c) Analogous to the Xorrish type I1 p r o ~ e s shydrogen ,~ is initially becoming bonded to oxygen rather than to carbon. By analogy to phot,oreduction of n-r* states, initial bonding to oxygen is far more likely than to carbon. The initially formed divalent carbon intermediate would, of course, immediately rearrange to acetaldehyde a t the pressure and temperature of the reaction., As perhaps a side issue, the lack of an effect of oxygen also raises the question as to the nature of the excited state responsible for the photodecarboxylation. Calculations show that at 50 mm. pressure of oxygen the mean lifetime between collisions of pyruvic acid molecules with oxygen molecules is 2.5 X 10-9 sec. It has often been assumed that if pressures of oxygen as low as 10-15 mm. have no effect on a photocheniical reaction (such as decomposition or isoiiierization of a ketone or conjugated olefin, etc.), then the triplet state is ruled out as the chemically active excited state. This may be quite valid in cases where chemical reaction is slow compared with collision lifetimes. HOWever, the very high quantum efficiency of the pyruvic acid decomposition (unity) means that chemical reaction must be nearly two powers of ten faster than any de-excitation path. Thus, it is highly reasonable that the lifetime for chemical reaction could be in the order of 10 -9-10-10sec. after the chemically active excited state i s reached. If this is the case, the lack of an effect of 15 or even 50 mm. of oxygen tells us nothing about the multiplicity of the reactive state. (4) R. Norrish and M. Appleyard, J . Chem. Soc., 874 (1934)
Volume 68, ,Vumbe? 8
August, 1964
2366
Porter claims evidence for the fact that intersystem crossing times for aromatic ketones such as benzophenone may be as short a t 10-Io sec.j This is substantially shorter than previous estimates in the order of lo-' to sec.'j Pyruvic acid undergoes exceedingly efficient intersystem crossing at 77°K. (the phosphorescence being even more intense than that of benzophenone2). Thus, if intersystem crossing time is comparable to benzophenone (and if the estimate cited by Porter is correct), intersystem crossing and subsequent decomposition of the triplet state of pyruvic acid could be occurring with combined lifetimes of less than see. Alternatively, decomposition could be taking place from an excited singlet faster than sec. Attempts mere made to carry out runs a t high oxygen pressures (500-700 mm.) where the presence or lack of an oxygen efl'ect would be definitive (with respect to establishing the multiplicity), but the results were not conclusive due to the slow and incomplete vaporization of pyruvic acid.
Experimental Materials. Pyruvic acid was llatheson Coleinan and Bell, 99yc reagent grade. It was distilled prior to use, b.p. 39--42O a t 1 nun. Oxygen and nitrogen were RIatheson high purity gases. Determination o j P m d u c t s and the Effect o j Added Gases. Irradiations were carried out in a 4-1. Pyrex vessel utiliziiig as a light source a Srinivasaii-Griffin (Rayonet) type of reactor. The reactor is cylindrical in design with twelve 8-w. Westinghouse black lamps mounted around the interior s u r f p . The radiation wave length was primarily 3660 A. Photolyses were carried out on 5O-pl. samples of pyruvic acid at 808 5 O , the elevated temperature being necessary to vaporize the keto acid. In the initial product runs pyruvic acid was placed in the reaction vessel and frozen out in a cold finger a t the bottom of the vessel at Dry Ice temperatures. The vessel n-as then evacuated, sealed off, heated to volatilizc the acid, and irradiated for 2 hr., and the products were subsequently frozen out a t liquid nitrogen temperature. Infrared analysis of the condensed products (utilizing a 10-cni. path length gas infrared cell) indicated the presence of only carbon dioxide and acetaldehyde. A similar sample of pyruvic acid was treated identically except that the irradiation was omitted. Xnalysis showed that no dark reaction ensued under the reaction conditions (SCiO, 2 h r ~ j . Quantitative analyses for the products mere obtained by vapor chromatography. Pyruvic acid samples Tvere frozen out, the vessel degassed, and the The Joirrnal of Physzcal Chemzstry
NOTES
system irradiated for varying periods of time (Fig. 1). Approximately 1 1. of helium was then injected into the system through a serum stopper, after which the gases were allowed to mix; 100-ml. samples were then withdrawn utilizing a Yale 100-nil. BD syringe with a blood stopcock between syringe and needle. The samples were injected into a dual column Aerograph gas chromatograph with Carbowax and silica gel colunins in series. The latter column was placed between the two exit ports causing the gas stream to re-enter the instrument and pass through the reference part of the detector. I n this way simultaneous analyses for both COS and acetaldehyde were obtained. Runs in the presence of added gases were carried out identically except that after initial evacuation of the reaction vessel, the desired quantity of gas was bled in. I n a separate experiment a 3.7-ni. silica gel column was employed to obtain the yields of carbon monoxide and methane. I n all cases authentic standards were injected to obtain absolute yields. Q u a n t u m Yield Determznatzon. The apparatus consisted of a standard optical bench, a P E K 500-w. high pressure mercury arc with appropriate power supply, columnatiiig lens, an$ a Corning 7-39 filter to isolate primarily the 3660 A. line. The amount of radiation absorbed by the pyruvic acid vapor was determined by an RCX 935 phototube in conjunction with a galvinometer. The reaction vessel was quartz, had a 30-cm. path length, and was wrapped with heating tape and asbestos to maintain elevated temperatur e (which was measured by a copper-constantan thermocouple). The light source mas calibrated using uranyl oxalate actinometry. Runs were carried out as follows : approximately 10 pl. of pyruvic acid was vaporized in the reaction vessel, heated to 80°, and irradiated for 15 min. The vessel was attached to a vacuum system, and the gases were pumped off. Acetaldehyde was condensed out in a pentane-liquid nitrogen bath at - 132O, and the other gases (mainly COz with traces of CO and CHJ were compressed in a sample cell by use of a mercury diffusion pump in series with a toepler pump.' The gases were analyzed quantitatively by vapor chromatography with a 2-in. silica gel column. A cknowledgment. This work has been supported by the Petroleum Research Fund of the American Chemical Society. Grateful acknowledgment is made to the donoins of said fund. ( 5 ) 4 Beckett and G. Porter, T r a n s Faraday Soc., 5 9 , 2038 (1963) (6) See, for instance, M . Kasha. Radzatwn Res. S u p p l . , 2 , 253 (1960). (7) T h e authors are indebted t o Professor James r\' Pltts for advice 011 the design of the gas analysls 55 stem
