FREDERICK D. LEWIS
3332 molecules to form n/IDPM's. We are further investigating the structure of the C7H7+ ions in the gas-phase radiolysis.
Acknowledgment. We thank Mr. Tamotsu Yamamot0 and Mr. Tomikazu Sawai in the Radiation Laboratory for assistance in the y irradiations.
On the Photoreduction of Acetophenone by Frederick D. Lewis Department of Chemistry, Northwestern University, Evanston, Illinois 60801 (Received April 83,1970)
The photoreduction of acetophenone in 2-propanol-benzene solution has been reexamined. The effect of 2-propanol concentration on quantum yields and apparent triplet decay rate constants is attributed to the presence of a quenching impurity in commercial 2-propanol. The value for the triplet decay rate constant in 0.1 iM 2-propanol, 3.4 X IO6 sec-l, is in accord with values obtained by phosphorescence decay studies and i s similar to the limiting value for benzophenone triplet decay in benzene solution. The photoreduction of aromatic ketones with 2-propanol is one of the most int,ensely studied areas of solution photochemistry. Kinetic studies and quantum yield determinations have provided information about the reactivity and lifetime of the triplet states of benzophenone, l-l0 a~etophenone,"-'~ and their derivatives. However, several types of evidence have been reported which indicate that the mechanism of these photoreduction reactions is far from simple.l6'l6 First, the quantum yield for disappearance of acetophenonel2 or benzophenone" is reported to increase with 2-propanol concentration at low concentrations, but to decrease at high (21 M ) concentrations of 2-propanol. This phenomenon has been related12 to the observation of highly absorbing and/or quenching by-products or intermediates formed in the photoreduction reactions of acetophenone12 and in concentrated 2-propanol solutions. Second, values for triplet decay rate constants (vide infra) determined in 2 M 2-propanol-benzene for acetophenone (1.6 X lo6 ~ e c - l ) ' ~ and benzophenone (7.3 X lo5 sec-l) l o are disturbingly greater than values obtained in recent phosphorescent decay studies (3.0 X lo6sec-' for acetophenone and 1.9 X 106 sec-1 for benzophenone). l7 Using benzhydrol as the hydrogen donor for benzophenone photoreduction a value of 3 x 106 sec-l is obtained.16 Finally, there is no a priori reason to expect the triplet decay rate constant for acetophenone to be substantially greater than that for benzophenone. We wish to present evidence obtained from a detailed reinvestigation of acetophenone photoreduction that both the anomalous quantum yields and rate constants are due to a quenching impurity in commercial 3-propanol. The Journal of Physical Chemistry, Vol. 74, N o . 18, 2970
The generally accepted mechanism for reduction of acetophenone (A) by 2-propanol (RH) in the presence of a quencher ( Q ) can be described by the following equations.
(1) J. N. Pitts, R. L. Letsinger, R. P. Taylor, J. M. Patterson, G. Recktenwald, and R. B. Martin, J. Amer. Chem. Soc., 81, 1068 (1959). (2) J. N. Pitts, W. H. Johnson, and T. Kuwana, J. Phys. Chem., 66, 2456 (1962). (3) A. Beckett and G. Porter, Trans. Faraday Soc., 59, 2051 (1963). (4) W. M. Moore and M. D. Ketchum, J. Phys. Chem., 68, 214 (1964). (5) S. G. Cohen and R. J. Baumgarten, J . Amer. Chem. SOC.,89, 3471 (1967). (6) S. G. Cohen and J. I. Cohen, Tetrahedron Lett., 4823 (1968); Israel J. Chem., 6, 757 (1968). (7) H. J. L. Backstrom, ( K . L. Appelgren, and R. J. V. Niclasson, Acta Chem. Scand., 19, 1555 (1965). (8) G. 0.Schenck, M. Cziesla, K. Eppinger, G. Matthias, and M. Pape, Tetrahedron Lett., 193 (1967). (9) N. Filipescu and F. L. Minn, J. Amer. Chem. SOC.,90, 1544 (1968); J. Chem. Soc., B , 84 (1969). (10) N.C. Yang and R. L. Dusenbery, Molecular Photochern., 1, 159 (1969). (11) S. G. Cohen, D. A. Lanfer, and W. V . Sherman, J . Amer. Chem. Soc., 86, 3060 (1964). (12) S. G.Cohen and B. Green, ibid., 91, 6824 (1969). (13) N. C. Yang, D. S. McClure, S. L. Murov, J. J. Houser, and R. Dusenbery, ibid., 89,6466 (1967). (14) N. C. Yang and R.L. Dusenbery, ibid., 90,5899 (1968). (15) Similar observations have recently been made in the case of benzophenone photoreductions.16 (16) P. J. Wagner, Molecular Photochem., 1, 71 (1969). (17) W. C. K. Clark, A. D. Litt, and C. Steel, Chem. Commun., 1087 (1969).
3333
ON THE PHOTOREDUCTION OF ACETOPHENONE (3)
1 0
I ~ P ~ / \ c HA , pinacol Assuming that the initially formed radicals (step 4) all proceed to products, the usual Stern-Volmer eq 7 and 8 are derived in the absence and presence of quencher, respectively.
(7)
The variation in quantum yield with 2-propanol concentration in degassed benzene so1ution18 was determined for acetone and pinacol formation and for loss of acetophenone using purified reagent grade 2-propanol. Acetone formation could be followed at low conversion (
