Halogenated Organic Intermediates

The production of chloromethane (methyl chloride), dichloromethane (methylene chloride), and chloroform is carried out in one process, the hot radical chlorination of methane [route (a) in Topic 5.3.5 for mechanistic details see Section 2.2]. The process yields a mixture of all the different chloromethanes including the least desired tetrachloromethane. The chlorination reaction is initiated at above 250 °C. This temperature represents the lower temperature limit for the formation of chlorine radicals Cl by thermal decomposition of CI2 in sufficient quantity. During the 

Production (mio. ta ) (unless specified data from Wiley-VCH, 2012) 

Dichloroethane is a key intermediate of all current processes to produce vinyl chloride. Dichloroethane is obtained by oxychlorination of ethylene [using HCl as chlorine source, route (b) of Topic 5.3.5] or by electrophilic addition of chlorine to ethylene [route (c) of Topic 5.3.5]. Dichloroethane is later converted into vinyl chloride by thermal elimination of HCl from dichloroethane. 

AUyl chloride is produced by hot radical chlorination [route (a) of Topic 5.3.5). To achieve high selectivity of chlorine substitution versus chlorine addition to the double bound, high reaction temperatures of around 500 °C are needed. In addition, alternative process schemes using HCl as the chloride source under oxychlorination conditions are known. 

Chlorobenzene and dichlorobenzenes are obtained by direct catalytic chlorination of benzene with chlorine. In the production process, gaseous chlorine is bubbled through a solution of the iron(III) catalyst FeCla in benzene. All chlorination reactions at the aromatic core are highly exothermic (e.g., AH= —131.5 kj mol for chlorobenzene formation from benzene and AH = —124.4 kJ mol for dichlorobenzene formation from chlorobenzene) and therefore appropriate reactor cooling (e.g., by internal coohng coils in the reactor) is required. Keeping the reaction temperature at a certain value is important to adjust the product distribution obtained from the process. For a high selectivity to monochlorobenzene the reaction temperature should be adjusted between 40 and 50 °C. Temperatures below 40 °C are unsuitable due to unfavorably low reaction rates. Temperatures above 50 °C, however, favor the formation of di- and even trichlorobenzenes. To maximize mono-chlorobenzene production it is, moreover, important to work with excess benzene such that the benzene conversion is limited to 65% at the desired full chlorine conversion. 

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Propylene oxide is a useful chemical intermediate. Additionally, it has found use for etherification of wood (qv) to provide dimensional stabiUty (255,256), for purification of mixtures of organosiUcon compounds (257), for disinfection of cmde oil and petroleum products (258), for steriliza tion of medical equipment and disinfection of foods (259,260), and for stabilization of halogenated organics (261—263). 

The reduction of conjugated nitroalkenes such as S-nitrostyrenes to oximes provides easy access to a large number of versatile organic intermediates. However, despite their potential utility, many of these methods suffer from the use of strongly acidic or basic conditions, requirement of anhydrous conditions, and incompatibility with halogenated arenes. Eurther, some of the methods are inefficient for the preparation of aldoximes due... 

The current primary uses of chlorobenzene are as a solvent for pesticide formulations, diisocyanate manufacture, degreasing automobile parts, and for the production of nitrochlorobenzene. Solvent uses accounted for about 37% of chlorobenzene consumption in the United States in 1981, nitrochlorobenzene production for 33%, and diphenyl oxide and phenylphenol production for 16% of consumption. Chlorobenzene is also used in silicone resin production and as an intermediate in the synthesis of other halogenated organics. The past major use of chlorobenzene was as an intermediate in phenol and DDT production (Hughes et al. 1983). 

It has been proposed that mediated electrochemical oxidation may be used for the ambient temperature destruction of hazardous waste. Using Co(III)- -mediated electrochemical oxidation in sulfuric acid, l,3-dichloro-2-propanol and 2-chloro-l-propanol were oxidized to carbon dioxide259. A series of studies is being conducted on the use of the anodic oxidation of barium peroxide to produce an intermediate that leads to destruction of halogenated organic compounds. 

Solvent in pesticide formulation degreasing agent intermediate in the synthesis of other halogenated organic compounds 111,3511,3512, 354,38... 

Halogenated organic compounds are important end products of organic syntheses in both academic research and industry, where they find use as solvents, plastics, drugs, dyestuffs, pesticides, and weedicides. Their role as intermediates is at least equally vital they are stepping stones in very numerous procedures for the junction of C—N, C—O, and C—C bonds and for formation of C—O, C=C, and C=C bonds. 

The identity and reactivity of the transient species produced im radiolysis and photolysis of halogenated organic confounds has been the subject of recent interest [20-24]. The recent advances in the mediods of gHmatitni and detection of transient intermediates have made the radical ion chemistiy an active area of research both in the gas and cmidensed phase [25-28]. The nature and leactims... 

Hypochlorous acid directly reacts with organic nucleophilic compounds (X = O, N, S) or via the intermediate formation of the chlorine cation Cl " (HOCl — OH-+ CC Gallard and Gunten 2002, Hanna et al. 1991). Such reactions might also explain the formation of halogenated organic compounds in soils from OM such as humic material where the volatile fraction (for example chloroform) emits into air. All olefinic hydrocarbons and aromatic compounds add Cl and OH from HOCl (see for example reaction 5.422). 

Therefore, organohalogens have been widely used as intermediates for the synthesis of oxygenated compounds, which has led to severe problems in waste treatment due to recalcitrant halogenated organic compounds. 

An alternative oxidation process is based on the electrooxidation of barium peroxide in aqueous surfactant suspensions [78] which produces the reactive intermediate barium superoxide. The system reaction has been applied to the oxidation of several halogenated organics, e.g. 1,2,4 trichlorobenzene, hexafluorobenzene, etc. Destruction is initiated by nucleophilic substitution of the halide by the superoxide ion, the resulting product is either chemically or electrochemically oxidised. The superoxide ion is stabilised by the barium ion and the surfactant. 

The electrochemical reduction of halogenated organic compounds (RX) in organic solvents is well known [7, 8]. The accepted general electrochemical mechanism starts with the transfer of one electron from the working electrode, followed by a fast elimination of the halide anion to form a radical intermediate R . This radical can either be reduced further to the carbanion R , which generally occurs at potentials more positive with respect to the reduction of the parent molecule RX, or it can couple to produce a dimeric product as a consequence of an RRC process. Therefore, the mechanism and the stmcture of the obtained products depend on the starting material and conditions selected for the electrochemical reduction. 

Organic Intermediates Functionalized with Oxygen, Nitrogen, or Halogens... 

Preparation of phlorogluciaol or its monomethyl ether by reaction of a halogenated phenol with an alkaU metal hydroxide in an inert organic medium by means of a benzyne intermediate has been patented (142). For example, 4-chlororesorcinol reacts with excess potassium hydroxide under nitrogen in refluxing pseudocumene (1,2,4-trimethylbenzene) with the consequent formation of pure phlorogluciaol in 68% yield. In a version of this process, the solvent is omitted but a small amount of water is employed (143). 

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