Mechanism of the photochemistry of p-benzoquinone in aqueous solutions. 2. Optical flash photolysis studies

Ononye, A. I.; Bolton, James R.

doi:10.1021/j100281a040

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“…[38] This is in contrast to data which indicate that in this system 1,4-benzosemiquinonetype radicals must be minor contributors to photohydroxylation (< 10 %) compared with a non-radical pathway. [37] Based on spin-trapping experiments, it has been suggested [39,44,45] that the first step is an H-abstraction from water [reaction (4)}, that is, that the reaction is analogous to reaction (1).…”

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confidence: 69%

“…Such processes have been observed before with many other systems, and with optically active sulfoxides an isomerization is observed; an exciplex as a very short-lived intermediate has been postulated. [87,88] The free-radical intermediates: The formation of free-radical intermediates has been noticed before and has been attributed to the formation of CQH and CQ(OH)H, [32] or only to CQH, [38] and, depending on pH, to their corresponding radical anions. From a flash photolysis study of a 0.4 mm Q solution at pH 7, it was concluded that F(CQH) = 0.47.…”

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confidence: 96%

“…This has been attributed to the formation of free radicals. [32] Since only one Q concentration had been studied, it escaped attention [32,38] that the radical yield is also a function of the Q concentration ( Figure 6, lower curve, left scale). In a flash photolysis study on duroquinone (2,3,5,6-tetramethyl-1,4-benzoquinone) in aqueous solution, [84] free-radical formation was only observed upon triplet±triplet annihilation, that is, at high triplet concentrations.…”

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Photohydroxylation of 1,4‐Benzoquinone in Aqueous Solution Revisited

Sonntag

Mvula

Hildenbrand

et al. 2004

Chemistry A European J

129

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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.

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confidence: 69%

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confidence: 96%

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confidence: 99%

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Photohydroxylation of 1,4‐Benzoquinone in Aqueous Solution Revisited

Sonntag

Mvula

Hildenbrand

et al. 2004

Chemistry A European J

129

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“…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.…”

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confidence: 99%

Kinetic studies of hydroxyquinone formation from water soluble benzoquinones

et al. 2014

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Table of contents entryThe kinetics of the reactions between 1,4-benzoquinones and hydrogen peroxide were studied.A systematic study of substituent effects revealed a Hammett-like correlation, where the rate of reaction is mainly determined by electronic effects. 3 AbstractThe kinetics and mechanisms of the redox reactions between hydrogen peroxide and 1,4-benzoquinone, 2-methyl-1,4-benzoquinone, 2,6-dimethyl-1,4-benzoquinone, 2-chloro-1,4-benzoquinone and 2,6-dichloro-1,4-benzoquinone were studied in aqueous media using spectrophotometric monitoring. The formation and decay of a hydroxylated 1,4-benzoquinone was detected. The formation of the intermediate was first order with respect to the parent 1,4-bezoquinone and hydrogen peroxide, whereas inverse first order dependence was revealed with respect to hydrogen ion. The decomposition reaction had two parallel pathways: one was first order with respect to the intermediate, while the other showed second-order dependence.The values of the rate constant measured for the formation step were successfully correlated with both the redox potentials of the substituted quinone/hydroquinone systems and the pK a values of the hydroxylated quinone derivatives. Therefore, electronic effects govern the reactivity of the quinones in this process. NMR and GC-MS measurements were carried out to identify the products in the system. Quantum mechanical calculations were also carried out in these systems.4

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“…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] . …”

Section: Figura 37 -Estruturas Do Fenol E Dos Compostos Formados Na Runclassified

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

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Mechanism of the photochemistry of p-benzoquinone in aqueous solutions. 2. Optical flash photolysis studies

Cited by 72 publications

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Photohydroxylation of 1,4‐Benzoquinone in Aqueous Solution Revisited

Photohydroxylation of 1,4‐Benzoquinone in Aqueous Solution Revisited

Kinetic studies of hydroxyquinone formation from water soluble benzoquinones

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

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