Temperature dependence of the rate of reaction of OH with some aromatic compounds in aqueous solution. Evidence for the formation of a π-complex intermediate?

Ashton, Lorraine; Buxton, George V.; Stuart, Craig R.

doi:10.1039/ft9959101631

Cited by 125 publications

(117 citation statements)

References 21 publications

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“…Curve (0) in Fig. 9 represents the spectrum recorded during pulse radiolysis of N 2 O-purged 1 mM benzene at alkaline pH indicating HO-adduct of benzene, i.e., hydroxy cyclohexadienyl type radical, which is having absorption in UV region (k max around 310 nm) and is similar to the reported one [42][43][44]. In contrast to hydroxy cyclohexadienyl radical in the benzene pulse radiolysis spectrum, a spectrum having two bands with maxima around 400 and 380 nm was observed during electron pulse irradiation of filtrate solutions obtained from photolysis experiments.…”

Section: Resultssupporting

confidence: 51%

Significant roles of oxygen and unbound •OH radical in phenol formation during photo-catalytic degradation of benzene on TiO2 suspension in aqueous system

Dey

2009

Res Chem Intermed

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Studies on photo-catalytic degradation of benzene using TiO 2 photocatalyst as a suspension in water is reported. Degradation studies have been carried out using 350 nm UV light. Phenol, a photo-catalytic product of benzene, was monitored under varying experimental conditions such as amount of TiO 2 , concentration of benzene, photolysis time, ambient (air, O 2 , Ar, N 2 O and N 2 O-O 2 mixture), etc. The phenol yields in both aerated and O 2 -purged systems increased with the photolysis time. In contrast to oxygenated systems, the yields of phenol in deoxygenated (viz. Ar-purged and N 2 O-purged) systems were quite low (*30 lM) and remained steady. H 2 O 2 yields in all these systems were also monitored, and found lower than an order of magnitude as compared to phenol yields for the respective systems. The rate of phenol production in aerated 1 mM benzene solution containing 0.05% TiO 2 suspension was evaluated at 12.3 lM min -1 which is lower than the rate obtained in an O 2 -saturated system (22.4 lM min -1 ). The low yields of phenol in both Ar-and N 2 O-purged systems, and also the increasing trends in oxygenated systems, together reveal that, for the phenol formation with an enhanced rate, oxygen is essential. In the present study, it is implied that the photo-generated hole, which is mainly an • OH radical, is either freely available in the aqueous phase or migrates to the aqueous phase from the catalyst surface, to react with benzene to produce HO-adduct radical. Later, following reaction with oxygen, the adduct produces phenol. On the other hand, h ? and surface adsorbed • OH radical, being trapped/bonded due to rigid association with the catalyst surface, were not able to generate phenol under similar experimental conditions. The mechanism of phenol formation with TiO 2 photolysis in an aqueous system is rechecked, on the basis of present results on h ? /surface adsorbed • OH radical/unbound • OH radical scavenging by benzene, collectively with previous reports on various systems.

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Section: Resultssupporting

confidence: 51%

Significant roles of oxygen and unbound •OH radical in phenol formation during photo-catalytic degradation of benzene on TiO2 suspension in aqueous system

Dey

2009

Res Chem Intermed

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“…Similar temperature dependences for reaction of hydroxyl radicals with aromatic compounds have recently been reported and discussed (20).…”

Section: Radiolysis Studies Of 1-hexyn-3-01supporting

confidence: 49%

Radiolysis studies on the corrosion inhibitors 1 -hexyn-3-ol, cinnamonitrile, and 1,3-diethyl-2-thiourea

Elliot¹,

Chenier²,

Ouellette³

1995

Can. J. Chem.

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Abstract:In this publication we report: (i) the rate constants for reaction of the hydrated electron with l-hexyn-3-01 ((8.6 ? 0.3) x lo8 dm3 mol-' s-' at lS°C), cinnamonitrile ((2.3 ? 0.2) x dm3 mol-I s-' at 2OoC), and 1,3-diethyl-2-thiourea ((3.5 ? 0.3) x lo8 dm3 mol-I s-I at 22'C). For cinnamonitrile and diethylthiourea, the temperature dependence up to 200°C and 150°C, respectively, is also reported; (ii) the rate constants for the reaction of the hydroxyl radical with 1-hexyn-3-ol((5.5 ? 0.5) x lo9 dm3 mol-I s-I at 20°C), cinnamonitrile ((9.2 ? 0.3) x lo9 dm3 mol-I s-I at 21°C), and diethylthiourea ((8.0 ? 0.8) x lo8 dm3 mol-I s-I at 22OC). For cinnamonitrile, the temperature dependence up to 200°C is also reported; (iii) the rate constant for the hydrogen atom reacting with 1-hexyn-3-01 ((4.3 ? 0.4) x lo9 dm3 mol-I s-' at 20°C).

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“…p-Complexes of radicals to aromatic compounds are known, and even for a radical as reactive as COH it has been shown that such a complex has to be postulated as an intermediate in its reaction with benzene to the hydroxycyclohexadienyl radical. [93] However, a p-complex of a strongly oxidizing radical such as COH with a strongly reducing radical such as CQH/QC À (as postulated in ref. [60]) will immediately collapse either by electron transfer of by recombination.…”

mentioning

confidence: 98%

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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Temperature dependence of the rate of reaction of OH with some aromatic compounds in aqueous solution. Evidence for the formation of a π-complex intermediate?

Cited by 125 publications

References 21 publications

Significant roles of oxygen and unbound •OH radical in phenol formation during photo-catalytic degradation of benzene on TiO2 suspension in aqueous system

Significant roles of oxygen and unbound •OH radical in phenol formation during photo-catalytic degradation of benzene on TiO2 suspension in aqueous system

Radiolysis studies on the corrosion inhibitors 1 -hexyn-3-ol, cinnamonitrile, and 1,3-diethyl-2-thiourea

Photohydroxylation of 1,4‐Benzoquinone in Aqueous Solution Revisited

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