4-chlorophenol reaction pathway

The reaction pathway of 2,4-dichlorophenol is illustrated in Figure 9.16. The CT reaction site leads to the product 2-chloro-4-hydroxyphenol (D Oliv-iera et al., 1993). Bond cleavage of the aromatic ring is observed to yield the minor products l-hydroxy-4-chlorophenol and 2-chloro-2,5-cyclodihexene-1,4-one, as shown in Figure 9.17. 

Ballschmiter K, Braunmiller I, Niemczyk R, Swerev M (1988), Chemosphere 17 995-1005.. .Reaction pathways for the formation of polychlorodibenzodioxins (PCDD) and -furans (PCDF) in combustion processes II Chlorobenzenes and chlorophenols as precursors in the formation of polychlorodibenzodioxins and -dibenzofurans in flame chemistry"... 

Figure 16 Reaction pathway suggested for the photocatalytic degradation of 4-chlorophenol in acidic medium.

Intermediates and Reaction Pathway. Identification of Inter mediates. The derivatization technique and GC-MS analysis were success fully used to detect organic pollutants in the aquatic environment (26, 27) This same technique was chosen to identify the partial oxidation products o 4-chIorophenol in Ti02 aqueous suspensions. Figure 5 and Table II show th< mass spectra of the derivadzed reaction products of 4-chlorophenol. Fou derivatized intermediate peaks [A, B, C, and D, with retention times (RTs at 10.32, 12.50, 13.72, and 14.11 min, respectively] were observed. Peak A i the acetylated parent compound, 4-chlorophenol. Peaks B, C, and D are ace tylated intermediate products. The identification of peaks B, C, and D wa based on the following observations ... 

Reaction Mechanisms. Our analysis of intermediates and reactions reported by other researchers leads to proposed reaction pathways describing the photocatalytic oxidation of 4-chlorophenol in TiOz aqueous suspensions. The photocatalytic oxidation reaction is brought about by OH radicals, which are formed mainly from water decomposition on the Ti02 surface upon UV light irradiation (9-13). The OH radicals can either directly react with the adsorbed organic species on the TiOa surface or diffuse to the solution and then react with the dissolved organic species in the solution phase. Both reactions lead to formation of hydroxylated products such as 4-chlorocatechol, hydroquinone, 4-chlororesorcinol, and hydroxyhydroquinone as the initial products (Figure 6). Eventually, the reaction will mineralize these interme-... 

Scheme I shows one of the possible reaction pathways involving OH radicals as the main reaction species in the Ti02-mediated photocatalytic oxidation of 4-chlorophenol. Only nonradical species were detected in this reaction scheme.

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 . ... 

Scheme 10.16 Reaction pathways of 4-oxocyclohexa-2,5-dienylidene (48) generated by photodecomposition of 4-chlorophenol (46)...

Both pathways have been shown to be relevant for PCDD/F formation in municipal-waste incinerations. Chlorophenols can be converted to PCDD by copper species known in synthetic chemistry as the Ullmann type II coupling reaction. By use of isotope labeling techniques in competitive concurrent reactions with both reactions performed in laboratory experiments it was shown that precursor theory pathways from chlorophenols may be more important compared to the de novo pathway, but either competing pathway strongly depends on such conditions as temperature, air flow rate, and residence time. It may be difficult to model the complex reahty of large incinerators using relevant laboratory experiments. 

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]. 

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 Ti02-mediated photocatalytic oxidation reaction can be described by the radical mechanism involving OH as the major reaction species. The reaction mechanism follows the ortho pathway, so that the main intermediate found is 4-chlorocatechol, whereas the formation of 4-chlororesorcinol and hydroquinone is only a minor pathway. Further degradation of 4-chloroca-techol leads to production of hydroquinone, which can be further oxidized and mineralized to carbon dioxide. In contrast, the direct photolysis of 4-chlorophenol follows the para pathway, which leads to the formation of hydroquinone and p-benzoquinone as the major products. 

A complete pathway for the mineralization of 4-chlorophenol can be described by the hydroxylation reaction through dechlorination, ring cleavage, and mineralization. Reaction kinetics of the 4-chlorophenol and its intermediates can be reasonably well approximated by using a complex parallel and consecutive first-order reaction mechanism. 

However, GC-MS and HPLC analysis indicate that the only detectable trihydroxybenzene is hydroxyhydroquinone, and no CTHB is detected. This result may indicate either that CTHB is not stable (it quickly degenerates to other species after it is formed) or that this pathway is not favorable. 4-Chlo-rocatechol has fewer hydrogen atom sites than 4-chlorophenol. Therefore, the chance of OH radical attack on the hydrogen atom site of 4-chlorocatechol will be smaller than the chance of a similar attack on the hydrogen atom sites of 4-chlorophenol. Conversely, there are more chances for OH radical attack on the chlorine atom site. The experimental results suggest that the dechlorination reaction is the main pathway to formation of hydroxyhydroquinone, although the formation of chlorotrihydroxybenzene still cannot be excluded. 

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