Photohydroxylation of 1,4‐Benzoquinone in Aqueous Solution Revisited

Abstract

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 approximately 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 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 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. 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 H(2)PO(4) (-) 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*/Q(2)* oxidizes DMPO (k approximately 1 x 10(8) M(-1) s(-1)) to its radical cation which subsequently reacts with water. Q*/Q(2)* 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 ( approximately 0.2 x 10(7)). DMSO (2.7 x 10(9)) and O(2) ( approximately 2 x 10(9)) act as physical quenchers.

Radioprotectant screening for cryocrystallography

“…4c(i) and 4c(ii) show the 30% EG/0.1 M cystine/0.5 M quinone sample, where quinone scavenging is clearly effective. For the control EG and 0.5 M quinone sample, a small peak was observed just above 400 nm which was due to the quinone radical, known to absorb at 424 nm (von Sonntag et al, 2004). This peak was sharper than that of the disulfide radical anion and much smaller in magnitude.…”

No Marked Kinetic Isotope Effect in the Peroxone (H2O2/D2O2+O3) Reaction: Mechanistic Consequences

“…A "pseudo COH radical" has been postulated before for another type of reaction but proved to be non-existing. [23] Further research will focus on the interesting chemistry of HO 3 À to decide, whether HO 3 À could contribute to COH production. …”

AOX-Bildung bei der sonolytischen Oxidation von Salicylsäure in Gegenwart von Chlorid

“…Die Bildung farbiger Zwischenprodukte durch oxidative Kupplung zu mehrkernigen Aromaten wurde bereits bei der Oxidation substituierter Aromaten durch Ozon beschrieben [22]. Zusätzlich führt auch die Bildung von Hydroxyhydrochinonen durch OH-Radikalreaktion mit substituierten Aromaten zur Verfärbung der Lö-sungen [23]. Des Weiteren wurden bei der SS-Sonolyse, wie von Keck beschrieben [24], höher hydroxylierte Produkte identifiziert.…”