178
WARRENV. SHERMAN AND SAULG. COHEN
Flash Photolysis of Benzophenone in 2-Propanol. Effect of Phenyl Disulfide
by Warren V. Sherman and Saul G . Cohen U.S. Army Laboratories, N a t k k , Massachusetts, and the Department of Chemistry, Brandeis University, Waltham, Massachusetts (Received July 19, 1966)
The effect of phenyl disulfide on the benzophenone-ketyl radical, formed by flash photolysis of benzophenone in 2-propanol, has been examined. The disulfide has no effect on the formation of the ketyl radical; it accelerates its disappearance and retards the photoreduction of benzophenone. Mercaptans and disulfides inhibit radiation-induced reduction of benzophenone by the previously proposed' hydrogen atom transfer reactions (eq. 1 and 2). The rate constant for transfer of hydrogen from benzophenone-ketyl to phenylthiyl radical is estimated to be >7 X lo5M-' see.-'.
Introduction Disulfides and mercaptans retard and inhibit by chemical reactions the photoreduction of benzophenone by 2-propanol. They may do this as effectively, or more effectively, than naphthalene2 even though the latter acts as an eficient physical quencher of the excited triplet of benzophenone in a diffusion-controlled reaction3-5 of rate constant -3 X lo9 M-' see.-'. M, The concentration of naphthalene, -4 X required to halve the rate of this photoreduction2 in undiluted -13 M 2-propanol leads to a value of -1 X 108 M-1 see.-' for k,, the rate constant for abstraction of hydrogen from 2-propanol by triplet benzophen0ne.~l59~The concentration of mesityl disulfide-2mercaptomesitylene required to halve the rate under the same conditions of photoreduction is -1.5 X M thiyl group present as 7001, mercaptan and 30% disuKde.lV2 The proposed mechanism of this retard& tion involves hydrogen transfer reactions between the intermediate radicals and the sulfur compounds, eq. 1and 2. (CeH5)2C-OH
+ AS. + (CeH&C=O
(CeH5)zC-OH
+ ASSA
(la)
~ ( ( C B H S ) & O H=) ~ksp((CeHs)&OH) (AS)
(3)
The rate of formation of the pinacol is given by
At the observed half-inhibited rate2 of disappearance of benzophenone, 1.4 X M sec.-', and with the reportede kq = 5.9 X lo7 M-' see.-', the concentration of the benzophenone-ketyl radical is calculated to be 5 X lo-' M , the value of ks,(AS) is calculated, 0.3 X (1) S. G. Cohen, S. Orman, and D. A. Laufer, J . Am. Chem. Soc., 84, 3905 (1962).
--f
(CeHs)zC=O (CH3)&0H
+ ASH
negating the chemical consequences of many quanta. These experiments do not allow separation of reactions l a and lb, and we will use for them the combined rate expression, ~ s , ( ( C ~ H ~ ) ~ C - (AS). O H ) In this, kgp is the nominal rate constant for reaction of the benzophenone-ketyl radical with thiyl radical and the disulfide under steady illumination in the photochemical reaction and (AS) is the concentration of sulfur moiety present at photoequilibrium as thiyl radical and disulfide combined, and had2 the value 0.5 X lod3 M . At half inhibition the following equality obtains
+ ASH +
+ ASH + AS.
(CH3)2CHOH
+ ASH
(lb) (2)
The sulfur compounds are regenerated in their alternate valence states and are used repeatedly, each molecule The Journal of Physical Chemistry
(2) 8. G. Cohen and W. V. Sherman, ibid., 85, 1642 (1963). (3) G. S. Hammond, M. J. Turro, and P. A. Leermakers, J. P h y s . Chem., 66, 1144 (1962); G. S. Hammond and P. A. Leermakers, ibid., 66, 1148 (1962). (4) W.M. Moore and M. Ketchum, J . Am. Chem. SOC.,84, 1368 (1962). (5) G. Porter and F. Wilkinson, Trans. Faraday SOC., 57, 1686 (1961). (6) A. Beckett and G. Porter, ibid., 59, 2038 (1963).
EFFECTO F PHEPI’YL DISULFIDEI N FLASH PHOTOLYSIS
OF
lo2 set.-', and the nominal value for ksp is 6 X lo4 M-l sec.-l. Evidence for the inhibition mechanism of eq. 1 and 2 has been reviewed recently.’ Deuterium exchange and racemization studies give unequivocal evidence for the formation of the alcohol-derived radical, and the occurrence of reaction 2. Other evidence in the photoreduction in the presence of alkali,2 in the B°Co y-ray-induced photoreductionla and in studies of photodecarb~nylation~ strongly imply but do not directly demonstrate the formation of the benzophenone-derived radical and the occurrence of reaction 1 during inhibition. We now report on the flash photolysis of 5.0 X M benzophenone in 2-propanol in the absence and presence of phenyl disulfide, in order to observe directly the benzophenone-derived radical.
179
BENZOPHENONE IN %PROPANOL
0.20
0.15
