Photochemistry of Phenyl Alkyl Ketones: The "Norrish Type 11" hot ore action An Organic Photochemistry Experiment

BronMaw Marciniak A. Mickiewicz University, 60-780 Poznali, Poland

The photochemistry of ketones has been extensively stud- ied for many years and the main factors that determine the photochemical properties of ketones are now well character- ized (la). The phenyl alkyl ketones with a hydrogen atom in the y position can undergo the photochemical intramolecu- lar hydrogen abstraction, the so-called "Norrish type 11" process (Fig. 1). This reaction is probably one of the best understood photochemical processes and can be an excellent example of the photochemical student's experiment. After absorption of incident light by ketone molecules (Fig. 1) and an efficient intersystem crossing process (%sc = 1) the low- est excited (n, s*) triplet states (3K) are generated. The key step of the "Norrish type 1I"reaction of phenyl alkyl ketones is the formation of a ',*-diradical (D) from the triplet state as the primary process. Further reactions of diradicals such as cleavage of C-C bond to produce enol (indicated as aceto- phenone) and olefin (Fig. 1, eq I), cyclization (eq 21, and disproportionation to regenerate the starting ketone (eq 3) can explain the observed results. The above mechanism has been proved by direct observation of the intermediates: trip- lets and diradicals in laser flash photolysis as well as by trapping experiments (la, 2).

The interaction of a variety of quenchers with the triplet states and diradicals of phenyl alkyl ketones has been the suhiect of extensive studies. The diradicals have been shown to undergo several types of reactions such as H abstraction, electron transfer, addition to double bonds, interactions with naramametic snecies. etc.. de~endina on the nature of . . . quenchers used ( 3 ) . ' ~ h e triplet states ut:'I~henylalkanones have been alsoshown to hequenched 11y different quenrhers via physical or chemical mechanism.; (4) .

The auenchine Drocess is iften analyzed by the help of so- called s t e r n - ~ o r i e r relation (see ref i b ) :

-

" = I + K [ Q ] 0

(4)

Flgwe 1. me Nonlsh type I1 process

where K is the Stern-Volmer quenching constant (M-I), C'O

and are relative efficiencies of photoreaction (or emission) in the ahsence and Dresence of a ouencher. IQ1 is the auench- . .. erconcentration (M), k, is thequenching rateconsfant (M-I s-I), and r is the lifetime of an excited state in theabsence of a quenrher(s~. The plot of W / + versus concentration 191 is uredicted to be a straiaht llne with the slope equal to K = . -

i ,r. If the lifetime is known, the quenching rate constant kg can be calculated.

In this paper a laboratory student experiment is described aimed ro study the "Norrish r w e 11" reaction of valerophen- one. The ad;antage of this experiment is that it can be performed with a simple irradiation system and with a gas

832 Journal of Chemical Education

chromatwraoh. which is readilv available in chemical lab- o r a t o ~ i e s . ~ ~ t brings students into contact with some prob- lems of organic photochemistry and teaches them t h e basic techniques used in this field of research.

Experimental Valeroohenone (Janssen) was distilled orior to use and shown to

he free irom scet'onhenone hv GC analkis. Methanol (Sneetro- ~ ~~~~ ~~~ ~ ~, ~ ~ . . ~ . cadel , acetophenone, pwpen, and triphenylphonphine are readily available commercial products and were used as received.

The gas-phase chromatographic (GC) analyses can be performed on any gas chromatograph. In the experiment described the Chrom IV chromatograph (Flame Ionization Detector) with 5% Carbowax 20M on 60-80-mesh Chromosorh G (3.5 m) was used, GC-MS ana- lyses were carried out with JEOL JMS-D-100 mass spectrometer coupled with a gas chromatograph (Carhowax 20 M column). MS spectra were obtained in electron-impact mode at 75 eV. Ultraviolet absorption spectra can he recorded on any spectrophotometer.

Irradiation Technique The irradiation can he carried out in small Pyrex tubes in any

"merry-go-round" apparatus with, for example, a high-pressure mercury lamp and a filter for excitation X > 330 nm. The irradiation system used in the proposed experiment is presented in Figure 2.

Figure 2. Irradiation system. (1) high-pressure mercury lamp HBO 200 (Nawa). (2) diaphragm, 13) thermal filter (10-cm quarlz cell wilh distilled water). (4) quartz or Pyrex lens, (5)glass filter(h > 330 nm). (6) "merry-gmund'' system (basket with six holes for phctotubes), (7) cylindrical protector.

Figure 3. Phototube used for deaeratlon and lrradlatlon. (1) Pyrex tube (I-cm diameter X 10-cm length). (2) Rotaflo stopcock. (3) rubber septum, (4) nee- dles

All experiments must he carried out under oxygen-free conditions because of the danger of interaction between triplet and diradical with oxygen (5). Therefore, the simple and easy-for-the-students deareating procedure is proposed. The phototuhes are equipped with a Rotaflo stopcock as in Figure 3.

The oxygen-free argon (or nitrogen) has been bubbled through the solution in the tube for about 15 min. Then the long needle was drawn over the stopcock but still below the ruhber septum, and argon was still running through the needle. Then the Rotaflo stop- cock was closed, and needles were taken out.

Procedure Three experiments are proposed showing the typical experimen-

tal procedures used in the organic photochemistry. In the first one the eeneral reaction oattern is studied (identification of oroducts). - Then. in the second exoeriment. the chances of efficiencies of orod- , ~~~ . "~ ~ ~ ~ ~ ~ - - -~ 7~ ~

uct fo;marionasafunctic,n oiirradintim timesarc mensured. In the third experrment the quenching i,f the photoreaction is studied using typical Stern-Volmer analysis.

Experiment 1: Identification of Photoproducts A methanol solution (4 mL) containing valerophenone (0.1 M)

was deareated in the phototube and irradiated. The irradiation time was chosen to cause about 20% of ketone conversion. The sample was analyzed by GC and the retention times of products were com- pared to those of authentic samples of acetophenone and propene. The solutions before and after irradiation were also analyzed by UV- absorption spectroscopy. The solutions were diluted 60 times in methanol, and their UV spectra were recorded in the region of 270- 400 nm in 1-cm quartz cells. For more advanced students the GC- MS analysis of reaction mixture is proposed. The photolysate is condensed under vacuum and injected into the GC-MS system.

Experiment 2: Efficiency of Acetophenone Formation as a Function of Irradiation Time

Three or more phototubes containing 4 mL of methanol solution of valerophenone (0.1 M) each were deareated and irradiated in the "merry-go-round" system. Irradiation times were chosen to eause 2- 15% of ketone conversion. After irradiation the samples were ana- lyzed by GC and the relative efficiency of acetophenone formation as a function of irradiation time was measured using nonanol-1 as an internal standard.

Experiment 3: Quenching of the "Norrish Type N" Reaction by Triphenylphosphine

Before the quenching experiment the students ran be asked to record the UV spectra uf methanol solutions of ralerophenone 10.1 Mjand triphenylphosphine (U U:lU MI topm\,e that the keton~is the only sprries absorhmg the exciting light.

l'hcphototuherunrsining4 ml.of0.1 M valerophenoneinmetha- no1 and five i lhototut,e~c~t~taininp: I ml.of methanol st,lutron of O I M valeroph~nane and concentrat& of PPh3 ranging from 0.005 M to 0.030 M were deareated and irradiated in the "merry-go-round" system. Irradiation time was chosen from the results of experiment 2 to cause about 5% of valerophenone conversion. After irradiation the samples were analyzed by GC using nonanol-l as an internal standard. The ratio of acetophenone efficiencies PI* (eq 4) was determined as an area ratio of acetophenone peak in the absence and presence of the quencher, measured relative to the standard peak.

Results

T h e photolysis of valerophenone in methanol results in t h e formation of acetophenone, propene and cis a n d t rans isomers of l-phenyl-2-methylcyclobutanol (Norrish type I1 products, Fig. 1). Typical results of GC analysis a re present- ed in Figure 4.

Acetophenone a n d propene were identified by a compari- son of their retention times with those of t h e authentic samples (two temperatures of column). Additionally, aceto- phenone and cyclobutanol derivatives were identified by GC-MS analysis:

acetophenonemle (relative intensity): 121(3.4), 120(M+, 41), 106(8), 105(100), 78(9), 77(83), 51(30), 50(11),

1-phenyl-2-methylcyelobutanol (isomer with shorter retention time, probably trans isomer) mle (rel. int.): 162(M+, 3) 135(9),

Volume 65 Number 9 September 1988 833

Figure 4. Typical example ol GC anaiysisafler photolyais of 0.1 M vaierophen- one in methanol (Carbowax 20M. temperature of column 150 'C, relative sensitivily in parentheses).

134(33), 133(25), 120(100), 105(16), 91(15), 78(43), 77(42), 51(18), 1-phenyl-2-methylcyclobutanol (isomer with longer retention time,

probably cis isomer) mle (rel. int.): 135(8), 134(7), 133(12), lZO(lOO), 105(56), 91(10), 78(40), 77(36), 51(12).

UV absorption spectra do not change significantly during irradiations (small conversion of valerophenone, formation of acetophenone as the main product). The progress of reac- tion, however, can be easily observed by means of GC analy- sis. The increase in the efficiencies of products as a function of irradiation time was measured in the experiment 2. The efficiency of acetophenone, determined as the area ratio of acetophenone to the internal standard, as a function of the irradiation time gave a straight line within the experimental error (for small conversion of valero~henone used in the experiment 2).

Twical resultsobtained for the quenching of Norrish type I1 reaction by triphenylphosphine are shown in ~igure-5. Experimental plot of PI@ vs. [Q] is linear, and the data satisfy eq 4 within the experimental error. The intercept is 1.0, and the slope is calculated to he K = 34 M-I. Assuming that only thevalerophenone triplet is quenched by PPh:, (see ref 6), the quenching rate constant can be calculated, from

Figure 5. Experimental Stem-Voirner plot of +'I+ M. triphenylphosphine concentration for Norrish type ii reaction of valerophenone in methanoi.

eq 5, as k, = 1.9 X lo9 M-1 s-', taking the lifetime Of valerophenone triplet in methanol as r = 16 ns (2).

The work of Y. L. Chow together with the author of this naDer (6. 7) ins~ired the latter to describe an experiment for &tochkmicalkducation.

For more advanced students, experiment 2 can be extend- ed to the determination of the quantum yield of Norrish type I1 reaction. In this case, however, monochromatic excitation light is required, and the light intensity absorbed by ketone under photolytic conditions should be measured by conven- tional chemical actinometer, for example, valerophenone in benzene (8). Additionally, instead of the efficiency of aceto- pheuone formation, its molar concentration should be deter- mined as a function of irradiation time. It is important to note that type I1 quantum yields of phenyl alkyl ketones depend on the conversion of ketone (9). Therefore, as sug- gested in this paper, small values of ketone conversion are required during irradiations. In order to be absolutely sure of a value, one must extrapolate the measured quantum yields to zero conversion of ketone (9).

Literature Cited 1. Turro. N. J. Modern Molerulor Phoforhemistry; Benjamin: Menlo Park. CA, 1978; (a1

pp 362-392,528-544; (b) p 247. 2. Srnal1.R. D., Jr.:Sesisno,J. C. Chem. Phys.Lett. 1977,50,431;J.Phya. Chem. 1977.81,

"."c *.*". 3. Scaisno. J. C. Acc. Chsm. Re*. 1982,15,252. 4. Scsiano, J.C. J.Pholochem. 1973/74,2,81; J. Phoforhsm. 1973/74,2,471.

C.A.;Encina,M. V. Reu.Cham.Inferm~d. 1978.2,139. 6. chow.^. L.;~~~~ikak,B. J.Olg. Chem. 1383.48.2910. 7. Msreiniak,B.:Chow.Y. L. J.Phoforhsm. 1986.32.165. 8. Wagner. P. J. Telrohedmn Left. 1967.1153: Wagner, P. J.; Kemppainen, A. E

Chom. Sor. 1972.94.7495. ten, A. F. J. Am. Chem. Soc 1972.94, 9. Wagnn, P. J.; Koehevar, I. E.; Kernppair

J. Am.

7489.

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834 Journal of Chemical Education