Chlorobenzene pyrolysis

Chlorobenzenes are stable compounds and decompose slowly only under excess heating at high temperatures to release some HCl gas and traces of phosgene. It is possible, under certain limited conditions of incomplete combustion or pyrolysis, to form polychlorinated dibenzo-/)-dioxins (PCDDs) and dibenzofurans (PCDFs) from chlorobenzenes (Cm OROCARBONS and cm OROHYDROCARBONS, toxic aromatics). 

Another development is due to the interest in polychlorodibenzofurans, spurred by their occurrence as environmental contaminants. Polychloro-phenols are manufactured in large amounts (150,000 tons per annum) and find a wide range of uses. The usual method of manufacture involves the hydrolysis of chlorobenzenes, and side reactions, favored by high temperature, can lead to the production of polychlorodibenzofurans and poly-chlorodibenzo-p-dioxins. The Seveso incident is well known." Polychloro-biphenyls are also widely used industrial chemicals, particularly in heat exchange systems, and their pyrolysis leads to the formation of polychloro-dibenzofurans. Polychlorodibenzofurans have also been detected in the fly ash and flue gases of incinerators and industrial heating plants. The most toxic of the polychlorodibenzofurans are 2,3,7,8-tetra-, 1,2,3,7,8-penta-, and 2,3,4,7,8-pentachlorodibenzofuran, and an extensive literature exists on the environmental pollution and the results of human exposure to these substances. A particularly tragic example of the latter occurred in 1968 in the Fukuoka prefecture of Japan after consumption of rice oil contaminated with a commercial polychlorobiphenyl. 

However, bromobenzene and chloroaromatics (chlorobenzene, 1,2-dichlorobenzene, 1,2,4-trichlorobenzene) are inert enough to be used routinely,131 181 222-223 except when the aryl cation generated during dediazoniation is stable enough to arylate the aromatic solvents.137 The advantage of pyrolysis in chlorobenzene over dry decomposition has been illustrated in the synthesis of mono- and difluorobenzo[c]phenanthrenes.230... 

The requirement for AlCh adds some obvious limitations, but a modest number of substituted arenes have been successfully coordinated to the [FeCp]+ unit. Photoinduced arene exchange has been described43 and is particularly effective with the -r -chlorobenzene-r -cyclopentadienyliron cation without the need for Lewis acids.42 More basic (electron rich) arenes readily replace chlorobenzene, leading to FeCp cation complexes of O-phenylethyl-p-toluenesulfonate and variously substituted thiophenes.42 The charged complexes are not purified by conventional organic techniques, such as chromatography, but re-crystallization is possible. The complexes are very air and heat stable again, methods of removal of the arene from the Fe are few. The simplest is pyrolysis at >200 C.46... 

The reaotion of benzyne from the pyrolysis of phthalic anhydride with chlorobenzene was expected to give chlorobiphenyls by insertion and naphthalene and chloronaphthalenes by 1,2- and 1,4-addition and rearomatization with respective loss of chloroacetylene and acetylene. 

Buser HR (1979a), Chemosphere 8 415-424.. .Formation of chlorinated dibenzofurans (PCDF) and dibenzo-p-dioxins (PCDD) from the pyrolysis of chlorobenzenes"... 

At low temperatures, PVC pyrolysis yields only traces of volatile chlorinated hydrocarbons. The main chlorine-containing hydrocarbons reported are methyl chloride, vinyl chloride, ethyl chloride and chlorobenzene. Heating PVC at 10°C/min up to 500°C in... 

On pyrolysis chlorobenzene gives phenyl radicals and a small amount of benzyne. ) The phenyl cation is the most intense fragment in the mass... 

Pyrolysis products of chlorinated polyethylene contain molecules similar to those found in polyethylene pyrolysates and, in addition, compounds similar to that obtained from vinyl chloride (significant amount of HCI). Chlorosulfonated polyethylene typically contains only about 1.5% sulfur, but sulfur-containing compounds such as SO2 can be detected among its pyrolysis products. The distribution of chlorine atoms in chlorinated polyethylene has been investigated using Py-GC [55, 56]. The polymer was considered equivalent with a terpoiymer poly[ethylene-co-(vinyl chloride)-co-(1,2-dichloroethylene)]. The level of specific degradation products such as aromatic molecules (benzene + toluene + styrene + naphthalene), chlorobenzene, and dichlorobenzenes correlates well with the carbon/chlorine ratio in the polymer. 

The p-scission is favored compared to other scissions by the strong effect of p-chlorobenzene group. Other reactions likely to occur during the pyrolysis are due to hydrogen transfer reactions followed again by p-scissions as shown in the following schemes ... 

Palchik [182] obtained nanosized amorphous iron oxide (Fe203) by the pyrolysis of iron pentacarbonyl, Fe(GO)5, in a modified domestic microwave oven in refluxing chlorobenzene as solvent under air. The reaction time was 20 min. Separate particles of iron oxide, 2-3 nm in diameter, were obtained together with aggregated spheres with a diameter of 25-40 nm. Differential scanning calorimetry measurements showed an amorphous/crystalline phase transition at about 250 °C. 

A reactitm related to the Schiemann reaction but conducted in aqueous solution rather than by pyrolysis is employed in a synthesis of p-dinitrobenzene from p-nitroaniline. Aryidiazonium fluoroborates containing nitro groups also decompose smoothly in chlorobenzene. ... 

I discussed my speculation with one of my assistants. Bob Cooper. He thought I was crazy. When I suggested that he should test out my speculation in the equipment he had set up for related pyrolysis reactions, he told me that 1 was out of my mind. 1 told him that so what if it was a crazy idea. If it didn t work, all he had to lose was two or three days of work. In order to stop me from pestering him, he did carry out the reaction. However, he added sufficient chlorobenzene to the system, in an order of 20 - 30%, so that there was a meaningful amount of it in the vapor. After a few days, he came to me smiling to report that my crazy idea worked. The added chlorobenzene significantly increased the percent of conversion per pass. After optimization studies on the amount of chlorobenzene to use, he was able to increase the percent conversion per pass of phenylphosphonous dichloride by more than three hundred percent (72). He also found that if he used too much chlorobenzene, there formed as a by-product chlorophenylphosphonous dichloride. Chlorobenzene was quickly introduced as a catalyst in our commercial production plant. 

The triazene 6, also derived from anthranilic acid, is more stable than 4 and can safely be stored in quantity. Benzyne, carbon dioxide, and dimethylamine are formed by decomposition of 6 in chlorobenzene at reflux temperature, or in benzene at 80°C in the presence of trichloroacetic acid.13 Benzyne is also produced on pyrolysis of diphenyliodonium-2-carboxylate (7) in various solvents at 160-220°C.14... 

Figure 4.24 Products bearing heteroatoms (N, O, Cl) formed in the ASR pyrolysis at different temperatures 98 propylene nitrile (1), pyridine (2), chlorobenzene (3),phenol (4), benzaldehyde (5), benzonitrile (6), benzeneacetonitrile (7). (Reprinted from J. Anal. Appl. Pyrol., 40-41, R. Rausa and P. Pollesel, page 383. 1997, with permission from Elsevier Science)...

As far as we are aware, there is only one investigation on a deamination of a chiral methylamine, namely Gautier s (1980) investigation of the pyrolysis of (i )- and (5)-A -pH, H]methyl-A -nitroso-4-toluenesulfonamide (7.30) to give methyl 4-toluene-sulfonate (7-13). The reaction was conducted in chlorobenzene at 95 °C (12 h, yield... 

Pyrolysis of tri-, tetra-, and penta-chlorobenzenes affords mixtures of chlorinated dibenzofurans and dibenzo-p-dioxins, " and chlorinated diphenyl ethers cyclize in the presence of palladium acetate to polychloro-dibenzofurans/ The action of malononitrile on tetrahydroxy-p-benzoquinone leads to the benzo-difuran (79), contrary to a previous report/ The acid-catalysed condensation of quinones with phenols has been studied p-benzoquinone and resorcinol, for example, afford compound (80)/ Naphtho[2,3- >]furan-4,9-diones (81 R = Me or Ph R = Ac, Bz, C02Et, or CN) are obtained from 2,3-dichloronaphtho-1,4-quinone and compounds R COCHaR in DMF that contains potassium fluoride/ The action of mineral acids on p-benzoquinone produces a mixture of complex benzofurans, which includes (82) and (83)/ Bases convert the ben-zofuranone (84) into the tetrameric compound (85), contrary to an earlier... 

PH3 was found to be one of the volatile thermolysis products of the arylphosphonous acids RCgH4P(0)(OH)H with R = H or CH3 in chlorobenzene in sealed tubes at 200 to 220 C after 20 h [30]. Pyrolysis studies of /-C4H9PH2 and f-C4HgPH2, intended to investigate the possible use of these compounds as precursors in the OMVPE process, revealed that PH3 is one of the first decomposition products [33, 34]. 

Finally, benzyne has been postulated to be formed in small amounts during the pyrolysis of chlorobenzene, " nitrobenzene, " benzotrifluoride, " and even benzene itself " " by the m ramolecular elimination of HCl, HNOj, HCF3, and respectively. Plasmolysis of halobenzenes, benzene, and phenyl-acetylene also provides arynes by a similar process. " /n ermolecular dehydrogenation either with other arynes [equation (11)] " or nitrenes [equa-... 

Buser (1979) has shown that pyrolysis of chlorobenzenes at 600 C in the presence of excess air yielded about 1% yield of tetra- to octa-CDFs and tetra- to octa-CDDs. It has also been shown (Buser and Rappe, 1979) that the pyrolysis of PCBs occurs intramolecularly by four alternative reaction routes to yield different isomeric PCDF products. 

Buser, H.R. 1979. Chemosphere, ,415 Formation of PCDFs and PCDDs from the pyrolysis of chlorobenzenes. 

Table 2 provided data on pyrolysis and combustion products from fluids which had been previously tested. This table indicated that information on partial decompositon products was available in the literature for siloxanes, phthalates and chlorobenzenes. 

There is now information available on combustion and pyrolysis products for eight of the fourteen PCB replacement fluids listed in Table 1. Of the eight materials which have been tested (polydimethylsiloxane, RTEmp, phthalates, phenylxylylethane, dixylylethane, isopropylbiphenyl, n-propylbiphenyl, and chlorinated benzenes), the chlorobenzenes appear to present the greatest human health hazard in the event of fire. This is primarily due to the formation of chlorinated dioxins, chlorinated furans, and other chlorinated aromatics during combustion. The other seven fluids appear to be better choices as PCB substitutes, with the nonaromatic fluids perhaps posing the lowest health risk. 

Thiiran that is produced by the addition of sulphur atoms to ethylene intramolecularly decays to vinyl mercaptan. High-temperature pyrolysis of thiiran leads to vinyl mercaptan, thiophen, and benzothiophen under milder conditions than are necessary for the pyrolysis of dimethyl sulphide. Its pyrolysis in chlorobenzene has also been reported. ... 

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