THE PHOTOCHEMICAL BEHAVIOR OF T H E A L D E H Y D E S G. K. ROLLEFSOX Department o j Chemistry, University of Calijornia, Berkeley, Californza Received October 2, 1936
The photocheniical behavior of the aldehyde? has been investigated by a number of people, with the net result that fairly good agreement exists concerning the experimental facts but two markedly different viewpoints are represented in the interpretation of these facts. The Anierican group, represented by Leighton (6), Kistiakon sky (4))Leerniakers ( 5 ) ,and Blacet ( 2 ) , has favored a mechanism in which the primary action of the light is considered to be a rupture of the molecule into an alkyl radical and a CHO radical, with the final products being produced by secondary reactions of these substances. Norrish (8) and his students have preferred a mechanism in which a molecule of carbon monoxide is ejected, leaving a hydrocarbon molecule. Recently Xorrish (9) has modified his mechanism slightly so as to permit some formation of alkyl radicals and hydrogen atoms, although still maintaining the direct formation of hydrocarbon and carbon monoxide froni activated molecules as the principal reaction. Each of these mechanisnis will explain a great many facts, but definite objections may be raised against each of them. In this paper it will be shown that the observed facts can be accounted for if we n ill aswnie that on activation b y light some of the molecules dissociate into free radicals, come decompose into hydrocarbon and carbon monoxide n ithout passing through a free radical stage, and some may enter into polymer formation, fluoresce, or return to the lowest state through collision processes. The clivi4ou between these different posibilities is assumed to be a continuous function of the n-ave length of the exciting light. For siniplicity let uq first consider t h c photocheniical behavior of acctaldehydr, for which the observations are most coniplcte, and then extciid the diqcuqsioii to include the other aldehydes. According to Leigliton and Blacet (7) the absorption spectrum of acetaldehyde begins at about 3480 A . U. and extends to about 2400 A. E. The spectrum appears to consist of a number of bands, some of them diffuse, beginning at 3480 A . L-. and 1 Presented at the Symposium on lIolecular Structure, held a t Princeton Uiiiversity, Princeton, New Jersey, December 31, 1936 t o January 2 , 1937 under the nuspices of the Division of Physical and Inorganic Chemistry of the American C'hemical Society. 259
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extending to 2666 A4.U. with a maxinium absorption at 3100 A. IT. and a continuous absorption extending from 3250 A. U. (estimated from tlie photometer curve (6)) to 2400 A. U. with a maximum at 2750 A. C . The usual interpretation of such a spectrum assumes that any diffuse structure or continuum corresponds to a dissociation of the molecule in a time of lo-“ sec., or less. An abundance of evidence has been secured which supports this interpretation in the case of diatomic or very simple molecules. However, extreme caution must be observed in extending this idea t o complex molecules, as a spectrum which is “experimentally continuous” may be obtained with polyatomic molecules without having any dissociation. I n order to see how such a spectrum can be obtained let us compare the transitions which may occur in a simple diatomic molecule, such as bromine, mlth those which occur in an aldehyde molecule, RCHO. From a consideration of the Bohr frequency relation and the uncertainty principle we know that a sharp line will appear in the absorption spectrum for every transition between two defylite quantized states in the molecule, provided the life of these states is of the order of see. or greater. In a diatomic molecule there is only one degree of freedom for vibration and one moment of inertia for rotation, so there are relatively few states possible and a fine structure should be observed in the spectrum unless dissociation occurs within a very short time. K i t h a polyatomic molecule, even if vie consider that we are limiting the electronic changes to a particular portion of the molecule, such as the carbonyl group in the aldehyde.;, n-e have many more vibrational and rotational states than exist in tlie diatomic molecule. To be qpecific, there are three moments of inertia corresponding to rotation about each of three axes and several degrees of freedom with reipect to vibration which would have to be considered. Furthermore, owing to the various ways in which the different modes of vibration may be combined, there will be a certain amount of variation in the moments of inertia. Even if this effect amounts t o only a few per cent it would be enough to reduce the rotational structure of the bands to a blur. If this blurring does not extend over too wide a range of rotational quantum nuniber.; it will still be possible to 5ec the structure due t o ribration. However, we muht also consider that the molecule may change rapidly by a process such a. is assumed to explain predissociation from the electronic state produced initially by the absorption of light t o various other states, with corresponding changes in the yibrational and rotational effects. With such transitions occurring within lo-” bee. or less of the absorption act we must expect to find an “experinieritally continuouq” spectrum. In the spectrum of acetaldehyde a comparison of the separation of tlie maxima of the discontinuous and continuous absorptionb and tlie separations of the long wave-length and short n-ave-length limith for these two types of absorption 5ug;g.e.t.; that 11 P arc’ oherving transfer\ from the loweat state
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to two higher electronic states separated by approximately 5100 cm-l If we will assume that in one of these excited states t h r nioleculc does not change readily to a different electronic structure by the predisqociation proceis but that it does from the other state, we can account for the observed spectrum without aisuming any discociation into free radicals. In order to reach a decision as to the nature of the action of tlie light n-e must consider other kinds of evidence. Definite evidence for the formation of free radicals by the action of light on acetaldehyde was obtained by Pearson (10). He employed Rice'? (11) method of the removal of metallic mirrors to demonstrate that illumination of the aldehyde vapor produced some dissociation of this type. The rriults are not quantitatire, but a comparison of the effect obtained u-ing acetaldehyde with that obtained with such substances as acetone indicatfd that the formation of radicals from the former is a rather ineEcieiit process. Another method of attack was used by Leermakers ( 5 ) , n-ho studied tlie photolysis of acetaldehyde at about 300°C. and fouiid quantum yields as high as 300. The behavior of the system under thece conditions could be explained by assuming that free radicals were formed by the action of the light aiid these caused the decomposition of other acetaldehyde inolccules in a chain mechanism of the type postulated by Rice. T h k mrchanirm receives further support from tlie recent observations of Fletcher and Rollefsoii ( 3 ) at a temperature of 140°C., n hich show that free radicals from the decomposition of ether or ethylene oxide can set up chains which result in the deconiposition of a thousand or niorr aldehyde molecules for each radical introduced. The experiments of Leermakers may be considered as showing that some free radicals are formed, but wpplies no information concerning the efficiency of the primary light process. The experiments juqt cited show that any complete mechanism for the decomposition of acetaldehyde must, include a reaction which yields free radicals. There are other experiments, however, which iiidicate that other processes must be considered as well. The recent work of Fletchrr and Rollefsoii (3) and of Staveley and Hinshelwood (13) has shown that the thermal decomposition of acetaldehyde, under conditions such that the rate is measurable, proceeds to methane aiid carbon monoxide without passing through a free radical stage. The actiration energy is 58,000 caloriec, which is far less than is supplied by light in the photochemical reaction, so we must consider the possibility of direct decomposition into the final products any time the molecule is actirated to a higher state than that reached in the thermal reaction. The recent careful analyseq of Blacet and Roof (2) have Show1 that a t least three equationc niuqt be written t o represent the chemical changes which occur n hen acetaldehyde is illuminatcd. The-e are: ( I ) CH3CH0 + CHI CO; ( 2 ) 71 CH3CHO + (CHSCHO),,; (3) CHaCHO + CO, H f ,
+
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and an unidentified hydrocarbon (possibly ethane or ethylene). The distribution between these three possibilities wa5 found t o vary approximately linearly with the wave length of the light absorbed, reaction 2 decreasing toward short wave lengths mid the other two increasing. Blacet and Roof attribute the variation in products to the increase in the velocity of separation of the radicals as the frequency of the absorbed light is increased. This does not seem to be very probable, as a simple calculation shows that these velocities vary by only a factor of two over the range of frequencies used. Furthermore, at the pressures uwd in the experiments the mean free path of the separating radicals would be a t least twenty or thirty diameters, which is sufficient to remove each from the field of influence of the other. Also the probability that they would be driven back toward each other is negligible, and their speeds would soon he reduced to that of ordinary thermal motion. Thereforr. t h r reaction\ which the radical3 undergo will in no n-ay be deterniiiied 1)ythe original velocity of separation. TABLE 1 Fracfions u j ilie actii~aleilaldehyde nzolecules reaclzng according to the tiariaus possible paths
I n a miilar iiianiier objection\ rail lit raised t o any niechaiiihin I$ hich assumes solely free radical- or iolely activated inoleculei. Let 115 abqunie that we have both free radicalh aiid activated inoleculez and that the relative number produced is a function of the exciting wave length. Applying this viewpoint t o acttaldehyde 11 f’ mu-t consider that a molecule which has absorbed light may (1) returii to the lowest statewith the emission of light or by collision> n-it11 other molecule\, (2) diqzociate into free radicals, (3) rearrangc t o gi\ e methanc and carbon monoxide, (4) react with another molecule or nioleciiles t o form a polymer. It is lmown from an extrapolation of Leermaker,’ n-ork to rooin temperatiire that it is unnecessary to consider any chain nic~chani~m+ under these conditionh, 50 we .hall assume that the biiin of the quantum yields of the four processes listed is unity. 111 addition let 113 c o i i d e r that the hydrogen is formed only by process 2. On this basis the data of Blacet and Roof may be interpreted as shown in table 1, in which are listed the fractions of the activated molecules which react according to each of the above-inentiontd paths (the path iiuniberb lihted i n tlic tablc correspond to the numliering above). The
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values given for the polymerization are one half of the quanturii yields given for that process by Leighton and Blacet ( 7 ) ,as Sniitli (12) has shown that the amount of polymer formed is a linear fiinction of the pressure for constant light abqorbed, $0 we may conclude that at least one niolecule of aldehyde in addition to the activated one goes t o form polymer. KO attempt has been niade to separate process 1 into deactivation by collision and fluorescence, a. no quantitative data are available for that purpose. From the fact that the quantum yields of the decomposition processes do not increase as the presqurc is decreased, it may be argued that process 1 is occurring primarily through fluorescence or at len>t that the rate-determining step in the deactivation is monomolecular. An inspection of table 1 hliowh that a i the diwociatioii procews 2 and 3 increase, the processeq 1 and 4,n-hich are ashociated with activated nioleculei, decrease. I t is not iurpriqiiig that the dccreaie is not exactly the same, as we have seen that we muqt consider two diffcreiit electronic levels for the upper qtate, and it is quite poqsible that they will not be equally efficient in causing polymerization. The iiicreases in processes 2 and 3 may be due to a difference in the behavior of t h e w two itates, as well as a change in the probability of decomposition \Tit11 increasing vibrational energy in the upper state. The chancc of these electronic levels fluorescing must be approximately the wine, as the absorption coefficients corresponding to the two transitions are not markedly different. This is probably not true of the other proceqse., or thc character of t h e abhorption spectrum would not be so different. The niechanirin of formation of the hydrogen from free radicals must be left indefinite on account of inhufficient data. It i. po-iblc for either the carhon-hydrogen bond or the carbon-carhon bond t o be broken. In order t o make a decibion it nould be dehirahlc t o know: (1) n-hether ethylene or ethane is formed, and ( 2 ) whether any products, c u c h as glyoxal, n-hich may be formed from CHO are prewit. Other tcbt4 if\-onld involve t h e detrrniiriatioii of the effect on the hydrogen yield of I arious hubstance?, such as nitric oxide, n-hich are kiioun to react readily nitli free radicalq, and a deterniiriatiori of the percentage of hydrogen in the products obtained, using light absorbed by one of the bands observed by Leightoii and Blacet as compared to the percentage ohtaiiied n it11 light from the adjacent regions of continuous absorption. The data for other aldehydes, except formaldeliyrlc, are 1csb complete than for acetaldehyde and therefore more difficult to interpret. Formaldehydc apparently qeparates into hydrogen and carbon monoxide without the formation of any free radicals, but this must be looked upon as a very special caie as the niolecule has a syriinietry which does not exist in the other aldehydes. Propionaldehyde (6) reactq m u c h the same as acctaldehyde, except that the hydrogen yield pa-es through a maximum at a wave
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length near 2i00 A. U. and thc polymerization passes through a niiriimum a t 3020 A. U. From tlic facts that fluorescence was observed at all wave lengths down to 2654 A. V. and that the polymerization is relatively more important than with acetaldehyde, it may be concluded that the life of a molecule in the activated state is greater than in acetaldehyde. The sum of the quantum yields is somewhat greater than unity at all n-ave lengths, according to Leighton and Blacet, but if we divide their value for the polymerization by 2 the discrepancy disappears except for the two highest frequencies (X = 2654 A. U. and 2537 A. U.). The high yield may be due to short chains, such as Staveley and HinshelTyood (13) have found a t high temperatures in the thermal decomposition. Crotonaldehyde is of special interest, since Blacet and Roof (1) have reported that it neither decomposes nor fluoresces and gives only a very slight amount of polymer. According to the views which have been expressed in this paper the stability of this aldehyde must be attributed to a rapid distribution of the energy absorbed into other degrees of freedom, followed by collisional deactivation and possibly some fluorescence of rather long wave length. Blacet and Roof suggest the formation of free radicals followed by recombination in preference to other possible reactions, owing t o the higher heats of activation of the latter. It seems improbable that the radicals obtained from this aldehyde should differ so much from those obtained from other aldehydes. If their theory is correct the decomposition should appear at higher temperatures and increase rapidly as the temperature is raised. According to the theory presented in this paper temperature qhould h a w little or no effect. The data on the other aldehydes are not sufficiently complete to warrant drawing any conclusions about the mechanism of decompositiori. I n general the views which have been expressed in this paper require that the composition of the products shall depend only on the wave length of the light absorbed and be practically independent of the temperature or pressure in the system. If the temperature is raised high enough to set up reaction chains, the coniposition of the product9 will be influenced by the nature of the chains set up. Fluorescence and polyinerizatioii will show similar changes and the decompositions will change iiidependently of each other, but in such a way that the sum of the decompositions will increase as fluorescence and polpmcrization dccrease. SUMMARY
It has been shown that the photochemical behavior of the aldehydes, especially acetaldehyde, is Such that it is necessary to assume that the photoactivated molecules can react in several ways a t comparable rates. Part of the decompositioii which occurq proceeds through free radicals. The relatire importance of the different proce*se5 is disciissed for acetalde-
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hyde. Sonit. experimental tests of the \-arious theories haye been suggest ed. REFERESC’ES BLACET ASD ROOF:J. Ani. Chem. Sac. 68, 73 (1936). RLACETA X D ROOF:J. Am. Chem. Sac. 68, 278 (1936). FLETCHER AND ROLLEFGON: J. Am. Chem. Soc., November, 1936. KISTIAKOWSKT: Cold Spring Harbor Symposia on Quantitative Biology 3, 46 (1935). ( 5 ) LEERMAKERS: J. Bm. Cheni. SOC.66, 1537 (1934). (6) LEIGHTON AXD BLACET: J. Am. Chem. Soc. 64, 3165 (1932). ( 7 ) LEIGHTOSAXD BLACET:J. Ani. Chem. Soc. 66, 1766 (1933). (8) NORRISH:Trans. Faraday Soc. 30, 103 (1934). (9) NORRISH:Acta Physicorhimica U. R. S.S. 3, 171 (1933). (10) PEARSON A N D PURCELL: J. Chem. Sac. 1936, 1151. (11) RICE ASD RICE: The Aliphatic Free Radicals. The Johns Hopkins University Press, Baltimore (1935). (12) SMITH,J . H. C.: Carnegie Inst. Wash. Pub. 27, 178 (1928). (13) STAVELEV ASD HINSHELWOOD: J . Chem. SOC.1936,812. (1) (2) (3) (4)
