June, 1963
PHOTOCHEMICAL DISAPPEARANCE OF BENZOPHESONE ISISOPROPYL ALCOHOL
134:l
THE EFFECT OF LIGHT INTENSITY AT 313 mp ON THE PHOTOCHEMICAL DISAPPEARANCE OF BENZOPHEKONE I N ISOPROPYL ALCOHOL BYA. C. TESTA Lever Brothers Company, Research Center, Edgewater, Ai. J . Received January 86, 196s Results obtained indicate that the quantum yield for the photochemical disappearance of benzophenone in isopropyl alcohol at 313 mp is intensity dependent. This effect is attributed to two competing processes. At high light intensities, the quantum yield approaches one, which arises from a predominance of the direct coupling of the ketyl radicals, (C6H&COH, formed by hydrogen abstraction from isopropyl alcohol by an excited benaophenone molecule. As the intensity is lowered, the contribution of an additional reaction path becomes significant, namely, one in which two hydrogen atoms are abstracted from each isopropyl alcohol molecule and two benzophenone molecules react for each quant>um of absorbed light. This latter scheme corresponds to @ = 2 for benzophenone disappearance. As a result of these competing reactions, the quantum yield for benzophenone disappearance has one as its lower limit and a maximum value close to two. I n the experiments described, the quantum yield varied between 1.12 and 1.62, while the intensity ?vas changed by a factor of one hundred. A plot of the reciprocal square root of light intensity 2;s.quantum yield was linear and a least squares calculation of the slope yielded a value of 2.43 i 0.25 X 106 molecules-quanta-'~~-sec.-'/2. The linearity of the plot is indicative of the predominance of radical recombination reactions.
Introduction The mechanism of the photochemical disappearance of benzophenone in isopropyl alcohol has been an active problem in the photochemical literature.1-6 There appears l,o be some disagreement as to whether the quantum yield for benzophenone disappearance should be one or two in degassed solutions. I n the results reported by Pitts, et uZ.,I the quantum yield a t 366 mp for benzopinacol formation in a 0.5 lkl benzophenone solution in isopropyl alcohol is 0.93, which corresponds to = 1.86 for benzophenone disap-, pearance. I n the experiments of Hammond, et u Z . , ~ the quantum yield for benzophenone disappearance was equal to one. The intensity jn these experiments was 4.66 X 10'' quanta/sec., in contrast to the value of 1.16 X loz5 quanta/sec. in Pitts' experiments. Preliminary experiments at 313 mp for a 1.32 X M solution of benzophenone in isopropyl alcohol were performed by the author16and quantum yields in nitrogen flushed solutions were found to be in agreement with results reported by Pitts, et uZ.l I n these experiments it was also observed that in air-saturated solutions the quantum yield for benzophenone disappearance was 0.31 with a standard deviation of 0.03 for nine determinations. It is noteworthy that changing the fractional disappearance of benzophenone from 4 to 67Oj, caused no significant change in the quantum yield, which would make the system suitable as an actinometer . Results obtained in degassed solutions indicated a tendency for quantum yields to be greater than one; however, data were insufficient to establish this with certainty. The aim of the present study was to determine if the quantum yield for benzophenone disappearance can be greater than one by investigating the intensity dependence of the quantum yield. (1) J. N. Pitts, Ji., R. L. Lotsingei, R. P . Taylor, J. A1. Patterson, G. Recktenweld, a n d R. B. Martin, J . A m . C h e n . Soc., 81, 1068 (1959). (2) G. 0. Ychenck. l?. Mader, a n d M. Pape, Proc. of the Intern. ConJ. Peaceful Uses At. Energy, Z9, 352 (1958). (3) V. Franren, Ann. Chem., 638, 1 (1980). (4) W. M. blooie, G. S. Hammond, a n d R. P. Foss, J . Am. Chem. S O L . , 83, 2789 (1963). ( 5 ) 0. Porter a n d r. Wilkinson, Tians. FaiadaU Soc., 57, 1686 (1961). (6) A. C. Testa, unpublished work.
Experimental Materials.-The benzophenone (Matheson, Coleman and Bell) used in this work was recrystallized from absolute ethanol and from hexane.' Spectrograde isopropyl alcohol was used as the solvent, and the concentration of benzophenone used in these M. experiments was 1.32 X Apparatus.-The 313-mp mercury line from a BH-6 mercury arc was isolated with a Farrand UV monochromator. The entrance and exit slits of the monochromator were set a t 2.0 and 0.75 mm., respectively. Photolyses were performed in 1 cm. fused silica cells with a ground glass joint fused on top to accommodate a stopcock attachment. Each sample was degassed three or more times on a vacuum line with alternate cycles of freezing and vacuum boiling. Experiments were performed a t 24.5 i: 0.5". Filters and Actinometry.-Intensity determinations were made using the potassium ferrioxalate actinometer described by Hatchard and Parker,? and the sensitive uranyl oxalate actinometer described by Pitts, et a1.8 The light intensity was changed over a range of one hundred and corresponded to intensities from 3.30 X lo1&to 3.40 X l O I 3 quanta/sec. Intensity determinations were made before and after each photolysis run, and the constancy was estimated to be approximately 2%. Analysis of Residual Benzophenone.-The benzophenone disappearance was measured by the method described by Pitts, et aL,l namely, by following the optical density changes a t 335 mM. The fractional change in benzophenone was kept below 10% in order to minimize interference, if any, from an absorbing intermediate formed during the photochemical exposure. A Beckman DE spectroph6tometer was used for absorbance measurements.
Results To eliminate the possibility of erroneous intensity measurements, it was found convenient to use two independent actinometers, which are sensitive over different, ranges and have different quantum yields. A comparison of intensities determined a t 254 and 313 mp with the uranyl oxalate and potassium ferrioxalate actinometers indicated a maximum difference of approximately 4%. Twenty-seyen determinations were made of the quantum yield for the disappearance of benzophenone. Results indicated no significant variation of this quantity when the fractiotisl change of benzophenone varied from 2.1 to 8.3%. It was further observed that the quantum yield increased as the intensity was lowered. (7) C . 0 . Hatchard a n d C . -4. Parher, I'rw. R u g . Sac. (London), A236, 518 (19.56). ( 8 ) J. N. Pitts, Jr., J. 11. Mrtrgeiurn, R. P. Taylor, a n d W. Brim, J . Am. Chem. S O C , 77, 5499 (1955).
A. C. TESTA
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Vol. 67
play a significant part in the over-all disappearance of benzophenone. On the basis of recent work by Scheiick, et aLj2and Franzen3 it seems reasonable to consider the formation of a hemiacetal radical as an intermediate iii the over-all disappearance of benzophenone (C&,)zCO
+ CH&OHCHa
--+
CH3
I I
(c~H~,)z~--O-c-oH
.-
2
4
6
I-'" x
8
IO
12
14
-1/2 IO8, ( Q U A N T A / SEC.)
16
18
CH, hemiacetal radical
.
Fig. l.-$'ariation of the quantum yield with the reciprocal q u a r e root of light intensity for the photochemical disappearance of benzophenone in isopropyl alcohol at 313 mp (1.32 X 10+ M benzophenone). Intensity range: 3.30 X 1016 t o 3.40 X 1013quanta/sec. Least squares calculation of slope: 2.43 0.25 x 108 niolecules-quanta-112-sec.-1/2.
Reference to Fig. 1indicates that a plot of the reciprocal square root of the light intensity us. quantum yield was linear and a least squares calculation yielded a slope of 2.43 0.25 X lo6 molecules-quaiita-i/a-sec.-'/z. Since the uncertainty in the quantum yield determinations is estimated to be approximately 10%) the dependence of the quaiitum yield on the reciprocal square root of tlie light intensity is probably real. If the quantum yield is inversely proportional to the square root of the light intensity, a recombination process second order with respect to the atoms or radicals concerned predominates in the mechanism by wliich the radicals d i ~ a p p e a r . ~ I n view of the results of Hammond, et and those reported in the present study, it is expected that above a limiting light intensity the quantum yield should be one. Similarly, as the intensity is lowered, the quantum yield approaches a value close to two.
Discussion Since the results described above show an inverse square root dependence of the light intensity with the quantum yield an explanation was desired to account for this behavior. In addition, it seemed possible to eliminate certain schemes in the literature which have been proposed to explain the quantum yields for benzophenone disappearance. For the case when the light intensity is sufficieiitly high, the dimerization of ketyl radicals, (C&)zCOH, predominates, and the quantum yield for benzophenone disappearance is unity. Under these experimental conditions, t$e destruction of the dimethylcarbinol radical, CH3COHCH3, occurs predomiiiantly by dimerization to form tlie following pinacol, (CH3)zC(OH)(OH)C(CH3)z. This reaction has been observed in the photolysis of an acetone-isopropyl alcohol mixture and in the photolysis of azodicarboxylic esters in isopropyl a1coho1.2,10 Some crossed product between the dimethylcarbinol and ketyl radicals also occurs according to the data reported by Hammond, et aL4 I n their work, the ratio of ( A benzopinacol/- A benzophenone) was 0.39 instead of the expected ralue of 0.5. Under the conditions of low intensity the formation of benzopinacol by one or more alternate paths must (9) W. A. Xoyes, J r . , and P. 1. Leighton, "The Photochemistry of Gases," Reinhold Puhl. Corp., Neu York, N. Y., 1941, p. 197. (10) G. 0. Schenck and H. Formanek, Anoew. Chem., 70, 506 (1968).
(1)
Further evidence regarding this intermediate species was noted in a recent publication by Pitts, et aZ.ll The hemiacetal radical couples with the ketyl radical formed from hydrogen abstraction from isopropyl alcohol.
+ (c~H~,)-z~o-c(CH,)zoH
(C&&)zCOH
--+
(C~H,)Z-C-C--O-C(CH,),OH
i I
(2)
OH (C6H5)Z hemiacetal and filially a hemiacetal cleavage results in a molecule of benzopinacol and one of acetone
It seems, therefore, that there are two coiicoinitaiit modes for the disappearance of benzophenone, i e . , (a) dimerization of ketyl radicals, (C&,)zCOH, and (b) reaction via eq. 1-3. According to the proposed reaction scheme, the quaiitum yield for benzophenone disappearance varies between one and two depending on the relative importance of each of the two possible niodes of benzophenone disappearance. Since the formation of benzopinacol .presumedly occurs from radical coupling of (C6H5)2COH with itself and with the hemiacetal radical formed in eq. 1, it is not unexpected that the reciprocal square root dependence of light intensity should be maintained. Although the reaction scheme proposed is qualitatively consistent with quantum yield determinations, this is not the only possible mechanism to account for the experimental results. Indeed, Scheiick and coworkers2 have postulated that intermediates I and I1 participate in the over-all disappearance of benzophenone. In viely of the quantum yields greater than one obtained in the present study, species I1 is not favored, since by this scheme two quanta are consumed, one (11) J. N. Pitts, Jr., T. I
