Chlorophenols reaction quantum yields

The carbene thus reacts with O2 to form an orffio-benzoquinone O-oxide, and with an aliphatic alcohol as H-donor to form a phenoxyl radical (plus an aliphatic radical not shown in Scheme 1). The ground state triplet electronic configuration of this carbene accounts for its reaction behavior, in particular for the fact that it reacts very slowly with the solvent, H2O. In agreement with the intrinsically faster intersystem crossing of 2-bromophenol compared to 2-chlorophenol, the quantum yield of the carbene pathway was higher for the former = 0.04) than for the latter compound (< = 0.003). In contrast, the quantum yields of photo contraction were comparable (< = 0.04). The transient absorption data were confirmed by photoproduct analysis, showing the formation of phenol from 4-bromophenol in the presence of H-donors [16]. 

Apparent Quantum Yield (0 ) of the Direct Photolysis of Chloro-phenols. The spectral data and sunlight photolysis rate constants of both actinometer valerophenone and the compound, and reaction quantum yield of valerophenone were used to calculate 0 of chlorophenols using GCSOLAR program (13). 

The mechanism of the aqueous photochemistry of 4-chlorophenol has been reviewed earlier [5,8]. Its basic features are the same as those of the carbene pathway described above for 2-bromophenol. The main differences are the fact that this is the only photolytic reaction of 4-chlorophenol and that its quantum yield is considerably higher than that of the 2-sub-stituted analogues

triplet carbene, 4-oxocyclohexa-2,5-dienylidene (A.max = 384 and 370 nm) from aqueous 4-chlorophenol (see Fig. 1) [20]. Photoproduct analysis yielded p-benzo-quinone (in the presence of O2), phenol (in the presence of an alcohol), hydroquinone and isomeric chlorodihydroxybiphenyls, which could all be accounted for by carbene reactions [20]. 

The photolysis of aqueous 3-chloroaniline was found to proceed via a clean photohydrolysis step to give 3-aminophenol with a quantum yield of

hydrolysis process or a fast reaction of a primarily formed aminophenyl cation with H20 (see Scheme 5). 

A charge transfer complex is involved in the photochemical reaction between 4-cresol and tetranitromethane. Irradiation at 350 nm yields the o-nitrated product 235 ° . Other phenols such as phenol, 2- and 4-chlorophenol and 2- and 4-cresol behave in a similar manner and irradiation yields 2- and 4-nitrated products (236, 237) ° . The quantum yields for product formation are in the range 0.12-0.31. Only the formation of 3-nitrophenol from phenol is inhibited, as might be expected from attack at the 3-position, and shows a low quantum yield. It has been reported that 2-hydroxy- or 4-hydroxybiphenyl and 4,4 -dihydroxybiphenyl are the primary products formed from the photochemical reaction of biphenyl with sodium nitrate in aqueous methanol . Apparently the hydroxybiphenyls are prone to undergo photochemical nitration as a secondary process and yield the biphenyls 238 and 239 as well as 4,4 -dihydroxy-3,3 -dinitrobiphenyl, originally reported by Suzuki and coworkers under heterogeneous conditions. 

Oxidation of 4-chlorophenol can be brought about by single photodecomposition by hydroxy radicals generated from Fenton s reagent (H2O2 plus Fe ions) . Irradiation in the 320-400 nm range with Fenton s reagent is also effective in the oxidation of 4-chlorophenol . Continuous irradiation at 365 nm has identified two different reaction pathways with formation of the 4-chlorodihydroxycyclohexadienyl radical and also of the chlorophenoxyl radical. The quantum yields of these processes have been determined to be 0.056 and 0.015, respectively . Reaction of 4-chlorophenol with ozone leads to the formation of 4-chloro-l,3-dihydroxybenzene and 4-chloro-l,2-dihydroxybenzene. The latter product is produced in quantity in the presence of hydroxyl radicals . ... 

Only one difference appecurs with 3-chlorophenol the photohydrolysis could be sensitized by phenol (37), whereas this reaction did not occur with chlorobenzene. The quantum yield of the sensitized photohydrolysis depended on the concentration of the acceptor and increased from 0.24 to 0.45 when the concentration of 3-chlorophenol increased from 4.4x10 5 to 6kl0 mol.l. ... 

The quantum yield of conversion of 4-chlorophenol was not measured with high accuracy ( 0.4 0.1 ). At low conversion rate (<0.1), the relative error on the conversion of 4-chlorophenol determined by HPLC measurement is about 25%. Moreover, the optical density at 254 nm increeised much with the progress of the reaction ( about a factor of 5 for 15% of transformation ). At conversion rates higher than 0.1, secondary reactions are not negligible trtien the solution was Irradiated at 296 nm. Nevertheless, the initial quantum yield of formation of Cl ( 0.25 0.05 ) appeared to be significantly lower than the quantum yield of conversion of 4-chlorophenol (37). 

Table II. Molar Absorptivities and Reaction Quantum Photolysis of Phenol and Chlorophenols Yields ((ir) for Direct...

Radiolysis and photodecomposition of chlorite and chlorate have been studied in aqueous solution. Reaction schemes, rate constants, and quantum yields are reported.The first stage in the reaction of chlorine with phenol is the second-order formation of [CI2 PhOH], with a rate constant 2 = 2.3 x 10 lmol- s at 5 °C. This [Clg-PhOH] intermediate, whose properties are similar to those of known charge-transfer compounds of halogens with aromatic compounds, decomposes by three parallel pathways. Chlorine reacts more slowly with chlorophenols than with phenol itself. The reaction of chlorine dioxide with phenol follows a comparable rate law ... 

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