Photohydroxylation of 1,4-benzoquinone in aqueous solution revisited

In water, photolysis of 1,4-benzoquinone, Q gives rise to equal amounts of 2-hydroxy-1,4-benzoquinone HOQ and hydroquinone QH(2) which are formed with a quantum yield of Phi=0.42, independent of pH and Q concentration. By contrast, the rate of decay of the triplet (lambda(max)=282 and similar to 410 nm) which is the precursor of these products increases nonlinearly (k = (2 --> 3.8) x 10(6) s(-1)) with increasing Q concentration ((0.2 --> 10) mm). The free-radical yield detected by laser flash photolysis after the decay of the triplet also increases with increasing Q concentration but follows a different functional form. These observations are explained by a rapid equilibrium of a monomeric triplet Q* and an exciplex Q(2)* (K=5500 +/- 1000 M-1). While Q* adds water and subsequent enolizes into 1,2,4-trihydroxybenzene Ph(OH)(3), Q(2)* decays by electron transfer and water addition yielding benzosemiquinone (.)QH and (OH)-O-. adduct radicals (.)QOH. The latter enolizes to the 2-hydroxy-1,4agreement with this, the yield of Ph(OH)(3) is most pronounced at low Q concentration. In the presence of phosphate buffer, Q* reacts with H2PO4- giving rise to an adduct which is subsequently oxidized by Q to 2-phosphato-1,4-benzoquinone QP The current view that (OH)-O-. is an intermediate in the photohydroxylation of Q has been overturned. This view had been based on the observation of the (OH)-O-. adduct of DMPO when Q is photolyzed in the presence of this spin trap. It is now shown that Q*/Q(2)* oxidizes DMPO (k approximate to 1 x 10(8) M-1 s(-1)) to its radical cation which subsequently reacts with water. Q*/Q2* react with alcohols by H abstraction (rates in units of M-1 s(-1)): methanol (4.2 x 10(7)), ethanol (6.7 x 10(7)), 2-propanol (13 x 10(7)) and tertiary butyl alcohol (similar to0.2 x 10(7)). DMSO (2.7 x 10(9)) and O-2 (similar to2 x 10(9)) act as physical quenchers.-semiquinone radical (.)Q(OH)H within the time scale of the triplet decay and is subsequently rapidly (microsecond time scale) oxidized by Q to HOQ with the concomitant formation of (.)QH. On the post-millisecond time scale, that is, when (.)QH has decayed, Ph(OH)(3) is oxidized by Q yielding HOQ and QH(2), as followed by laser flash photolysis with diode array detection. The rate of this pH- and Q concentration-dependent reaction was independently determined by stopped-flow. Ibis shows that there are two pathways to photohydroxylation; a free-radical pathway at high and a nonradical one at low Q concentration. In agreement with this, the yield of Ph(OH)(3) is most pronounced at low Q concentration. In the presence of phosphate buffer, Q* reacts with H2PO4- giving rise to an adduct which is subsequently oxidized by Q to 2-phosphato-1,4-benzoquinone QP. The current view that (OH)-O-. is an intermediate in the photohydroxylation of Q has been overturned. This view had been based on the observation of the (OH)-O-. adduct of DMPO when Q is photolyzed in the presence of this spin trap. It is now shown that Q*/Q(2)* oxidizes DMPO (k approximate to 1 x 10(8) M-1 s(-1)) to its radical cation which subsequently reacts with water. Q*/Q2* react with alcohols by H abstraction (rates in units of M-1 s(-1)): methanol (4 7 x 10(7)), 2-propanol (13 x 10(7)) and tertiary butyl alcohol (similar to0.2 x 10(7)). DMSO (2.7 x 10(9)) and O-2 (similar to2 x 10(9)) act as physical quenchers.