Organic Photochemistry with 6.7-eV Photons: Irradiation of Molecular Oxygen in Hydrocarbon Solution

Abstract

Irradiation of a solution of oxygen gas in cyclohexane with 185-nm light from a mercury resonance lamp leads to cyclohexyl hydroperoxide as the principal product. Control experiments show that (i) the radiation is absorbed by oxygen to give the observed product, (ii) there is no impurity in the solvent that is responsible for either the initiation or propagation of the process, (iii) a chain reaction is not involved, (iv) the reaction is distinct from thermal autoxidation, and (v) ozone is not an intermediate in the process. It is mostly likely that, on absorption of a photon, oxygen is excited to the3Σu-state and reacts directly with the solvent. Irradiation of oxygen in methylcyclohexane under identical conditions leads to mixtures of nearly all possible methyl cyclohexyl hydroperoxides. The product composition changes very little over the temperature range -10 to +41°C. The lack of selectivity in the formation of these products also supports the intermediacy of an energy-rich species. Photolyses of these solutions with the unfiltered light from a mercury resonance lamp lead not only to hydroperoxides but also to the corresponding alcohols and ketones. The latter are mostly, if not exclusively, formed by the secondary photolysis of the hydroperoxides at long wavelength. © 1979, American Chemical Society. All rights reserved.