The photochemical a cleavage and reduction reactions of several aryl alkyl ketones have been investigated. Electron-releasing aromatic substituents decrease the excited state reactivity of ieri-alkyl aryl ketones toward both primary photoprocesses to a similar extent. The aromatic carbonyl 3( , *) state is concluded to be responsible for a cleavage as well as hydrogen abstraction reactions. The rate constant for intermolecular hydrogen abstraction by the carbonyl triplet state decreases with -methyl substitution. Pinacol and carbinol products are formed via the combination and disproportionation of ketyl radicals. A primary deuterium isotope effect on isobutyrophenone ketyl radical disproportionation of 2.7 is observed. -Methyl substituents increase the percentage of carbinol products due to steric hindrance of free-radical combination.Synthetic8'4 and mechanistic5-7 aspects of aryl alkyl ketone photoreduction have been extensively investigated. A number of «-substituted acetophenones form pinacols upon irradiation in 2-propanol or other suitable hydrogen-donor solvents;3•4 however, quantitative data on such ketones was unavailable prior to our investigation.7 The elegant studies of Yang and coworkers5 on aryl-substituted acetophenones estab-

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Spectroscopy has been used as a unifying theme to teach basic chemical principles and laboratory skills to talented high school students.

Für die photolytische Spaltung des Desoxybenzoins (I) gilt die G1.