4-chlorophenol conversions

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. 

The advantages of microreactors, for example, well-defined control of the gas-liquid distributions, also hold for photocatalytic conversions. Furthermore, the distance between the light source and the catalyst is small, with the catalyst immobilized on the walls of the microchannels. It was demonstrated for the photodegradation of 4-chlorophenol in a microreactor that the reaction was truly kinetically controlled, and performed with high efficiency [32]. The latter was explained by the illuminated area, which exceeds conventional reactor types by a factor of 4-400, depending on the reactor type. Even further reduction of the distance between the light source and the catalytically active site might be possible by the use of electroluminescent materials [19]. The benefits of this concept have still to be proven. 

As discussed in the earlier survey (1), a biogenic source of polychlorinated dibenzo-p-dioxins and dibenzofurans is peroxidase-catalyzed transformation of chlorophenols as first reported by Oberg and Rappe (2041-2044). More recent studies confirm these observations (2045-2048). In addition to lactoperoxidase and horseradish peroxidase, human leukocyte myeloperoxidase catalyzes in vitro formation of dioxins and dibenzofurans from chlorophenols (2046, 2047). Formation rates are in the pmol/mol range (Scheme 3.6) demonstrating that a human biosynthesis of dioxins and furans is not only possible but also likely. These observations are reinforced by the reported in vivo (rats) conversion of the pre-dioxin nona-chloro-2-phenoxyphenol to octachlorodibenzo-p-dioxin (OCDD) (2049), and the production of hepta- and octachlorodibenzo-p-dioxin in the feces of cows fed pentachlorophenol-treated wood (Scheme 3.7) (2050, 2051). 

Figure 8 Toxicities of chlorophenols (CP), methylphenols (MP) and phenol before and after treatment with HRP and SBP. [Initial substrate] = 1 mM, [Initial peroxide] = 1.5 mM. All reactions were conducted at pH 7.0 and 25°C and sufficient enzyme was provided to achieve greater than 95% conversion of phenolic substrate. Residual peroxide was destroyed before toxicity was assessed. (Adapted from Ref. 101.)... 

In some cases, substrates and enzymes are not soluble in the same solvent. To achieve efficient substrate conversion, a large interface between the immiscible fluids has to be established, by the formation of microemulsions or multiple-phase flow that can be conveniently obtained in microfluidic devices. Until now only a couple of examples are published in which a two-phase flow is used for biocatalysis. Goto and coworkers [431] were first to study an enzymatic reaction in a two-phase flow in a microfluidic device, in which the oxidation ofp-chlorophenol by the enzyme laccase (lignin peroxidase) was analyzed (Scheme 4.106). The surface-active enzyme was solubilized in a succinic acid aqueous buffer and the substrate (p-chlorophenol) was dissolved in isooctane. The transformation ofp-chlorophenol occurred mainly at... 

The starting point for the metallation sequence was /7-chlorophenol, whose directing power was first maximised by conversion to the carbamate 3. Ortholithiation and reaction with N,N-diethylcarbamoyl chloride gave the amide 4. The second ortho carbonyl substituent was then introduced simply by allowing the lithiated carbamate 5 to undergo an anionic ortho-Fries rearrangement to the bis-amide 6. The phenol was protected as its methyl ether 7. 

Dichlorophenol has a large-scale use in pharmaceutical and herbicide synthesis, but is classed as a priority recalcitrant environmental pollutant conversion to 2-chlorophenol, which can be recycled, is therefore a desirable reaction. Ni/Si02, Au/SiC>2 and NiAu/SiC>2 have been prepared from diaminoethane complexes and tested for this reaction their activities rising in the sequence... 

Hydrolysis of chlorobenzene and the influence of silica gel catalysts on this reaction have been studied by Freidlin and co-workers (109). Pure silica gel gave up to 45% phenol from chlorobenzene at 600°C. When the silica gel was promoted with 2% cupric chloride, up to 75% phenol was obtained (381). A number of other salts were tested by Freidlin and co-workers as promoters, but they exerted an adverse effect on the activity or selectivity of the catalyst. With 0.2% cupric chloride and 6% metallic copper, the activity of silica-gel was doubled (389). At 500° under the above conditions, the halides were hydrolyzed at rates decreasing in the following order chloride, bromide, iodide, fluoride (110). The specific activation of aryl halides by cupric chloride was demonstrated by conversion of chlorobenzene to benzene and of naphthyl chloride to naphthalene when this catalyst was supported on oxides of titanium or tin (111). The silica promoted with cupric chloride was also found to be suitable for hydrolysis of chlorophenols and dichlorobenzenes however, side reactions were too prominent in these cases (112). 

The nature of the cathode has been found to have major effect on the efficiency of electrochemical HDH of halogenated compounds. For instance, the HDH of 12 mM chlorobenzene at carbon cloth or lead cathodes gave conversions up to 95% with a current efficiency of 20%, lower conversion and efficiency (<5%) were observed using platinum, titanium or nickel cathodes (Zanaveskin et al. 1996). A 100% electrochemical HDH of 153 ppm 4-chlorophenol to phenol was achieved using a palladium-coated carbon cloth cathode (Balko et al. 1993). Unfortunately, several environmentally unacceptable materials, such as Hg and Pb, have also been used as cathodes (Bonfatti et al. 1999 Kulikov et al. 1996). 

Chapter 7 reports a scaling-up procedure for photocatalytic reactors. The described methodology uses a model which involves absorption of radiation and photocatalyst reflection coefficients. The needed kinetics is obtained in a small flat plate unit and extrapolated to a larger reactor made of three concentric photocatalyst-coated cylindrical tubes. This procedure is applied to the photocatalytic conversion of perchloroethylene in air and to the degradation of formic acid and 4-chlorophenol in water. 

Chlorophenol is also reactive and irradiation in water leads to its conversion into resorcinoP" or in methanol to yield 3-methoxyphenol in 94% yield. Photoamidation with N-methylacetamide of 3-chlorophenol is also efficient and resnlts in the formation of the phenol 241 in a yield of 77%. Intramolecnlar amidation arises on irradiation of 242 in basic methanol. This resnlts in the formation of the indole derivative 243 as well as the methoxylated prodnct 244. More complex halophenols such as 245 are also photochemically reactive, but this yields a complex mixture of products including a benzofuran. The formation of this must be similar to the cychzations described earlier and involves the attack of a radical, produced by the C—I bond fission, on the other ring . 3-Nitrophenol is converted on irradiation in aqueous solution into a variety of products such as nitrocatechols, nitroresorcinol and resorcinol itself... 

Equations (7-9), (7-10) and (7-11) allow modeling the photo-catalytic conversion of 2-chlorophenol and 2,4 dichlorophenol, under this special case and, as a result, without the requirement of having to define q . Only the groups k cuhq. m and K q are assessed with experimental data. 

A similar photohydrolysis occurs with bromobenzene and polychlorobenzenes (18,19), but e]q>eriments were limited to di- and trichlorobenzenes ( for solubility reasons ). As shown by HPI , the primary products in the photoreaction of dichlorobenzenes and trichlorobenzenes were respectively chlorophenols and dichlorophenols. The accumulation of the primary products was much smaller with trichlorobenzenes than with monochlorobenzene, because photochemical secondciry processes occurred even at a very low rate of conversion. Hie following photohydrolysis qucintum yields were obtained (18,21). 

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