Photolysis of aqueous solutions of p-benzoquinone: a spectrophotometric investigation

Abstract

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.

Photohydroxylation of 1,4‐Benzoquinone in Aqueous Solution Revisited

“…[19] From this, a quantum yield of F(Q consumption) = 0.31 was obtained. [19] The quantum yield was found to be independent of pH in the range of pH 0.4±8. We have recently shown that HOQ is a fairly strong acid (pK a 4.2).…”

“…We had used the electrochemical oxida- 442 FULL PAPER tion of 1,2,4-trihydroxybenzene (Ph(OH) 3 ) to generate HOQ, [71] but its oxidation by Q [19] (reaction 8; for the kinetics of this reaction see below) is an alternative and more convenient procedure that was now used for calibration.…”

“…[5] In water and in the absence of hydrogen donors, however, Q is efficiently photohydroxylated (quantum yield of photodecomposition F = 0.31, [19] F = 0.5, [37] ), and as resulting products 2-hydroxy-1,4-benzoquinone, HOQ, and hydroquinone, QH 2 , are formed in equal amounts (F = 0.25). [37] This type of reaction is also given by other quinones, for example, anthraquinonesulfonates, [21, 52±59] 2-methyl-1,4-benzoquinone, [60] 2-methoxy-1,4-benzoquinone [47] and 2,6-dimethylbenzoquinone.…”

Kinetic studies of hydroxyquinone formation from water soluble benzoquinones

“…The concentrations of the stock solutions in our experiments were typically 1×10 -3 mol dm -3 . 1,4-Benzoquinones are photosensitive compounds in solution, 7,8,[31][32][33][34][35][36][37] therefore fresh solutions were prepared for all experiments in brown glass volumetric flask.…”

Síntese, caracterização e estudo de propriedades catalíticas de hidróxidos mistos lamelares contendo Cu(II)/Mg(II)/AI(III)

“…Esse provavelmente é o principal motivo para a não detecção, ou detecção de valores extremamente baixos, de p-benzoquinona em estudos já publicados envolvendo hidroxilação de fenol monitorados por cromatografia gasosa. Além disso, a p-benzoquinona, em meio aquoso e sob luz, é capaz de clivar as moléculas de água, formando o radical hidroxila [99], [100], [101] . …”