MECHANISMS OF PHOTOCHEMICAL REACTIONS IN SOLUTION .58. PHOTOREDUCTION OF CAMPHORQUINONE

The photoreduction of camphorquinone (1) in methanol and isopropyl alcohol solution has been studied in detail. The products are the isomeric ketols 2 and 3, plus, in methanol, adducts 4 and 5. The quantum yields for disappearance of the diketone are 0.018 ± 0.003 in methanol and 0.057 ± 0.006 in isopropyl alcohol. The intermediate semiquinone radical, 7, has been characterized by electron spin resonance spectrophotometry. The radical appears either to have a symmetrical structure or to oscillate rapidly between two equivalent unsymmetrical structures. The rate of termination for 7 in isopropyl alcohol was found to be 1.6 × 107 M−1 sec−1. The reaction is sensitized with either m-methoxyacetophenone or benzophenone. Reaction in the presence of the former is unexceptional but use of benzophenone results in much increased quantum yields which increase as the concentration of 1 decreases. This anomalous effect is attributed to “chemical sensitization” in which benzophenone ketyl radical, formed by hydrogen abstraction from the solvent, transfers a hydrogen atom to camphorquinone. © 1969, American Chemical Society. All rights reserved.