Abstract:
When aqueous solutions of anthraquinone-2-sodium-sulphonate (D) are photolyzed the quinone becomes hydroxylated and (in the absence of oxygen) reduced. On the basis of the kinetics of the photohydroxylation in air or oxygen saturated solutions (pH 2–14), two alternative mechanisms are proposed, the 3D*/S (S = H2O or OH–) and the 3D*/D. In the former, D attacks S to produce an adduct DOH– which is converted by ground-state D to the radical DOH·. In the latter, 3D* attacks D to form a solvent caged radical pair {D·+ D·–}, the D·+ constituent then reacting with S to give OH· radicals which with D yield DOH·. This intermediate is converted by oxygen to hydroxy product. Neither continuous irradiation kinetics nor conventional µs flash photolysis permits discrimination between these schemes. The thermodynamic feasibility of OH· production by the 3D*/D reaction is demonstrated.
