When aqueous solutions of anthraquinone-2-sodium-sulphonate (D) are photolyzed the quinone becomes hydroxylated and (in the absence of oxygen) redud. On the basis of the kinetics of the photohydroxylation in air or oxygen saturated solutions (PH 2-14), two alternative mechanism are proposed, the 3D*/S (S = H 2 0 or OH-) and the 3D*/D. In the former, D attacks S to produce an adduct DOHwhich is converted by ground-state D to the radical DOH-. In the latter, 3D* attacks D to form a solvent caged radical pair (Do+ De-), the Do+ 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 p flash photolysis permits discrimination between these schemes. The thermodynamic feasibility of OH. production by the 3D*/D reaction is demonstrated.
Anthraquinone-2-sodium sulphonate in " aqueous " solutions within the range pH 14 to 100 % H2S04 (and also in oleum) is both reduced and hydroxylated when irradiated with U.V. or visible light. The major hydroxy-photoproducts were isolated and identified as a 3 : 2 mixture of isomeric a-and /3-hydroxy derivatives of the starting material in the range 40 % H2S04 to pH 9 ; outside this region the relative amount of CL isomer produced falls off rapidly and in strong acid or alkali only the /3-hydroxy product is obtained. ProIonged irradiation produces also small amounts of polyhydroxy derivatives. The pH-dependence of the ratio of a-and fl-hydroxy product yields is correlated with ionization of hydroxyl radical adduct intermediates.
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