2576
J. Phys. Chem. 1980, 84, 2576-2579
Mechanism of Benzophenone Ketyl Radical Formation in Acid Alcohols Studied by Pulse-Radiolysis and Rigid-Matrix Techniques Mlklo Hoshlno, Shigeyoshi Arai, Masashi Imamura, The Institute of Physical and Chemical Research, Wako-shi, Saitama 35 1, Japan
Kiyoshl Ikehara, and Yoshimasa Hama Science and Engineering Research Laboratory, Waseda University, Shinjuku-ku, Tokyo 160, Japan (Received: February 20, 1980; In Final Form: May 30, 1980)
The mechanism of the benzophenone ketyl radical formation in acid methanol, ethanol, and 2-propanol was studied by using pulse-radiolysis and rigid-matrix techniques. When a 0.1 M ethanol solution of benzophenone containing hydrogen chloride (1.2 M) was irradiated at 77 K by y rays from 6oCo,the absorption spectrum of the trapped intermediates was ascribed solely to benzophenone ketyl radicals. The pulse-radiolysis study of the solution at 100 K revealed that the ketyl radicals are produced by protonation of presolvated benzophenone anion radicals. At 153 K, the ketyl radicals were observed to be produced also by hydrogen-atom transfer from CH3CHOH and CH3CH(OH)CH3to benzophenone;the temperature dependence of the transfer rate constant was studied.
Introduction A series of low-temperature pulse-radiolysis studies carried out in our laboratory have been proved of great use for investigation of the rapid kinetic behavior of ionic transients in organic media.1-5 In ethanol solutions of aromatic ketones at 100 K, the transients observed immediately after pulse irradiation are presolvated anion radicals, which gradually change to solvated anion radic a l ~ For . ~ the benzophenone anion radical, the shift of A, takes place from 780 to 640 nm. The solvated benzophenone anion radical (A, = 640 nm) has frequently been observed in y radiolysis of ethanol solution at 77 K and in pulse radiolysis and flash photolysis of alkaline alcohol solutions at room temperature.610 The solvated anion radicals (Bwc)are in equilibrium with ketyl radicals (BH) B s o+~ H+
BH
and its equilibrium constant has been r e p ~ r t e d . ~The ?~ forward reaction of the above equilibrium, however, has not yet been investigated. One of the aims of the present study is to find whether there is any difference in the protonation reaction between the solvated and presolvated benzophenone anion radicals. The ketyl radical is a key intermediate in the photochemical reaction of benzophenone in organic solvents. The established mechanism is that the triplet benzophenone abstracts a hydrogen atom from a solvent molecule to produce the ketyl radical and a free radical of solvent. Benzpinacole, the final product, is formed by recombination reaction between ketyl radicals. When 2-propanol was used as a solvent, the quantum yields of benzophenone disappearance increased with increasing concentration of benzophenone. This result was interpreted in terms of hydrogen-atom transfer from the solvent radical (CH,C(OH)CH3) to benzophenone.* In ethanol solutions, however, the quantum yield is almost 1.0 and is independent of the concentration of benzophenone. It was concluded, therefore, that no hydrogenatom transfer from CH3CHOH to benzophenone occurs a t room temperature.8 In the present study, we examined the mechanism of the benzophenone ketyl radical formation in acid alcohols at various temperatures and found that CH,CHOH as well 0022-3654/80/2084-2576$0 1 .OO/O
as CH3C(OH)CH3reacts with benzophenone to produce ketyl radicals.
Experimental Section Benzophenone was purified by recrystallizing three times from ethanol solutions. Reagent-grade ethanol was used without further purification. Methanol and 2-propanol were purified by distillation before use. Hydrogen chloride gas generated from a mixture of sodium chloride and sulfuric acid was dried on concentrated sulfuric acid. The acid alcohols were prepared by dissolving the dry gas of hydrogen chloride into pure alcohols or by adding aqueous hydrogen chloride (12 N) in pure alcohols. Essentially the same results were obtained for these two acid alcohol solutions. The pulsed electron beams were generated from a Van de Graaff accelerator. Pulse width and energy were 1.0 y s and 2.7 MeV, respectively. The apparatus for the lowtemperature pulse radiolysis was described e l ~ e w h e r e . ~ ~ ~ The technique employed in the rigid-matrix experiments was the same as that used in the earlier s t u d i e ~ . ~ The ,~ sample cells were 0.5 and 1.0 mm in optical path length. The dose rate of y-rays from @'Co was 58 600 rd/min. The absorption spectra of trapped intermediates were recorded on a Cary 14 R spectrophotometer at 77 K. Results Acid Ethanol Solutions of Benzophenone. Figure 1 shows the absorption spectra of the trapped intermediates in y-irradiated ethanol solutions of benzophenone with and without hydrogen chloride a t 77 K. In the absence of hydrogen chloride, the solvated benzophenone anion radical (Amm = 630 nm) is a major intermediate, and the ketyl radical is only a minor one. The relative amount of the ketyl radical (& = 555 nm) was observed to increase with increasing concentration of hydrogen chloride. The absorption spectrum of the y-irradiated ethanol solution of benzophenone containing 1.2 M hydrogen chloride at 77 K is due solely to the ketyl radical; neither solvated electron nor benzophenone anion radical was observed. When the irradiated sample was warmed for 1.0 min by withdrawing it from liquid nitrogen into the atmosphere, the absorption due to the ketyl radical increased in intensity 1.5 times. The observed increase in the ketyl radical concentration indicates that some reducing species 0 1980 American Chemical Society
The Journal of Physical Chemistry, Vol. 84, No. 20, 1980 2577
Mechanism of Benzophenone Ketyl Radical Formation
0.4
0.3
I -
OD
700 800 WAVELENGTH ( nm )
600
I \ 400
500
600 700 WAVELENGTH( nm)
1 800
Figure 1. Absorption spectra observed for y-irradiated 0.1 M ethanol solutions at 77 K (A) no hydrogen chloride ackied; (6) hydrogen chloride at 0.1 M; (C) hydrogen chloride at 1.2 M.
900
Flgure 3. Time-dependent absorption spectra obtained for a pulseirradiated 0.1 M ethanol solution of benzophenone containing 1.2 M hydrogen chloride at 100 K. A, B, and C were determined immediately, and 200 and 1600 ps after a pulse, respectively. 0.3 CONCENTRATION OF HCI
0
0 0
A
A
0.2 OD
600
700 8co WAVELENGTH ( nm '1
900
Figure 2. Time-dependent absorption spectra obtained for a pulseirradiated 0.1 M ethanol solution of benzophenone containing 0.1 M hydrogen chloride at 100 K. A, B, and C were determined immediately, and 200 and 1600 ps after a pulse (1.0 ps width), respectively.
has been present in the irradiated ethanol solution before warming. A low-temperature pulse-radiolysis technique was applied to acid ethanol1 solutions in order to elucidate the mechanism of ketyl radical formation in the y-irradiated solutions at 77 K. Figure 2 shows the transient absorption spectra obtained with the 0.1 M ethanol solution of benzophenone containing 0.1 M hydrogen chloride at 100 K. The transient appearing immediately after the electron pulse is the presolvated benzophenone anion radical with an absorption peak a t 780 nm. Within a few milliseconds, the presolvated anion radical is solvated and the absorption peak shifts to 640 nm. This spectral change was essentially similar to that of neutral solution reported previously.2 The solvated anion radical is not stable in acid ethanol and changes gradually to the benzophenone ketyl radical at 100
M
0.22 M
0 5 3 ~
8,
A
9
0.93 M 18 M
'e
400
0
800 T I M E (P s J
1200
1600
Figure 4. Decay profile of presolvated benzophenone anion radicals observed for a 0.1 M ethanol solution of benzophenone containlng hydrogen chloride at concentrations as indicated at 100 K. The optical densities determined at 780 nm.
' L
t 1.o 2.0 CONCENTRATION OF HCI ( M )
K.
Figure 5. Relative yields of presolvated benzophenone anion radicals in a 0.1 M ethanol solution of benzophenone as a function of concentration of hydrogen chloride. Yields determined at 1.0 ps after a pulse at 100 K.
The transient spectrum at wavelengths shorter than 580 nm could not be detlermined because of significant accumulation of benzophenone ketyl radicals which absorb at 500-600 nm. The similar time-dependent absorption spectra for 0.1 M ethanol solution of benzophenone containing 1.2 M hydrogen chloride at, 100 K are shown in Figure 3. The absorption spectrum observed immediately after the pulse is due solely to the presolvated benzophenone anion radicals, which decay within a few milliseconds without any spectral shift as observed for 0.1 M solution of hydrogen chloride. The benzophenone ketyl radicals (555 nm) were observed to be produced concurrently with decay of the presolvated benzophenone anion radicals. This fact indicates that the presolvated anion radicals, in the 1.2 M
solution of hydrogen chloride, undergo protonation and form ketyl radicals. Figure 4 shows a decay profile of the presolvated anion radicals at various concentrations of hydrogen chloride at 100 K. The decay profile is independent of acid concentration, indicating that the protonation reaction does not accelerate the decay of the presolvated anion radical. In other words, the protonation reaction is unlikely to be competitive with solvation of the presolvated anion radicals. Neither the first- nor the second-order kinetics apparently applies to the decay of the presolvated anion radicals. Figure 5 shows the relative yields of the presolvated benzophenone anion radicals obtained with the pulse ra-
2578
The Journal of Physical Chemistry, Vol. 84, No. 20, 1980
Hoshino et al.
0.1
0.2
CONCENTRATION( M )
TIME
Figure 6. Formation of ketyl radicals in a pulse-irradiated 5.5 X IO-* M ethanol solution of benzophenonecontaining 1.2 M hydrogen chloride at 153 K: monitored at 555 nm.
diolysis of the ethanol solution of benzophenone at several concentrations of hydrogen chloride. The yield obviously decreases with increasing acid concentration. Since the initial yield of electrons is considered to be independent of acid concentration, the decrease in the relative yield is ascribed to capture of electrons by protons. The pulse-radiolysis study of acid ethanol solutions of benzophenone was also carried out at 153 K. The observed intermediate was the benzophenone ketyl radical. Figure 6 shows the formation curve of ketyl radicals monitored at 555 nm after pulse irradiation of a 5.5 X M ethanol solution of benzophenone containing 1.2 M hydrogen chloride. The curve shows that the ketyl radicals are produced by a fast process followed by a slow one. Judging from the results obtained from the rigid-matrix study at 77 K and the pulse-radiolysis study at 100 K, we regard the fast process to be due to protonation of the presolvated benzophenone anion radicals and the slow process to be due to hydrogen-atomtransfer from some reducing species to benzophenone molecules. The first-order kinetics is applied to the slow process of the ketyl radical formation: In ( D , - Dt) = -ht constant
+
where D, and D, represent the optical densities at 555 nm at an infinite time and at time t, respectively; k is the product of bimolecular rate constant, hb, and the concentration of benzophenone. From the kinetic plot for the 5.5 X M benzophenone solution, the same rate constant, k = 2.7 X lo2 s-l, was obtained at acid concentrations of 0.6 and 1.2 M, indicating that k is independent of acid concentration. Figure 7 shows the relationship between k and the benzophenone concentration. The slope of the line gives the bimolecular rate constant of 3.9 X lo3 M-'s-l at 153 K. The Arrhenius plot of h obtained with a 8.0 X lo-' M ethanol solution of benzophenone containing 1.2 M hydrogen chloride yields an activation energy for the slow process of 2.9 kcal/mol. If one uses the bimolecular rate constant at 153 K and the activation energy, kb is represented as kb = 5.44 X lo7 exp(-1460/T) Acid 2-Propanol and Methanol Solutions of Benzophenone. The solvated benzophenone anion radical was oberved when glassy 2-propanol and methanol solutions were irradiated with y-rays at 77 K. In the presence of 1.2 M hydrogen chloride, however, the absorption spectrum of the trapped intermediate is due solely to the benzophenone ketyl radical. When the irradiated sample is warmed, as in the case of acid ethanol solutions, the in-
Figure 7. Dependence' of the rate constant k on benzophenone concentration (M) at 153 K.
50
0
-1 -
Figure 8. Time profile of the absorption of ketyl radicals at 555 nm for a pulse-irradiated 1.0 X lo-' M methanol solution of benzophenone containing 1.2 M hydrogen chloride at 153 K.
tensity of the absorption due to ketyl radicals increased 1.7 times for the acid 2-propanol solution, but not for the acid methanol solution. These facts suggest that reducing species responsible for ketyl radical formation have been produced in y-irradiated 2-propanol, but not in methanol. The pulse-radiolysis studies for a 1.0 X M 2propanol solution of benzophenone containing 1.2 M hydrogen chloride at 183 K shows that the ketyl radicals are produced by the fast and slow processes as for the acid ethanol solutions. The fast process must be due to protonation of benzophenone anion radicals, and the slow process to hydrogen-atom transfer from reducing species produced by pulse irradiation of 2-propanol, The firstorder kinetics was applied to the slow process, and, from the relationship between k and benzophenone concentration, the bimolecular rate constant, kb,was determined to be 1.06 X lo4 M-l s-l for 2-propanol solutions at 183 K. The activation energy obtained from the temperature dependence of k is 5.25 kcal/mol. The conventional Arrhenius expression of kb is
kb = 2.06 x lolo exp(-2650/T) Figure 8 shows the time profile of the absorption intensity of the benzophenone-ketyl radicals monitored at 555 nm after pulse irradiation of the 1.0 X M methanol solution of benzophenone containing 1.2 M hydrogen chloride at 153 K. The time profile was invariant with the concentrations of benzophenone and hydrogen chloride, and no slow formation was observed at the temperature range from 120 to 200 K. These results indicate that ketyl radicals are formed solely by the fast process in methanol solutions. Discussion Ketyl Radical Formation from Anion Radicals. In a previous study: presolvated benzophenone anion radicals were observed for y-irradiated ethanol solutions of ben-
Mechanism of Benzophelnone Ketyl Radical Formation
zophenone at 4 K. Tlhe presolvated anion radicals undergo solvation on warming the irradiated sample from 4 to 77 K, accompanying the formation of a small amount of the ketyl radicals. The amount of the ketyl radicals did not increase or decrease as long as the sample was preserved a t 77 K. The absorption spectrum appearing in the warmed sample was in good agreement with that obtained by y irradiation of ethanol solutions at 77 K. In the present study, ketyl radicals are found to be produced in y-irradiated ethanol, 2-propanol, and methanol solutions of benzophenone containing 1.2 M hydrogen chloride at 77 K. From the pulse-radiolysis study of ethanol solution of benzophenone at 100 K, the precursor of the ketyl radical was confirmed to be the presolvated benzophenone anion radical. The ketyl radical formation observed for y-irradiated acid 2-propanol and methanol solutions at 77 K may also be interpreted as protonation of the presolvated anion radical i(BprJ
Bpr;
+ ROH2+
-+
BH -I- ROH
where ROH2+ and BH represent a protonated alcohol molecule and the ketyl radical, respectively. This reaction is expected ta take place between the adjacent ions in highly viscous alcohol solutions at 77 K. Probably, the presolvated anion radical allows a protonated alcohol to reorient to make a proton close to the CO- site. The decay profiles of the presolvated benzophenone anion radicals are ementially similar irrespective of the presence or the absence of hydrogen chloride at 100 K. This fact suggests that the reorientations of neutral and protonated alcohol molecules which are rate-determining steps for solvation aind ketyl radical formation at 100 K occur with similar frequencies. From these results, it is concluded that geminate recombination between the presolvated anion and proton is responsible for the formation of a small amount of ketyl radicals observed for y-irradiated ethanol solutions in the absence of hydrogen chloride, in which protons are produced by ion-molecular reaction between a cation radical of ethanol and an ethanol molecule. Ketyl Radical Formation from Reducing Species. The radiolysis mechanism of liquid ethanol is represented by11-14
CH3CH20H
--
CH3CH20H++ e,
CH&HZOH+ -t CH3CHZOH CH3CH20,CH3CHOH + CH3CH20H2+ +
e,
-+
e,, e,
+ CH3CH20H + CH3CEIzOH H2 + CH3CHOH CH3CHZO + CH3CH20H CHSCHOH + CH3CH20H e,
-+
H
-
+
where the subscripts m, s, and t stand for mobile, solvated, and trapped states, respectively. In acid ethanol, e, and et may react with protons to produce H.
The Journal of Physical Chemistry, Vol. 84, No. 20, 1980 2579
The additional formation of ketyl radicals found by warming of y-irradiated glassy acid ethanol solution is probably due to reduction of benzophenone by some reducing species. The candidates for the reducing species are H, CH3CH20, and CH,CHOH. H and CH3CH20, however, react rapidly with adjacent ethanol molecules to form CH3CHOH,12J3 In fact ESR spectra show that the trapped intermediates observed in the y-irradiated acid ethanol are principally CH,CHOH at 77 K.15 Consequently, a possible reducing species may be concluded to be CH,CHOH. The slow formation of the ketyl radicals observed in the pulse radiolysis of acid ethanol solutions at 153 K is also due to the same reaction as in the warmed rigid matrices. The ketyl radical formation may thus be represented as B + CH,CHOH BH + CH,CHO -+
where B is a benzophenone molecule. The hydrogen-atom transfer from the reducing species to benzophenone was also observed for acid 2-propanol solutions, in which CH3C(OH)CH3is the principal product of y radiolysis at 77 K.15 In the acid methanol solution, however, no hydrogenatom transfer from the reducing species to benzophenone was observed.. Since the major product in y-irradiated acid methanol is CH20H,15this result indicates that CHzOH does not reduce benzophenone at the temperature range from 77 to 200 K. Becket and Porter1" reported that benzophenone reacts with CH3CH(OH)CH3but not with CH3CHOH at room temperature. The present study revealed that both CH3C(OH)CH3and CH3CHOH can reduce benzophenone at low temperatures. If the values of IZb at low temperatures are extrapolated, the rate constants of 4.2 X lo5 and 3.0 X loe M-l s-l are obtained at 300 K for CH3cHOH and CH3C(OH)CH3,respectively. The former value is one order of magnitude lower than the latter one. Acknowledgment. We thank I. Kinoshita and K. Nakano for their generous assistance during the study.
References and Notes S.Arai, S.Tagawa, and M. Imamura, J. phys. Chem., 78, 519 (1974). M. Hoshino, S.Arai, and M. Imamura, J. Phys. Chem., 78, 1473 (1974). S.Arai, M. Hoshino, and M. Imamura, J. phys. Chem., 79, 702 (1975). M. Hoshino, S.Arai, and M. Imamura, J. Phys. Chem., 80, 2724 (1976). S. Arai, A. Kira, and M. Imamura, J. Phys. Chem., 81, 110 (1977). G. E. Adams and R. L. Willson, J . Chem. Soc., Faraday Trans. 1 , 89, 719 (1973). G. Porter and F. Willson, Trans. faraday Sac., 57, 1686 (1961). A. Becket and 0.Porter, Trans. Faraday Soc., 59, 2038 (1963). M. Hoshino, S.Arai, A. Namiki, arid M. Imamura, Chem. phys. Left., 28, 582 (1974). f. Shida, S.Iwata, and M.'Imamura, J. phys. Chem., 78, 741 (1974). L. Kevan, Actions Chim. Blol. Radlat., 13, 57 (1969). M. Shiotani, S.Murabayashi, and J. Sohma, Int. J . Radlat. Phys. Chem., 8, 483 (1976). G. R. Freeman, Actions Chim. Bid. Radiat., 14, 73 (1970). G. R. Freeman, Natl. Stand. Ref. Data Ser. (U.S., b t l . Bur. Stand.), No. 48 (1974). M. Hoshino and K. Ikehara, unpublished results
