546
I\-. -1LBERT NOSES, JR
SOAIE XSPECTS OF THE PHOTOCHE;\IISTRIy OF KETOSES’ U’ -1LBEIIT S O I T S JR. Depnrtirient oj Cheniisit y, 17niz,eistty oj R o c h e s f e l H o ~ l i c s / e i .Vex Yolk Ii’eceiicri Ocfobe) 23. 1947
The ketones have been the subject of numerous photochemical investigations, largely because they absorb in a convenient part of the spectrum and because it seemed, from early work, that the products would be simple and easily identifiable. The simple aliphatic ketones all absorb, in the vapor phase, from about 2300 to about 3200 8. Other absorption regions are present below 2000 i., but few studies of the photochemical behaviors of these compounds have been made a t these short wave lengths. The photochemistry of the simple ketones has been reviewed (6) recently, and there is no point in repeating all references t o the literature of this subject a t the present time. Licetone does show some evidence of banded structure in its spectrum in the region from about 2900 t o about 3200 -1.(25, 26, 27). Available information indicates that such a structure is not found in the nearultraviolet spectra of any other ketones, although discrete bands are found for those investigated in the far ultraviolet (10, 11, 12, 13, 19, 27, 28, 29). It is impossible from existing data and from the present state of knowledge t o draw any precise concluiions from spectroscopy concerning the primary process during optical absorption. Acetone does fluoresce both in the blue and in the green parts of the spectrum, the latter result being due undoubtedly to biacetyl (2, 20, 21), which is a common impurity in acetone and which is synthesized photochemically when acetone is exposed to ultraviolet light. The &reenfluorescence seems to be excited by all wave lengths between 3130 and 2637 A. (5, 17), but no corresponding information exis$ as regards the blue, which is only known definitely t o be excited by 3130 A. radiation. The absence of fluorescence when radiation below 2000 is used (lT), the excitation a t least of green fluorescence by 2537 A. radiation where all indications of structure in the absorption spectrum have disappeared, and the existence of weak fluorescences in methyl ethyl ketone (21) and in diethyl ketone (21), neither of which has been shown t o have a discrete absorption spectrum, all tend t o prove that the appearance of the spectrum of a polyatomic molecule is not a safe guide as t o whether the absorption of light leads t o excitation, to predissociation, or t o immediate dissociation. The weak blue fluorescence observed in acetone is probably t o be ascribed t o the acetone molecule itself for the following reasons (18): ( a ) It is observed in acetone in a flow system which does not permit of accumulation of products of reaction (2); ( b ) it is observed with oxygen present and thus under conditions which prevent biacetyl synthesis (2, 5) ; (c) it is observed a t temperatures up to 20OOC. a t which no biacetyl is formed (16). Since the fluorescence efficiency for
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1 Presented a t the Symposium o n Radiation Chcmistry and Photochemistry which was held a t the University of S o t r e Dame, S o t r e Danie, Indiana, June 24-27, 1047.
PHOTOCHEMISTRY OF I X T O S E S
54i
the blue is independent of the intensity of the exciting light (18), it seems probable that the radiation is from excited acetone molecules and not from a recombination spectrum of some sort. Part of the radiation absorbed, a t least at 3130 8. and probably also a t other wave length,, must produce excited molecules. Xt room temperature the quantum yield of acetone decomposition is lorn ( G ) , but it has been shown ( 5 ) that fluorescence cannot be mainly responsible for this low yield. The fact that there is an appreciable blue fluorescence a t 135°C. (18), where the quantum yield of carbon monoxide formation from acetone is well within the experimental error of unity, is further proof either that the yield of excited molecules in the primary process is very small or that the majority of these molecules dissociate under ordinary experimental conditions. More information is needed concerning the character of the blue fluorescence in acetone. The only photographs have been obtainecl (2. 5 , 18) under conditions which would make detection of any btructure most unlikely. Owing to the low intensity, spectrographs of l o r dispersion have been used, often with wide slits. present no evidence of structure exist', although there i, one slight maximum near 4600 .f. The limits of emission are from about 3850 -1.to about 5000 -1. If the emission is truly continuous (i.e., if the apparent absence of structure is not due t o a great deal of overlapping structurc) when observed under high dispersion with high resolution, either the upper or thc lower state must be repulsive. Airepulsive upper state is improbable bccuusc blue fluorescence is observed ut temperatures where the C'fI3c'O radical i q w r y unatnblc (16, 18). -1repulsive lower state must q x t r a t e into ethane and carbon monoside, since insufficient energy remains after emission of any of the abovc \raw lengths t o permit any other mode of disociation (18). The effect of pressure on the quenching of the blue fluorescence (18) indicates that a t least two upper levels must be involved, one of much longer life than the other. The long-lived state must be insensitive to collisions with acetone, although it must be deactivated or dissociated n-ith greater probability a t high temperatures than at low. I n Tien- of the photochemical data at high temperatures disociation is more probable than simple deactivation The following conclusions roncerning the primary proces- in xetone are derivable from all of the facts a t our disposal: (a)Some e x i t e d molecule. are formed at 3130 5. and perhaps a t other iva1-e length-. (0) Eitliw the number of excited molecules formed is very miall, or they must dissociate, particularly at high temperatures. (c) -1very high cfficienTy of production of methyl radicals i j probable, because a t high temperatures about 1.4 moleculPs of methane are produced per quantum absorbed by acetone (1, 7.(& Direct dissociation into ethane and carbon monoxide cannot be prowd until furthcr effort5 have been made to detect structure in the emission bands. ( e , -1primary dissociation into (?€I3 and CHSCO a t rooin temperature is indicated by the svnthesis of amounts of biacetyl dependent on experimental conditionq. (f)The instahility of CH,CO depends both on wave length and on temperature (6, 30). ( 9 ) S o conclusive evidence for or against a primary dissociation into ethane and carbon monoxide has been found ( 6 ) .
51s
11.. ALBERT SOYES, J K .
X great many secondary steps have been suggested to account for the photochemical decomposition of acetone. The general picture is n o v quite clear, although there are some disagreements as to details. I n view of excellent reviews recently published there seems to be n o point in discussing these steps in detail. One matter, however, needs further attention. There is evidence from several sources ( 2 , 5 , 1 8 ) that biacetyl does not accumulate indefinitely in acetone exposed to radiation at room temperature. Since the spectrum of biacetyl is excited to a, high intensity when its pressure is quite low ( 2 , 20), it probably receives energy from excited acetone molecules. This may cause it t o dissociate part of the time. That this is the case is indicated by two effects: (a) collisions enhance rather than quench the green fluorescence; (6) the green fluorescence of “acetone” grows weaker rapidly as the temperature is raised. Pure biacetyl excited by the 3660 -1.line of mercury shows an abnormal change of fluorescence efficiency with pressure, i.e., the efficiency increases as the pressure increases (3, 15). This same effect is observed for the green fluorescence in acetone (18). The lifetimes of the green emitter in acetone and of the emitter in biacetyl have been shown to be the same (2). Excited biacetyl molecules produced under these conditions must be able to dissociate (or at least pass into a state incapable of fluorescence) unless they lose some energy by collision. If this is true, collisions involving sufficient energy should be able to raise metastable or excited biacetyl molecules to states capable of dissociation. I n this way the absence of green fluorescence a t 135OC. in acetone, even when biacetyl is present, can be explained. From the above discussion it can be concluded that dissociations of excited or metastable molecules must be considered in any complete picture of the photochemistry of acetone and of biacetyl. The distinction between molecules which absorb when they are in higher vibration levels of the ground state and those which acquire energy by collision after ihey are artiuated is one which cannot be investigated except with diatomic and very simple polyatomic molecules. The temperature effect on fluorescence can be explained qualitatively either way, providing predissociation is more probable in higher than in lower vibration levels of the upper state. The character of the blue fluorescence in acetone is such that the initially formed state is quenched by collisions, whereas the green requires the reverse to be true for the initially formed excited state of biacetyl. These facts are best explained as due to the effect of collisions on transfer between upper levels. Since this explanation is equally effective in dealing with the temperature eflect, it is tentatively accepted. The long lifetimes of the sec. ( 2 ) ) and of acetone ( > l o p 5 see. (18)) upper state of biacetyl (1.5 X are ample for collisional effects on the upper state to be important. Brief mention will be made in conclusion of only two other ketones: diethyl ketone and methyl n-butyl ketone. mhile diethyl ketone does show a weak fluorescence (21) (probably due t o bipropionyl), the quantuni yield of carbon monoxide formation is very close to unity and independent of temperature (8, 14). The primary dissociation must have a yield very close to unity. If the products of the primary dissociation are C2H, and COCnHj, the latter radical
PHOTOCHEMISTRY O F KETOSES
549
must be sufficiently unstable even at room temperature t o give carbon monoxide virtually 100 per cent of the time. The fluorescence, which is probably identical with that of propionaldehyde (21), may be due to bipropionyl and be excited by a very small number of excited diethyl ketone molecules produced in the primary process. S o detailed studies of this fluorescence have been made. The decomposition of methyl n-butyl ketone has been studied by Sorrish and hii con-orkers (4, 22, 23, 24), who gave strong evidence that a substantial portion of the primary process yields acetone and propene without there being intermediate free radicals. This point of view is further supported by the invariance of the yields of these compounds with temperature from 25°C. t o 300°C. (9). ,Inalogous behavior has been found for other ketones all of which have a t least one alkyl group three or more carbon atoms in length. These molecules conntitute the best examples of primary dissociation into completed molecules as distinguished from free radicals. For steric reasons, events of this type arc more probable with long alkyl groups than with short, particularly since the carhoncarbon bond broken is not the one adjacent to the carbonyl group. I n this brief discussion we have made little or no mention of secondary processes. Even in the case of acetone so many have been suggested that a study of the kinetics becomes hopelessly complex, and definite conclusions concerning the complete mechanism cannot be reached on such a basis alone. Other mean? of proving or disproving individual steps must be used: mirrors for reaction with radicals; use of iodine, nitric oxide, unsaturated compounds, etc. for reaction Trith radicals; introduction of radicals from other sources; fluorescence; wall effects; spectroscopy, t o mention only a fen.. Even these aids to reaction kinetic studies do not always furnish proof free from all doubt concerning individual steps. Seither experimental evidence nor theory predicts much about 1,eactions hetween excited molecules and other molecules. Data on the photochemical reactions of complex molecules must cover many variables. The precision is often poor because of experimental difficulties. *Isa result most mechanisms cannot be established in detail beyond a reasonable doubt. However, one can and should avoid as far as possible the postulation of steps which violate common sense and which are so vague and unsupported by evidence that no experimental method can be devised either to prove or to disprove them. Generally speaking, one should start irith a simple mechanism and complicating steps should be added only when such steps have heen proven absolutely necessary. SiURIM I R T
1. Alcetoneis the only ketone which shows p2sitive evidencr of structure in its spectrum at wave lengths longer than 2500 -1. 2. The appearance of the spectra of complex molecules (particularly those of a low order of symmetry) is not a safe guide t o the nature of the primary procesz, 3. Studies of the fluorescence of acetone indicate that, a t least at 3130 -1. and perhaps at other wave lengths, some activated molecules are produced in the primary process.
550
\I-. ALBERT
XOPES, JR.
4.Both excited acetonc and excited biacetyl molecules can dissociate as a result of additional energy acquired by collisions, t'hus making the yield of the primary dissociation dependent on temperature and, t o some extent, on pressure. 5 . S o positive evidence exists that' simple ketones can dissociate directly into completed molecules, but. a further study of the fluorescence may provide additional information on this point. 6. Ketones possessing long alkyl groups apparent'ly do dissociate into completed molecules in the primary process. 7 . The difficulties of proving all details of the mechanisms of ketone decomposition beyond a reasonable doubt are pointed out. 111.
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( ~ S )ALJIY, '2. A[., .&SI> ( ; I L L E T T E , 1'. lt. : J . C ' l i ~ ~ 1 111 , (41 hJIFORU, (.'. H., .\A-I) ~ O E E l h i i I, t . (;. \v,: .I.
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( T i D - ~ v I sWALL.\O:. , Jit. : I7~ipublis1icdn ~ s u l t r011 :icc'toiic. a1 eltivaied temperatures. (Si DAVIS, \YALL.LC,E.J R . : ~~ii1)iit)IisIied wsults O I I dicili~-1lictonc. (:I) L)avis, \VALL.~CE.J K . ..\SI) SOI-I:~. \Y.-1,.,JR.: J . . h i . Chem. Soc. 69, 2153 (194T). (10,I ~ S C A N .I. , 11. F.:,J. C'1ic.m. PIi>.s.3, 131 (1935~. . 1 7 . : ,I. CIie111. l'hys. 8, 444 (1910~. (11) Dusuax, ~ 1 13. (12h I)r;xc,as, A . 13. I.',, F;I.I.-. 1 , 1 1 . . . i s 1 1 SO YE;.^. M'..\ . . J H . : .I. -\in. C'lieni. SCJC.68, 1454 (1936). V . I t . : J . .111i.('h(*in.Suc. 60, 1YC4 ( I O X W . S o ~ t . : s .IT. J R .: J . .11n. ( k m S w . 60, 2031 (1938). . C'.. J H . .AII) SOYES. \\'. .\.. J K . : J . . h i , Chcni. SOC. 62, 1038 (l94Oj. (161 JIERH,z).S.,.is11 , \Y.X.,J R .: J . -\ni. ('hem. Sot. 62, 2052 (1940). (17) I ~ O W, L J . ,l'.. .ISI) S \V. .i..,In.: J . .41ii. ( ' h e m . SOC.68, 1404 (19361. (18) I1cs.r. 11. I:.,.\XI) . \V. .4., J K . :J. .ini. ('hern. S O C .70, (1048). ( l g ! I,:\\TVSOS, 1\I.iRTH.I. .?SI) I ) V X ( ' . i S . .\, 13, F , :J .('hcni. l'hys. 12, 329 (1914!. (2Oj A[ATHESOS. A I . S . . &SI) SOYES, \ V ,:\.. , J R , : .J. =\III. C'heIi1. SOC.60, 1862 (1938 1 . (21 I AI.iwmsos, AI. S , . .
