534
F. E. BLACET
(3) HOIST.G . : %. physik. C'heni. A179, 172 (1937); A182,321 (1938). (4) HPRI).F.. .ISD LIVISGSTOS,R . : ,J. Phys. Chcni. 44, 865 (1910). ( 5 ) J ~ s t o f i s ~ tAr , : d c t a I'hys. l'olon. 4, 311 (1935). (6) KAUTGKY, H., HIRXTI.A , . ASD FLESCH. W .: Her. 66,401 (1932 ; 68, 132 (1936). ( 7 ) LEWIS,G., ASD C A L V I K11.: , J . .!ai. C'lieni. SOC.67, 1232 (1945). (8) LEWIS,G., A N D KASHA, 11.:J . Cliern. SOC.66, 2100 (l!U (9) LEWR,G . , LIPKIS,D . . A S D ~ L I G E I T , . . : J . .\in. Chrm. Roc. (10) LIVISGSTOS,R . : *J. P h (11) ~ ~ B R A D J . ,Y. 4 S. D LTVI d Chrni. 62, S o . 4 (1948:, (12) SOYES, IV. A , , A K D LE citristry o,f Gases. 1). 133. Reinhold Publistling Corporation, Sex\- l o r k (1941) , (13) PRIKGSHEIJI! 1'. : Fluorescerlz a n d Phosphoresceiii, 11. 160. J . Springer. I3erlin (1!)2S). (14) P R I N G S H E I J I , p . . AND VOC:EI,R. H . : J. Chem. Phys. 33, 31q5 (1936). (15) RABIKOWITCH, F:,: Photosynthesis, Vol. I. p. 494. Interscience Press. I n ? . . Sen. York (1945). (16) RABINOWITCH, E.. A K D PORRET, P . : S a t u r e 140,321 (1937). ( l i ) RIDEAL,E., A K D WILLIAMS, 2:. G . : J . Chem. Sac. 127, 258 (1925). (18) SCHISCHI,O~-SKY, A .I> \-A\-ILOV, S.: Z.Physik (F,S.S,R.I6, 379 (1934). (19) TELLER, E . : J. Phys. Chem. 41, 109 (1937). (20)VAVILOT,S., AKD LEVSHIK, W.:Z . Physik 36,920 (19261. (21) WEISS. J . : Trans. Faraday Soc. 36,48 (1939).
T H E PHOTOCHEMISTRT O F T H E ALDEHYDES' F. E. BLBCET Department of Chenrzsiril, l - r n t e r s z f u of C a l z f o m z a , /,os .-ltigeles, Cal7jornza R e c e z i c d Octobei 2 3 . 1947
The absorption spectra and vapor pressures of the aldehydes are such as to invite photochemical studies of them in the gaseous phase. They all have a first region of absorption in the near ultraviolet with a span of approximately 1000 A. On the long-wave-length side this region is always discontinuous in character, showing some fine structure in the case of the more simple molecules. The fine structure blends into a diffuse or what has been designated a predissociation spectrum, and this in turn changes over to a continuum on the short-wavelength fringe of the region. second absorption region soon starts in below this and continues perhaps another thousand L%ngstromunits well into the Schumann region. A few investigators have ventured into this second region, but for the most part it remains unexplored, waiting for the development of better equipment and better technique for quantitative studies. We shall return in this discussion, therefore, to the first absorption region, where much work has been done. It is believed that here at least a beginning has been made toward an appreciation of the chemical processes which follow the absorption of radiant energy. 1 Presented at the Syniposium on Radiation ('henustiy and l'hotochemistij held a t the L-niveisity o f S o t r c Ilanic, S o t r c Dame. Indi:ma, .June 24-27,1947.
I\
hich was
PAOTOCHEMISTRY O F THE ALDEHYDES
535
In figure 1 are given microphotometer tracings of acetaldehyde, propionaldehyde, and isobutyraldehyde (21). The spectrum of normal butyraldehyde coincides almost exactly with that of isobutyraldehyde showing, however, slightly more pronounced bands. Similarities and differences in structure are apparent. Detail structure becomes less evident with decreasing wave length and with increasing molecular weight. .Ittention is called particularly to acetaldehyde. One can see the distinct structure reported by Henri and Schou (16) as extending to 3300 8. The bands which they reported as becoming more and more diffuse at shorter wave lengths extend well beyond the limit of 2820 8. set by them. These bands are most intriguing, but they are not of sufficient detail to mak possible a mathematical interpretation of the spectrum. However, from t h
c
.-0
n L
0 v)
n
a
t Wave l e n g t h , Angstrom u n i t s
F I G ,1, Alirrophotorneter tracings of the absorption spectra of acetaldehyde, propionaldeliyde. and isohutgraldehyde. The positions of the principal lines of the mercury arc a r e indicatPtl.
interpretation which had been given to somewhat similar spectra of diatomic molecules, a physical concept of such spectra of polyatomic molecules was formulated by Henri (15) and others somewhat as follows: Bands having distinct see. With structure indicate electronic excitation with lives of the order of the aldehydes these occur on the long-wave-length side of this region of absorption. The continuum which lies on the short-wave-length side was conceived to represent molecular dissociation, which takes place within the period of one vibrational cycle, i.e., in approximately sec. The intermediate diffuse bands were designated as predissociation spectra by Henri. Predissociation has been described in various mays, but we can do no better than follow the lend of Burton and Rollefson (13) and consider predissociation as a decomposition process which occurs after a time greater than one vibration period has elapsed following absorption. The mercury arc lines make possible the study
536
F. I:. RLACET
of the aldehydes with monochromatic radiation in each absorption type (ser figure 1) Such chemical studies should serve to extend and amplify our general interpretations of the absorption spectra of complex molecules. For formaldehyde (16), acrolein, and crotonaldehyde (10) the first absorption region is shifted a few hundred -!ngstrom units t o longer wave lengths and in general shows more detailed structure. The ketones have absorption spectra similar to the aldehydes in this region. Since the carbonyl group is common t o all these compounds, absorption is thought t o represent the excitulion of an electron associated with this group. As supporting evidence for the processes attributed to the types of spcctra in this region, some of the aldehydes at any rate show pronounced fluorescence a t long wave lengths. This fluorescence diminishes in intensity as the bands become less evident and is no longer discernible in the continuum. The ahsorption and fluorescence spectra, with their interpretation, led to predictions twenty or more years ago that no dissociation of these compounds would take place if they were irradiated with monochromatic light falling in the wavelength range of fine srructure bands (15). I n the diffuse range some dissociation might occur, while in the continuum all absorption leads to dissociation in a period of time far shorter than that between molecular collisions These predictions,-namely; that radically different chemical results were to be expected at different wave lengths,-stimulated investigation and early results were reported to be in general agreement with them. However, since most early esperiments were little more than qualitative in nature, it would perhaps have been better to say that they were not in disagreement with spectral interpretations. As indicated in figure 1, the mercury arc is a convenient light source for detailed study of the aldehydes. With proper equipment individiial lines of sufficient intensity can be isolated to study the chemical effect of absorption in each region. This has been done with several aldehydes under various other experimental conditions. I n general, marked rhanges in chemical resultq have not been found as consistently, on going from one type of absorption to another. as one might have supposed. For example, in figure 2 decomposition quantum yields of three aldehydes are shown to vary rather uniformly over m w t of the spectral region shown in figure 1 (19, 20). Because of its markedly different types of absorption, crotonaldehyde, CH,CH=CHCHO, was thought originally to be a good substance to investigate in the hope of correlating absorption spectra with chemical processes (10). I t was soon learned, however, that at room temperature no photolysis occurs even with the full force of the mercury arc (6). -It elevated temperatures some decomposition will take place, as shown in figure 3 (5). In a sense it may be said that the early interpretation of spectra is substantiated, since no measurable decomposition was obtained in the region of many bands However, the fact that the quantum-yield curve extends o w r two electronic regions of absorption indicates, perhaps, that it is the amount of energy absorbed and not the absorption type that is most important. -1study of the photochemical oxidation of crotonaldehyde at room temperature has given results which conform to this
537
PHOTOCHEMISTRY O F T H E ALDEHYDES
idea (9). I n this case the smooth oxidation curve cuts across three types of absorption spectra. Acrolein, CH2=C€ICH0, like crotonaldehyde, dissociates very little a t room temperature (2). Unlike crotonaldehyde, however, it may photopolymerize extensively, as shown in figure 4. A small amount of photodecomposition was found at room temperature. The high polymerization quantum yields found a t T a w lengths which fall in a region of absorption which appears continuous r a y be accounted for by assuming that the free radicals produced in photodecompoqif ion wrve as niiclei around which polymerization occurs.
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3130
Wave l e n g t h , A n g s t r o m u n i t s FIG.2 . I)cconiposition quantum yields of :tcetnldf~liytle,propioiiRlclClISdc, and isobiutgraldehytic, at scl-cr:rl \ v : i v ~Iciigi hs diatrihiit ~d n w ~t h' c absorption region.
Of the saturated members of the aldehyde family acetaldehyde has received more photochemical attention than all the others put together. It is the typical aldehyde of the general formula RCRO, and, in a broad sense, information obtained from a study of it may be applied to the other compounds of the series. Exceptions to this statement may be found, of course, but if one is to obtain an over-all concept of the photochcmistry of the aldehydes in a short time he must not mire down in details. It is reasonable to suppose that most apparent exceptions will turn into logical variations, as more and better data are obtained. We shall follow the lead, therefore, of Rollefson and Burton (23), Yoyes and Leighton (22), and finally, Steacie ( 2 5 ) , all of whom have given a prominent
538
F. E. BLACET Wave length, A. 3130 2804
2654
2537
2380
0.6
0.5
e v;
0.4
5.-
x
0.3
5Ei d
0.2
0.1
26000
30000
34000
42000
38000
Cm-' FIG.3. Showing the variations of the molecular extinction coefficients (irregular curve) of crotonaldehyde and quantum yields of decomposition (smooth curve) \$-it h wave length. The quantum yields x e r r obtained at 245'.
a.
3660
Wave length, 31303020280426542537
1.50
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PHOTOCHEMISTRY OF THE ALDEHYDES
539
place to acetaldehyde in their books on photochemical and free-radical reactions, and focus our attention for the remaining time on this compound. Two important chemical processes are reported to occur Jvhen acetaldehyde is irradiated. One is referred to as decomposition and the other as polymerization. There is a strong tendency among us to treat the polymerization as an unfortunate nuisance, to ignore it as much as possible, therefore, and to confine our attention to the cleaner and more easily handled gaseous products. Only under certain circumstances is such an approach justified. 310"C., where thermal disOne of these is high-temperature photolysis. sociation may be neglected, Leermakers ( l i )found that approximately 300 acetaldehyde molecules decompose for each quantum absorbed. This is evidence that a chain mechanism is operating, involving most certainly some free radicals. Leermakers chose to assume that the primary dissociation is into a methyl and a formyl radical, followed by one or more of the chain possibilities which can he formulated from the following equations:
+
+ +
CH3CHO h~ -+ CH, CHO CHO-+CO+H H CH,CHO + HP CHiCO CHSCO + CH3 CO CH, CHZCHO -+ CHa CH3CO CH3 + C2H6 CH, CHO CH3CHO -+ CH, CO CHO CH3 CHO --+ CH, CO
+
+ + + +
+
+
+
+
+
These equations are alniost identical with those proposed by Rice and IIerzield for the thermal dissociation of this compound; as we review the subject it n-ould be surprising if this were not so. High quantum yields, vithout condensation, are difficult to visualize without a radical mechanism. Once radicals are formed in a primary process there is little reason to suppose that the influence of this photochemical act will be felt very far down the chain. -1chain process, in order to go far, must be self sustaining, and it makes little difference whether it was initiated by the absorption of a photon or by an unusually energetic molecular collision. At elevated temperatures the only decomposition products definitely established are methane and carbon monoxide. Hydrogen was included by Leermakers partly to account for a residual gas volume and partly because it had been found in the room-temperature photolysis of aldehydes (19, 20). -11 best, only a small percentage of hydrogen can be expected at high temperatures, percentages so small as to be within the probable limit of error of ordinary chemical methods of analysis. I-et a qualitative proof of the presence or absence of traces of hydrogen might throw considerable light on reactions 2 and 3. In the laboratory of the author, such a proof is t o be sought with the aid of the mass spectrometer. If hydrogen is found, we can only say that one or more of several reactions, including 2 and 3, are not ruled out. The complete absence of hydrogen gas
540
F. E. BLACET
would indicate that atomic hydrogen is not formed in the bystem either as a primary or as a secondary product. Leermakers' equations include two cyclic processes, equations 4 and 5 constituting one, and equation 7 the other. If hydrogen is found in an amount equivalent to the photons absorbed, the likelihood of equation 7 being important mould diminish. I n any event there is little evidence to be found in support of number 7. -4s chain-breaking reactions, two alternatives are given-equations 6 and 8. It has been shown by chemical analysis that ethane is not a photolysis product a t room temperature (8). l l r . R. K. Brinton in our laboratory has confirmed thib result in recent weeks by means of the mass spectrometer. It does not follow, however, that reaction 6 is not important a t elevated temperatures where reaction 4 doubtless almost always occurs. Severtheless, after consideration of a number of factors, Leermakers was inclined to favor equation 8 over 6 as a chain-terminating step. Primary dissociation into other radicals, such as hydrogen atoms and acetyl, mould give secondary reactions similar to the ones used above and would explain the experimental facts equally well. Also, as far as over-all products are concerned, some dissociation directly into the stable molecules methane and carbon monoxide would make no difference at elevated temperatures. Such dissociation would, however, have a bearing on the average chain length. For example, if no such process occurred in Leermakers' experiments quoted above, the average chain length was 300. If, on the other hand, 50 per cent of the primary dissociation Jyas by a non-radical mechanism, the chains averaged 600 cycles in length. It is important to know, therefore, more about the initial photochemical steps. From studies at lower temperatures considerable has been learned about the probable primary proceeses. Much work has been done in a temperature range of from -40°C. to +150"C. Some of the data are conflicting; nevertheless, the over-all picture seems to be developing in a logical manner and there is no sharp demarcation between high-temperature and low-temperature mechanisms. One mechanism blends into the other as the various secondary reactions change in relative importance with temperature. &isthe temperature is decreased the photolysis products become more complex, and at room temperature a condensed product must be reckoned with. For the most part this is a polymer of the original aldehyde. Its formation appears to be promoted by free radicals in the manner indicated previously for acrolein. -4 complete study of this so-called polymer has not been made. However, a partial analysis of the acetaldehyde condensate ha5 revealed the presence in relatively small amounts of biacetyl, glyoxal, and formaldehyde (1). The amount5 of these compounds relative to carbon monoxide found at three different temperatures are given in table 1. The variation of products with temperature should be noted. Tests for acetone and methylglyoxal, which can be postulated as other products, were negative. It may be noted that carbon monoxide, methane, and hydrogen were the only non-condensible products
54 1
PHOJ?OCHE1\lIYTRT O F I?HE ALDEHYDES
found. An extensive search again revealed no ethane and, as stated before, in a study which is in progress now, the ma55 spectrometer has substantiated the chemical analyses. S o unsaturated hydrocarbons were detected. On the basis of these rcsults and many others involving quantum yields for the most part, T 1131,l i Relatrip
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