_____ SONMRMATEONAL EFFECTS ON THE PBUTOCHEMISTR‘I’ OF SOME Z-PHENYECYCLG- HEXANONES A DEsser‘!a%Eon . iter- fi'ze Degree 04‘ pk. D. MECEEGA‘N SYA'E‘E WWEBSZTY Thomas C Stratton 1975 u,_~v, h ‘9 1:S$s . ’u..\§‘\r .42.; .‘ r ' ‘w. o - 0~ {E 5; 3" :l I - 1.- A 4 A .1 g .5 j . \ P .. i - c: ' .I ' . ‘- o W.‘ ' \ f- " i 2 A 54 918(‘31 ~-’ '- .‘ ‘ ‘3 it: If '. - .’ ’v ..., ‘ '3 .- v 0"“: g: ’ «:3; ."- ' ’nL- V erg" 's'} ' ‘ I ’ .' - x 9;” "1".“ r... - . _ This is to certify that the thesis entitled “\. COIih‘UiLI?x\'I‘IOI‘=£AL hlii‘li‘ifiTS 0N TREE i-‘ilQ’I‘OCEléLBIIb’i .Y 0;" SOL-21:1 2-1’:i£‘£YLC‘z'Cl'JOJEXAEiQNiQS presented by Thomas C. Stratton has been accepted towards fulfillment of the requirements for P113 degree in Chemistry Mfr professor” Date 54"” 11(7'7/ {J / 0-7639 ABSTRACT CONFORMATIONAL EFFECTS ON THE PHOTOCHEMISTRY OF SOME 2-PHENYLCYCLO- HEXANONES by Thomas C. Stratton A careful study of the photochemistry of 2-phenylcyclohexanone revealed an interesting phenomenon. The major photoproducts. cis and trans-G-phenyl-S-hexenal were quenched by dienes at different rates, k T values 1.7 and .41 H". respectively. By measuring the singlet :ifetime (4.4 x 10" sec), intersystem crossing quantum yield (.88) and fluorescence intensity (2 .01) it was determined that the singlet state of 2-phenylcyclohexanone was not one of the quenchable states. Tnans-4-t-butyl-2-phenylcyclohexanone was found to be photoinert while cis-4-t-butyl-2-phenylcyclohexanone formed the expected alkenals cis and trans-4-t-butyl-6-phenyl-5-hexenal, qu values 18.0 and 2.85 M'l, respectively. It was postulated that formation of the cis alkenals could only occur from a twist-boat conformer of the cyclohexanone ring and that the twist-boat and chair conformers formed distinct triplet states of different life- times. CONFORMATIONAL EFFECTS ON THE PHOTOCHEMISTRY OF SOME 2-PHENYLCYCLO- HEXANONES By ev‘l Thomas CfoStratton A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Chemistry 1975 To Dr. David A. Bak for his help, encouragement and understanding ii ACKNOWLEDGEMENTS The author wishes to thank the Department of Chemistry for its support through teaching assistantships and for the outstanding facilities which were made available for conducting research. The author is appreciative of the research funds provided by the NSF and administered by Dr. Wagner. I would like to express special thanks to Drs. Grubbs, Farnum and Eick for their help through many of the difficult times. The friendship of all the many people who I've had the oppor- tunity to meet and know during my stay at MSU has indeed been re- warding. The ups and downs of life have inevitably been shared most closely with the members of "the Wagner group" and to them a special kinship is felt. iii TABLE OF CONTENTS Page INTRODUCTION ..... . . . . . . . . . . . ......... l A. Introductory remarks . . . . . . .......... l 8. Electronic transitions ............... 2 C. Stern-Volmer kinetics . . . . . . . . . . . . . . . . 4 D. Some factors controlling the rates of a-cleavage in 6 cyclohexanones . . . . . . . . . . . . . . . . . . . E. Research objectives . . . . . . . . . . . . ..... l2 RESULTS . . . . . . . . . . . . . . . . . ....... . . . 14 A. Product identification . . . . . . . . . . . . . . . 14 B. Quantum yields . . . . . . . . . . . . . . . . . . . l4 C. Stern-Volmer quenching . . . . . . . . . . . . . . . 16 D. Intersystem crossing quantum yields . . . . . . . . . 16 E. Singlet sensitization . . . . ............ 22 F. Ultraviolet spectra . . . ..... . ........ 24 G. Emission spectra . . . . . . . . . . . . . . . . . . 24 H. Isomerization of B-methylstyrene . . . . . . . . . . 26 DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . 27 A. Products . . . ..... . . . ..... . . . . . . 27 8. Differential quenching . . . . . . . . . . . . . . . 28 C. Singlet reaction . . . . . . . . . . . . . . . . . . 29 D. Conformationally different molecules ........ 32 w TABLE OF CONTENTS (Continued) E.Summary ....... ....... F. Future Work . . . . . . . . . . . .......... EXPERIMENTAL ........ . . . . . . ........... A. Chemicals ................ . . . . . . l. Ketones . . . . . . . . ...... . ..... a. Z-PhenyTC‘YCTOhexanone o o o o o o o o o o o b. Cis and trans 4-t-butyl-2-phenylcyclohexanone c. Valerophenone . Internal standards . a. Tetradecane . . b. Pentadecane . . c. Heneicosane . . d. cis and trans-propenylbenzene . . . . . Quenchers a. l,3-pentadiene (piperylene) . . . . . . b. 2.5-Dimethyl-2,4-hexadiene . . . . . . c. 2.3-Butanedione (biacetyl) . Sensitizers a. l-Methylnaphthalene . . . ......... b. Acetophenone . . c. Acetone ..... . Solvents . Page 38 39 4O 40 4O 4O 41 41 41 42 42 42 42 42 42 42 42 42 42 42 42 TABLE OF CONTENTS (Continued) a. Benzene . . . . . . . . . . . . . . . . . . b. Cyclohexane . . . . . . . . . . . . . . . . B. Instrumentation . . .’. . . . . . . . . . . . . . . . l. Vapor phase chromatography (VPC) . . . . . . . . a. Instrumentation . . . . . . . . . . . . . . b. Columns . . . . . . . . . . . . . . . . . . c. Standardization factors . . . . . . . . . . d. Integration . . . . . . . . . . . . . . . . 2. Infrared spectroscopy . . 3. Ultraviolet spectroscopy . . . . . . . . . . . . 4. Nuclear magnetic resonance spectroscopy . . . . 5. Fluorescence and phosphorescence spectroscopy . C. Techniques . . . . . . . . . . . . . . . . . . . . . l. Irradiations . . . . . . . . . . . . . . . . . . 2.Actinometry.................. 3. Intersystem crossing quantum yields . . . . . . 4. Identification of photoproducts ........ D. Photokinetic data . . . . . . . . . . . . . . .,. . . BIBLIOGRAPHY O I O O O I O O C O O C O O C I O C C ...... vi Page 42 43 43 43 43 43 43 44 44 44 44 44 44 44 45 46 46 48 53 TABLE 8 LIST OF TABLES Page Effect of ring strain, radical stability and product stability on rates of a-cleavage in cycloalkanones . . . 9 Lifetimes and Quantum Yields for 2-Phenylcyclohexanones l5 Piperylene Quenching of 2-Phenylcyclohexanone . . . . . 49 2,5-Dimethyl-2,4-hexadiene quenching of 4-t-butyl-2- phenylcyclohexanone . . . . . . . . . . . . . . . . . . 50 Biacetyl quenching of 2-phenylcyclohexanone . . . . . . 5l l-Methylnaphthalene sensitization of 2-phenylcyclohexanone 51 Intersystem crossing quantum yield for 2-phenylcyclo- hexanone O I O O I O O O I O O O I O O 0 O O O O O O O O 52 Standardization factors . . . . . . . . . . . . . . . . 52 vii FIGURE 1 2 3 10 ll 12 13 LIST OF FIGURES Page Norrish type I and II photocleavages ........ 1 Modified Jablonski diagram ............. 5 Bond rotations necessary for alkenal and ketene formation ..................... 7 Intermediate in alkenal formation from cyclohexanone showing axial and equatorial position of CHQ group . ll Stern-Volmer plot of piperylene quenching of etc (A) and trans (. ) 6-phenyl-5-hexenal products from 2- phenylcyclohexanone ................ 17 Stern-Volmer plot of biacetyl quenching of trans-6- phenyl-S-hexenal from 2-phenylcyclohexanone . . . . 18 Stern-Volmer plot of 2,5-dimethyl-2,4-hexadiene quenching of trans-G-phenyl-4-t-buty1-5-hexenal from cis-4-t-butyl-2-phenylcyclohexanone ........ 19 Stern-Volmer plot of 2,5-dimethyl-2,4-hexadiene quenching of cis-G-phenyl-4-t-butyl-5-hexenal from cis-4-t-butyl-2-pheny1cyclohexanone ........ 20 Intersystem crossing quantum yield determination of 2-phenylcyclohexanone ............... 21 1-Methnaphthalene sensitization of cis and trans-6- phenyl-S-hexenal formation from 2-pheny1cyclohexanone 23 Ultraviolet absorption spectra of 0.01 M 2-pheny1- cyclohexanones in cyclohexane ........... 25 Alkenal formation from the chain formation of trans- 4-t-butyl-2-phenylcyclohexanone showing t-butyl group (top) and carbonyl group (bottom) hindering required rotation of the phenyl group ............ 34 Cis-alkenal formation from chair conformation of cis- 4-t-buty1-2-phenylcyclohexanone .......... 35 viii LIST OF FIGURES (Continued) FIGURE Page 14 Gig-alkenal formation from the twist-boat conformation of cia-4-t-butyl-2-phenylcyclohexanone . . . ..... 36 15 Alkenal formation from the twist-boat fOrm of trans—4- t-butyl-Z-phenylcyclohexanone . . . . . . . . . . . . 37 INTRODUCTION A. Introductory remarks. During the last decade perhaps no area of chemistry has 'generated as much interest as photochemistry. From the opening verses of Genesis to viable solutions for the energy crisis of the 1970's. light and its interaction with matter have been of prime interest and importance to man. Like other reagents, light can create a bewildering array of products from rather simple starting materials. One of the first steps in understanding the whys and wherefores of photochemical reactions is the classification of the various reaction types. During the 1930's R. G. N. Norrish, while studying gas phase photodecarbonylation of aldehydes and ketones. characterized two new photochemical reactions of particular interest to us.1 These reactions are now referred to as the Norrish type I and type II cleavages. The type I cleavage results in breaking the bond between the alpha carbon and the carbonyl, while the type II cleavage results in breaking the alpha-beta carbon carbon bond. 0 2 Type I R-U-CHz-CHzérJHz-CH. ——-> R-C' + 'CHz-CHz-CHz-Cll3 P 8 Type II R-C-CHz-CHz-CHz-CHa ———-> R- -CH3 + CH2=CH-CHa Figure 1. Norrish type I and type II photocleavages. Mechanistic studies have been undertaken on both the type I and II cleavages.2 Rather complete understanding of the mechanistic details of the type II cleavage has been gained through the exhaus- tive studies with phenyl alkyl ketones by Wagner.3 The type I cleavage has been studied most conveniently with five and six membered cycloalkanones.“ This thesis deals with some of the mechanistic aspects of the Norrish type I cleavage in 2-phenyl- cyclohexanones. Pertinent to our understanding of the mechanistic data relating to these photochemical reactions are brief descrip- tions of the following subjects: electronic transitions, Stern— Volmer kinetics, and some factors which affect the rates of a- cleavage in cyclohexanones. 8. Electronic Transitions The absorption of light by an aliphatic ketone results in the promotion of an electron from the ground state to an excited state. Most stable ground state molecules have their electrons paired. The net electron spin is zero and therefore the multi- plicity is one, (28 + 1). Selection rules require that the tran- sition between energy levels occur without a change in multi- plicity.5 Although forbidden by quantum mechanics, the electron can change its spin with the help of spin orbital coupling, pro- ducing an excited state molecule with a net electron spin of one and a multiplicity of three. The molecule is thus referred to as being in a triplet state. The triplet state is lower in energy than the singlet state according to Hund's rule.6 Due to very rapid internal conversion (10"12 sec) from higher to lower vibrational levels, both the singlet and triplet states rapidly reach their lowest vibrational state.7 The radiationless process involving electron spin changes is called intersystem crossing. Triplets are generally longer lived than singlets, since triplets require another forbidden spin flip before they can return to the ground state. An excited state may be characterized by the origin of the electron which was promoted as well as by the molecular multi- plicity. Aliphatic ketones have only one readily available source of electrons for excitation, the n (nonbonding) electrons on the oxygen of the carbonyl. ‘Excitation of these electrons to the n* antibonding orbital is design as n,n* excitation.a Aromatic ketones. ketones which have their carbonyl conjugated with an aromatic system, show another type of transition due to the availability of n electrons, which is designated n,n*.° The n,n* transition is for- bidden by symmetry and therefore occurs with less intensity than the allowed n,n*. The nature of an n,n* triplet state ketone has been compared to that of an alkoxy radical.’ The typically nucleophilic charac- ter of oxygen, often seen in its ground state reaction. has been re- versed in the n,n* state. Excitation of the n electron on oxygen has decreased the electron density of the oxygen and thus oxygen behaves as an electrophile in the excited state. The similarities between alkoxy radicals and n.n* triplet ketones towards hydrogen abstraction is a striking and well studied phenomenon.‘° Rapidly, by a variety of pathways, molecules in the excited state return to the ground state. From the singlet state a molecule may intersystem cross, fluoresce, radiationless decay or undergo chemical reaction. From the triplet state a molecule may phosphoresce, radiationless decay or react. These processes are graphically represented by the Jablonski diagram,11 Figure 2‘. The quantum yields for intersystem crossing are high for many carbonyl compounds.12 Photochemical reactions can occur from either the singlet or triplet state. The rate of reaction has to compete with the rates for all other processes occurring from the excited state. The quantum yield of singlet reaction, in carbonyl compounds, is many times limited by the rate of intersystem crossing. The rates of intersystem crossing in aliphatic ketones have been measured by Yang et al.13 It was found that the rates of inter- system crossing for many aliphatic ketones were in the range of l - 5 x l0° sec.". C. Stern-Volmer kinetics. Under steady state conditions one may measure directly only one quantity, the quantum yield.‘“ The quantum yield may be de- fined as the number of molecules reacting in a given way divided by the number of photons that have been absorbed. Excited state lifetimes may be measured indirectly by quench- ing experiments. Assuming a steady state concentration of excited state molecules, one may by quenching of the excited state determine the relative decrease of photoproduct as a function of quencher con- centration. The Stern-Volmer equation ¢0/¢ = l + qu[Q] expresses this relationship.15 o0 equals the quantum yield of photoreaction without quencher present, ¢ equals the quantum yield of photo- reaction with quencher present, kq is the rate constant for INCREASING ENERGY (1°) (ISC) (ISC) WV _L g 1 ILL] I Tx11L So :iérfi:fF—§E:3E:; GROUND.STATE ICS intersystem crossing IC internal conversion Figure 2. Modified Jablonski Diagram quenching, which is usually assumed to be diffusion controlled and thus obeying a modified Debye equation, kq = 8RT/2000n.16 Tau, r, the lifetime of the excited state is defined to be equal to 1/ (k1 + k2 + ‘°°kn) where k represents rate constants for all other processes that occur from that excited state. Stern-Volmer plots of oo/o versus quencher concentration should be linear with the slope equal to k I. By using a previously determined value for k q q calculate T from the slope of such plots. The excited state life- one may time may be equated to the reciprocal rate constant for reaction if the rate constant for reaction is large compared to all competing rate constants. Steady state kinetics for various excited state situations have been delineated by Wagner.‘“ 0. Some factors controlling the rates 9: a-cleavage and the efficiengy_gf product formation lg gyclohexanones. The major products formed from the photolysis of cyclohexanone in inert solvents, 5-hexenal and l-hexenal, may be accounted for by a simple biradical mechanism arising from a-cleavage of the parent ketone. The 1,6-biradical formed by o-cleavage may disproportionate by intramolecular hydrogen abstraction. The suggestion that cyclo- hexanones reacted by a concerted mechanism appears to have been based upon inaccurate data.17 The acyl radical can form 5-hexenal by abstracting a hydrogen alpha to the alkyl radical. Conversely, l-hexenal can be formed when the alkyl radical abstracts a hydrogen alpha to the acyl radical, Figure 3. Ketenes, such as l-hexenal, are usually trapped by alcohols as the corresponding esters or by amines as amides. Figure 3. Bond rotations necessary for alkenal and ketene formation. As the number and functionality of substituents on the cyclo- hexanone ring increase or as the reactivity of the solvents increases, the number and complexity of the photoproducts increase. Indeed, a bewildering array of products are possible. However, simply substi— tuted cyclohexanones, photolyzed in inert solvents, provide a con- venient vehicle for the study of the factors affecting the rates of a-cleavage and the efficiency of product formation.from the biradical without untold complications. Disappearance of cyclohexanone, upon photolysis in inert solvents such as benzene, has been accounted for by formation of alkenal and ketene products.18 Any discrepancy between ketone dis- appearance and product appearance has been attributed to minor side products such as caused by photoreduction and/or decarbonylation. Surprisingly few quantum yields for ketene formation have been re- ported along with the rather more numerous quantum yields for alkenal formation.“’1" The alkenal/ketene ratio would be expected to be particularly sensitive to substitution in the 2 and 3 positions. The hydrogen abstracting ability of both the acyl and alkyl radicals are pre- sumably related to their own stability and the strength of the carbon-hydrogen bond which is to be broken. The stability of the acyl radical should remain relatively unchanged by substitution on the cyclohexanone ring.thile alkyl radical stability will in- crease with substitution in the 2-position. The strength of the carbon-hydrogen bond decreases as substitution changes the secon- dary 2 or 3 position carbon into a tertiary carbon. The rates of o-cleavage in cyclohexanones may be interpreted as depending primarily upon two factors; 1. ring strain and 2. alkyl radical stability. The efficiency of product formation, as indi- cated by quantum yields, will reflect both the efficiency of a- cleavage and the ability of the 1,6-biradica1 formed from a-cleav- age to disproportionate to products. The rates of a-cleavage and the efficiency of disprOportionation are strongly affected by sub- stituents on the cyclohexanone ring. It has been found that sub- stituents as simple as methyl groups in the 2,3, or 4 positions can affect both the rate of a-cleavage and/or the efficiency of product formation. The effect of ring strain on the rates of a-cleavage can be seen in the following series of ketones, cyclohexanone, cyclopenta- none, 2-methylcyclopentanone and norcamphor, Table 1. Since nor- camphor a-cleaves to a secondary alkyl radical the rate of a-cleavage should be compared to that of 2-methylcyclopentanone. Clearly as the degree of ring strain increases so do the rates of o-cleavage. Alkyl radical stability is increased by substitution at the 2-position. The rates of a-cleavage increase with the increase in alkyl radical stability. The effect of alkyl radical stability is clearly and simply seen in the following series of 2-substituted cyclohexanones; cyclohexanone, 2-methylcyclohexanone, 2,2-dimetbyl- cyclohexanone, Table l. The rates of o-cleavage in 3-methyl, 3,5-dimethyl, and-3,3,5-tri- methyl cyclohexanone were found by Wagner and Spoerke8 to be virtually identical to each other. The rate of a-cleavage from these three 8- Table 1. Effect of ring strain, radical stability and product stability on rates of a-cleavage in cycloalkanones."’“”2°’21 EEEQDE- Rate a—cleavage, ¢alkenal ¢disappearance Cyclohexanone 1.1 x 107 .08 .20 2-Methylcyclohexanone 2.5 x l0a .42 .50 2,2-Dimethylcyclohexanone 1.8 x 109 .41 .52 2,2,6,6-Tetramethylcyclo- 7.8 x 10’ --- --- hexanone Cyclopentanone 1.1 x 10° .24 .28 2-Methylcyclopentanone 3.6 x 109 .26 --- 2,2,5,5,-Tetramethyl~ 5.0 x 101° .61 --- cyclopentanone Norcamphor 5.0 x 101° .28 --- substituted cyclohexanones was found to be approximately twice as fast as the rate of a-cleavage from cyclohexanone. Approximately the same rate increase towards a-cleavage compared to cytlohexanone was found with 4-methylcyclohexanone . All a-cleavage rates, are assumed to be 10 equal to the reciprocal of the ketone lifetime, were measured by Stern-Volmer quenching of alkenal products. These rate increases in d-cleavage, caused by substitution in the 3 and 4 position are not reflected in larger quantum yields for product formation. In fact the quantum yields for alkenal formation from these ketones (.033, .005, .002, respectively) are smaller than for cyclohexanone and markedly smaller than for many 2-substituted cyclohexanones, Table l. The lack of correlation between rates of a-cleavage and efficiencies of alkenal formation suggest that there are signifi- cant substituent effects on the biradical behavior. Wagner and Spoerke from their study with B-substituted cyclohexanones found that the low quantum yields for alkenal formation were consistent with equally low quantum yields for ketone disappearance, .083, .003, .024 for 3-methyl, 3,5-dimethyl, 3,3,5-trimethyl cyclohexa- none, respectively. Regardless of the direction of o-cleavage, with these three 8-substituted cyclohexanones, primary alkyl radicals are produced. It was conceivable that there might be a preference for a-cleavage towards the more substituted side. Alkenal stability is in- creased by substitution in the 3-position by increasing double bond stability and by decreasing B-carbon hydrogen bond strength by substitution in the 3-position. Both Wagner and Spoerke18 and Agosta and Schreiber23 report some preference for a-cleavage away from the substituted side, although the degree of preference is not at all clear. The surprising preference for a-cleavage away from ll the substituted side, along with the marked decrease in the efficiency of product formation suggest that the presence of even one B-methyl group seriously impaires the rotation of the biradical. By deuterium labeling experiments with 3 and 4-methylcyclo- hexanone Agosta and Schreiber23 determined the relative amounts of axial and equatorial hydrogen transfer from C3 carbon to the acyl radical. It was found that two-thirds of the aldehydic hydrogens in the alkenal products were originally from the axial position and one-third from the equatorial position. In Figure 3 rotation (a) would be favored over rotation (b) by a 2:1 margin. Intermediate 1, Figure 4, where the CH; group can maintain its equatorial position, would then be favored over intermediate 2, with substitution of a 3-methyl group then either rotation would require that the methyl group or the CH; be in an axial position. -C 96 mgd H a CH2 1 Figure 4. Intermediates in alkenal formation from cyclohexanone showing axial and equatorial positions of the CH; group. Wagner and Spoerke reported that for the series of ketones which they studied all alkenal formation occurred from the triplet l2 state. Turro et al.21 have shown that the rate of a-cleavage from the singlet is at least two orders of magnitude slower than for a- cleavage from the triplet state. As the number of a—alkyl sub- stituents increase, the rate of a-cleavage increases. Singlet a- cleavage should start to compete with intersystem crossing when the rate of a-cleavage from the triplet approaches 101° sec". Eastman and Beard2° have suggested that the singlet state of 2,2,6,6-tetra- methylcyclohexanone is sufficiently reactive to a-cleave in competi- tion with intersystem crossing. E. Research Objectives l). The major product from the photolysis of 2-phenylcyclohexa- none was unidentified by Wagner and Spoerke.18 Baum31 later reported that the product was trans-G-phenyl-S-hexenal. Since the VPC separa- tion of the reportedly cis and trans isomers of 6-phenyl-5-hexenal was extraordinarily large, unlike that of other eta-trans isomers, Baum's report was somewhat puzzling. 2). The published rate of o-cleavage from 2-phenylcyclohexanone was inconsistent with the rates of a-cleavage from other 2-alkyl- cyclohexanones. 3). Wagner and Spoerke reported that the unidentified product was unquenchable, while a minor product was quenchable. This minor product was assumed by Wagner and Spoerke to be 6-phenyl-5-hexenal. Baum later confirmed that the product was the cis isomer. The possibility that the cis-alkenal was quenchable while the trans- alkenal was not suggested that perhaps competing photochemical mechanisms were operating in the photolysis of 2-phenylcyclohexanone. l3 4). If the trans isomer proved to be the major product, then this reaction would represent a very stereospecific photochemical reaction since the trans to cis alkenal ratio would be 45/1. A careful study of 2-phenylcyclohexanone was undertaken to sort out the nature of these apparent anomalies. RESULTS The unraveling of the mechanistic details of 2-phenylcyclo- hexanone photochemistry began with the identification of the major photoproduct. Careful quenching experiments and quantum yield determinations on the two major photoproducts revealed surprising results. Further work determined the quantum yield for intersystem crossing and the singlet lifetime. A hypothesis to explain the surprising results of the quenching experiments was tested by studying the photochemistry of cis and trans-4-t-butyl-2-phenyl- cyclohexanone. Product identification, quantum yields and quench- ing studies were determined on these two ketones. The results from all of the ketones studied are presented in detail below. A. Product Identification The major photoproduct from 2-phenylcyclohexanone was found to be trans-G-phenyl-S-hexenal while cis and tnan3-6-phenyl-44t-butyl~5- hexenal were identified as the major photoproducts from cis-4-t- butyl-Z-phenylcyclohexanone. Photolysis of the parent ketones in benzene at 3130 A followed by careful chromatographic separation of the resulting photolysis mixture yielded the photoproducts. IR and NMR analysis confirmed the identity of the products. B. Quantum Yields Quantum yields for alkenal formation and ketone disappearance were measured for both 2-phenylcyclohexanone and I4 15 Table 2. Lifetimes and Quantum Yields for 2-pheny1cyclohexanones a Piperylene quenching_9f_2-Pheny1cyclohexanone Product kq 1' gb Cia-G-phenyl-S—hexenal 1:69i,15 .Ol Trans-G-phenyl-5-hexenal 0.41:,04 .45 (Disappearance) .65 T-..S_e_