1544
On the Photoreduction of Benzophenone in Isopropyl Alcohol Nicolae Filipescu and Fredrick L. Minn Contribution f r o m the Department of Chemistry, George Washington University, Washington, D. C. 20006, Received October 14, 1967 Abstract: 1-(Phenylhydroxymethylene)-4-(dimethylhydroxymethyl)-2,S-cyclohexadieneis proposed to be the stable intermediate (In) in the photoreduction of benzophenone in pure isopropyl alcohol. The phosphorescence and phosphorescence excitation spectra of In were obtained by matrix isolation in frozen isopropyl alcohol at 77°K. The absorption spectrum of In determined by the use of actinometry and verified by the excitation spectrum was used to follow the kinetics of the dark reaction following irradiation. The proposed mechanism is consistent with all observed data and explains the erratic quantum yield results reported previously.
T
he nature of the intermediate in the photoreduction of benzophenone (PhZCO) in isopropyl alcohol in the absence of air has remained a controversial subject for roughly 10 years. Different adducts between diphenylhydroxymethyl (PhzCOH) or dimethylhydroxymethyl (Me2COH) free radicals and benzophenone (PhzCO) were first suggested. 1-4 Later, charge-transfer complexes between free radicals Me2COH and P h 2 c 0 H or between MezCOH and the Me2COHPhzCO adduct were proposed by Backstrom, et a1.j However, absence of esr signals has eliminated a freeradical formulation for the long-lived colored intermediate.++ More recently Schenck, et a1.,9v10 interpreted a number of experimental observations by assu.ming the intermediate to be a simple adduct of two PhzCOH radicals, “isobenzopinacol” (I or 11).
I
OH I1
Unlike the photoreduction of PhzCO in benzene in the presence of benzhydrol, 1 1 - 1 3 the quantum yield in 100% isopropyl alcohol varied erratically, 1 ~ 8 ,l 3 depending on concentration of PhzCO, irradiation time, and interval allowed before admission of air. These facts, together with the observed properties of the intermediate, namely, its stability, reducing capability, indicator behavior, diamagnetism, and rate of reaction with Oz, were difficult to explain simultaneously. (11 J. N. Pitts. R. L. Letsineer. R. P. Tavlor. J. M. Patterson. G. Recktenwald, and R. B. MartinrJ. i m . Chem.-Soc., 81, 1068 (1959). ’ (2) V. Franzen, Ann., 633, 1 (1960). (3) H. Mauser and H. Heitzer, Naturwissenschafren, 50, 568 (1963). (4) H. Mauser, U. Sprosser, and H. Heitzer, Chem. Ber., 98, 1639 (1965). ( 5 ) H. L. J. Backstrom, I
