J. W. Haas, Jr. Gordon College Wenharn, Massachusetts 01984
The Photoisomerization of Cyclic Ketones A n experiment in organic chemistry
The recent growth of photochemical knowledge and current high level of research interest are little reflected in laboratory texts in organic chemistry. Typical experiments are usually limited to excitation in the visible region of the spectrum by sunlight or a tungsten lamp which may involve often lengthy exposure times and heat dissipation problems as well as the inherent restriction to relatively low energy processes. The recent appearance of a low cost uv photochemical reactor stimulated our interest in designing an experiment which would illustrate some major themes in photochemistry a t higher energy levels. This experiment deals with parameters such as the nature of the excited state, effect of triplet quenchen on product formation, chemical structure and reaction rate, and quantum yield when cyclopentanone and cyclohexanone are irradiated at 254 nm. These cyclic ketones provide a varietv. of nhotolvsis information in a short time . span, are conveniently analyzed by gas chromatography, and are readily available at the requisite level of purity. Background
The photochemistry of ketones is the most widely investigated of any class of compounds ( I ) . Photolysis of cyclic ketones was reported over 50 years ago and a large body of detail is known for the gas phase reaction (2). However, a systematic study of cyclic ketones in the liquid phase has been undertaken only within the last decade (3-6). This recent work has defined the nature and rate of product formation and provided insight into the reaction process. On liquid phase irradiation a t 254 or 313 nm, gas chromatoma~hicanalvsis shows 4-~entenalto he the onlv sienificak broduct from cyclopentanone with 5 - h e x e d an> smaller auantities of 2-methvlcvclonentanone found for cyclohexaione. The rate of prod;ct fbrmation is 3-5 times more rapid for cyclopentanone than cyclohexanone. Formation of 4-pentenal is linear with time to a t least 10% conversion while the rate of 5-hexenal formation decreases markedly with time (3). The 2-methylcyclopentanone, in contrast, does not show this effect forming a t a constant rate (3). Oxygen, 1.3-pentadiene, and napthalene significantly quench product formation while yields are insensitive to the addition of hexane, benzene, and allyl alcohol (6-8). These data are consistent with a reaction pathway illustrated for cyclohexanone.
'Bradford Scientiftc, Inc., P.O. Box 275, Marblehead, Mass. 01945. 346
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The Drocess resulting in ring contraction is not clearly nndersiood. Because the r e i t i o n is insensitive to triplet quenching Srinivasan and Cremer proposed a concerted skeletal Garraneement from an initial sinelet state involving "transfer of a proton from the 3- to the 2-position in a configuration in which the ring is substantially intact (3)." However, there is some evidence that ring contraction may stem from a secondary biphotonic rearrangement of 5-hexenal instead of from a primary photoprocess (4). Although some ketene may he found in the reaction process it is not seen due to a lack of stability under the in gas chromatographic conditions tvdcallv emnloved . . analysis of a product. A ring contraction product is found in low yield upon irradiation of cyclohexanone and higher ketones (3). Although the details of this reaction are not known, the effect of trinlet ouenchers clearlv indicates that the carhonyl triplet state'is a precursor both in ring contraction and alkenal formation. the hieher vield " The observation that product formation is more rapid for cyclopentanone is in marked contrast to typical ground state reaction rate comparisons between five- and sixmembered ring ketones (9). It is well established that a sp2 hybridized carbon atom is incorporated into a fivemembered ring more readily than into a six-membered ring. However, the triplet (and singlet) excited states for formaldehyde (and by analow cyclic ketones) appear to he pwamidal species similar to amines thus affording the possibility that the carhonyl carbon exhibits some sp3 character, resulting in easier incorporation into a sixmembered ring (5). This rate effect is paralleled in the photoreduction reaction of cyclic ketones and illustrates the marked difference which may exist between the geometries (and reactivities) of the ground and the excited state
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(4).
Experimental
Chemicals Aldrich 99+% cyclopentanone and 99.8% cyclohexanone were found suitable for use without further purification; Zmethyleyclopentanone was obtained from Chemical Samples Co. The eommercially unavailable 4-pentenal and 5-heaenal may be prepared by extended photolysis of the ketone and purification by preparative gas chromatography. We found n-pentanal and n-hexanal to be useful substitutes for the alkenals in gas chromatographic peak identification as retention times are almost identical. Other reagents were obtained from standard sources. Apparatus Excitation was carried out in a Bradford semi-micro photochemical reactor using the 254 nm lamp. The reaction ve3sel was either a 17 x 150 mm fused silica test tube used for bulk reaction or where oxygen was introduced or a smaller 5 x 150 mm tube. The latter tube was of advantage in allowing use of smaller sample sizes and enabling the simultaneous use of up to six samples in the reactor port as well as in providing enhanced product cancentration per unit time when compared to the larger tube. If stirring was desired the smaller tubes were affixed with plastic tape to a thin glass rod which was connected to the chuck of a variable speed stirrer. Oxygen for quenching experiments was admitted to the reaction vessel through a Kontes K-95600 gas dispersion tube.
Gas chromatographic analysis was carried out with an F&M Scientific Model 700 using the thermal conductivity detector under the following conditions: column, 6 ft x 0.125 in., 10% U C W-98 on 80-100 mesh Chromosorb W; gas, helium st 60 ml/min; injector, 150°C;column, 1 W C ; detector, 210°C;sample, 0.54; attenuation, XI. Approximate retention times (in min) were observed for the following compaunds: air, 0; 4-pentenal, 0.6; cyelopentanone, 1.2; 2-methylcyclopentanone, 1.5; cycloheaanone, 2.25.
Discussion of the Experiment
The approach to this experiment may vary widely in order to meet course objectives and limitations in time and equipment. We have developed this study as a quasiresearch problem with students working in teams of two to four purposing to find out as much as possible about the reaction systems in a given time allotment. Emphasis is placed on experimental design and mechanistic considerations. The student is introduced to photochemistry in a pre-lab session dealing with excitation energies, the properties of excited states, and terms used in describing photochemical reactivity (10). Qualitative and some quantitative studies may be carried out using the liquid ketone. For comparison of relative reactivities, 5 M solutions of the ketone in benzene or hexane may be employed. Care must be taken to achieve reproducible reaction vessel geometry and sample volume in comparison studies. At appropriate time intervals samples are removed by syringe from the reaction vessel for gas chromatographic analysis. Liquid and solid quenching agents may be added a t concentration levels ranging from 10-2-1 M. The effect of varying rates of oxygen bubbling may be of interest. Reaction solutions were routinely gassed with nitrogen for 10 min before use.
The time required to collect meaningful data will depend in part on the intensity of the light source and gas chromatographic equipment sensitivity. We have found 45-60 min to he sufficient for obtaining a broad range of product concentrations for cyclopentanone. The slower reacting cyclohexanone requires a 2-3 hr period for study of alkanal formation and 6-12 hr for the ring contraction product. Students may wish to expand the study to include the four., seven- and eight-membered cyclic ketones. Cycloheptanone and cyclooctanone provide a variety of products at relativelv low rates which may be rationalized in terms of a-cleavage following the formation of triplet carbonyl. In contrast, cyclobutanone provides markedly different products and is not influenced by triplet quenchers (4-6). Plots of retention volume versus time for product formation under specific reaction conditions provide a format from which the student may draw conclusions concerning reaction pathways. Literature Cited (I) c n l w n , J. G.. and ~ i t t a J. . N., " ~ h a t & m i ~ t r y . ' ~ o h nwiey 8 sons. ~ n e .N, ~ W York, 1966, p. 377. (2) Sti"i"..... R.. in "Advances in Phofoehemistri," Wiley-1nfcmeiene. P"biih.ra, New Yark. 19P. Vol 1. p 8 3 (3) Stinivsan,R.,andCremor, S.E., J A m e r . Cham. Sor., 87.1€41(1966). (4) Dalton. J. C., and Turn,N. J.,in "Annual Rcviea.of Physical Chemistry," Annual Review, Ine. Palo Alto, 1970, Voi. XXO,p. 499. ( 5 ) Tuna. N. J.. Dslfon, J. C., Dawes. K., Farrington. G.. Hautala, R.. Morton, D., NiemcryL.M.,sndSehcae, N..AccountsChem.Res., 5.92 119721. (6) Tuna, N . J., end Morton,D. R., J Amer Chom Soc., 95,3947 (1973). (7) Dunion,P.,and~mbore,C.N..J . A m m Chsm. Soc.. 87.4211 (1966). (8) Wagner, P.J.,andSpoerke,R.W., J.Amer Chem. Soc.. 91,4437(1969). (91 Elid, E. L., Allinger. N. L., h g y a l , S. J., and Moniaon. G , A., "Canformational Analysis: Wiley-lntcncience Puhliihen, New York. 1965. p. 196. (101 Tuno,N . J., J. CHEM.EDUC.44,636(19671.
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