The photoenolizations of 2.4-dimethylbenzophenone, 2-isopropylbenzophenone, and 2-benzylbenzophenone have been investigated by conventional and laser flash photolysis. The hydrogen-abstraction step occurs from the triplet state, the lifetime of which is 30 ns or less. The second transient, formed a t the same rate as the triplet disappears, has not been assigned unambiguously but, while the decay rates vary with compound and solvent, they are not affected by oxygen. Each of the ketones produces two isomeric enols. The rate constants for decay (return to ketone), as well as reaction with oxygen and dimethyl acetylenedicarboxylate, have been measured and illustrate the high degree of reactivity of these molecules. Photolysis of the enols result in the formation of another transient which has been assigned a dihydroanthrone structure. Three transients are observed in the photolysis of 2.6-dimethylbenzophenone but these are not similar to those detected in the other ketones and no assignments are suggested.THE photoreduction of benzophenone to benzopinacol in hydrogen-donor solvents is one of the best studied of all photochemical reactions. The reaction has been shown to be initiated by abstraction of hydrogen from the solvent by the triplet state (m*) of the ketone. Substitution at the ortho-position by an alkyl group containing an a-hydrogen (-CHR1R2) results in a marked suppression of the pinacol-forming reaction. Yang and Rivas showed that this was not due to a decrease in reactivity of the triplet state but was due to an internal hydrogen abstraction, rather than abstraction from the solvent, to yield an enol isomer of the ketone instead of a ketyl radical. Thus, irradiation of 2-methylbenzophenone (1) gave, as an unstable molecule, the enol (2). The transitory existence of the enol was demonstrated by deuterium exchange in MeOD (to give -CDR1R2) and it was trapped by a dienophile, dimethyl acetylenedicarboxylate. It has recently been demonstrated that the trapping reaction is a true Diels-Alder reaction since addition of maleic and fumaric acids occurs stereospecifically.2 Irradiation in propan-2-01 containing the dienophile has demonstrated that internal hydrogen abstraction (enolization followed by trapping) is considerably more efficient than abstraction from the solvent (photoreduction) .3Two further reactions have been reported which seemed to involve the enol. Ullman and Huffman irradiated 2-methylbenzophenone in degassed propan-2-01 at -80 "C for several minutes and observed the formation of a ' deep yellow' colour. When the solution was warmed to 25 "C most of the yellow colour disappeared but some residual colour remained (T+ ca. 36 h). Addition of oxygen caused this weakly coloured transient to disappear and from the reaction the authors isolated a small yield (ca. 1%) of 9-anthrone. They suggested that the two coloured transients were the enol

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The photoenolization of 2-methyl-, 2,4-dimethyl-and 2,5-dimethylacetophenone has been investigated using conventional flash photolysis. It was found that the initial concentration of photoenol in aprotic solvents increased with the addition of oxygen. The results are interpreted in terms of the formation of an oxygen quenchable enol triplet which can decay either to the enol or the ketone ground state. The data are used to calculate relative quantum yields of en01 formation of ethanol (l.O), diethyl ether (OS), and cyclohexane (0.1). A model is proposed which leads to the conclusion that the en01 triplet is quenched at a near diffusion-controlled rate. No photoenolization was detected in 2-isopropyl-and 2,4-di-isopropylacetophenone. These results are interpreted in terms of the above model.

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SynopsisConstants for the Mark-Houwink-Sakurada relation can be established in principle from GPC measurements on broad distribution polymers. The method requires use of two samples with dif- on the GPC measurement of Vn. The simplest and safest way to use GPC data to estimate the MHS constants involves the measurement of GPC chromatograms of two polymer samples with different intrinsic viscosities. The method is not confined to the solvent used as the GPC eluant. The MHS constants derived from GPC appear to reflect the molecular weight range of the calibration samples and may not be as widely applicable as those from the more tedious classical methods which employ a series of fractionated samples.