Photohydroxylation of 1,4‐Benzoquinone in Aqueous Solution Revisited

AbstractIn water, photolysis of 1,4‐benzoquinone, Q gives rise to equal amounts of 2‐hydroxy‐1,4‐benzoquinone HOQ and hydroquinone QH2 which are formed with a quantum yield of Φ=0.42, independent of pH and Q concentration. By contrast, the rate of decay of the triplet (λmax=282 and ∼410 nm) which is the precursor of these products increases nonlinearly (k=(2→3.8)×106 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 Q2* (K=5500±1000 M−1). While Q* adds water and subsequent enolizes into 1,2,4‐trihydroxybenzene Ph(OH)3, Q2* decays by electron transfer and water addition yielding benzosemiquinone .QH and .OH adduct radicals .QOH. The latter enolizes to the 2‐hydroxy‐1,4‐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 QH2 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. This shows that there are two pathways to photohydroxylation; a free‐radical pathway at high and a non‐radical 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 is an intermediate in the photohydroxylation of Q has been overturned. This view had been based on the observation of the .OH adduct of DMPO when Q is photolyzed in the presence of this spin trap. It is now shown that Q*/Q2* oxidizes DMPO (k ≈1×108 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 (4.2×107), ethanol (6.7×107), 2‐propanol (13×107) and tertiary butyl alcohol (∼0.2×107). DMSO (2.7×109) and O2 (∼2×109) act as physical quenchers.