The oxidation of organic compounds by “singlet” oxygen

Abstract

The normal or “triplet” ground state (³Σg^–) of molecular oxygen is that of a biradical, ·O–O·, but “singlet” oxygen (¹Δg) OO, which is structurally similar to ethylene, is the initial product of heterolytic decompositions of hydrogen peroxide or of per-acids. “Singlet” oxygen reverts to normal “triplet” oxygen by a bimolecular reaction that produces chemiluminescence but is sufficiently long-lived to be able to react as a dienophil; it combines with anthracenoid hydrocarbons at their meso positions, or adds to suitable 1,3-dienes.

Singlet oxygen has been produced (i) by the reaction of alkaline hydrogen peroxide with either sodium hypochlorite or bromine and (ii) by decompositions of alkaline solutions of per-acids, either alone or with added hydrogen peroxide. By these methods the endo-peroxides (the “photoperoxides”) of a number of 9,10-di-substituted anthracenes have been prepared in the dark, whilst 9-substituted anthracenes have given typical hydrolysis products of the primary endo-peroxides. The most effective route uses the H₂O₂–Br₂ reaction, but gives side-products owing to concurrent bromine addition unless a two-phase reaction mixture is used. With the H₂O₂–NaOCl system the dienophilic addition of oxygen to lumisteryl acetate and to 2,4-cholestadiene has also been established.

Similar evidence is given to show that the chemiluminescent Trautz reaction also yields singlet oxygen.