The photolysis of aqueous solutions of p-benzoquinone is investigated spectrophotometrically over a wider range of pH, concentration, and the wavelength of irradiation than before. Experimental evidence has been obtained for the formation of benzene-I ,2,4-triol as the sole primary photochemical product at all values of pH. Quinol and 2-hydroxy-I ,4-benzoquinone are produced in equimolar quantities as secondary products both in acid and alkaline solutions, when the reaction between p-benzoquinone and benzene-I ,2,4-triol is very fast. 2-Hydroxy-I ,4-benzoquinone at higher concentrations polymerizes to the so-called ' humic acid '. Between pH 4 and 6 benzene-I ,2,4triol is the chief photoproduct owing to suppression of the secondary reaction.BRUCE has reviewed the light-induced reactions of quinones and has pointed out that no solvent so far tried is entirely inert toward quinones, especially under illumination. Leighton and Forbes reported that 9-benzoquinone (I) in aqueous solutions when irradiated with mercury light was converted into quinol (11) and an unknown product. Later Poup6 reinvestigated this system using polarography and confirmed that one of the photoproducts was (11) and that the other was 2-hydroxy-1,d-benzoquinone (111). Poup6 suggested a reaction mechanism which assumes that the primary photochemical process is the conversion of (I) into benzene-1,2,4-triol (IV). According to his results, the photolysis was independent of pH in the range 5-8 and of wavelength in the range 3600-4600 a : at higher pH and at high initial concentrations of (I) the photolysis was more complex. Poupg was unable to investigate the system at lower values of pH because his polarographic technique was not applicable to acid solutions. More recently Joschek and Miller irradiated solutions of (I) with 2537 A light and, using chromatographic techniques, detected quinol (11) and the triol (IV) as the chief products in the absence of oxygen, while (11) alone was the chief product in the presence of oxygen. The photochemistry of 9-benzoquinone in aqueous solution is thus not yet fully clarified. Since spectrophotometry not only allows investigation of this system at all pH values but also records the structural changes taking place, it was decided to use this method.

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