Chlorination, of benzene

Chlorination of benzene is an electrophilic substitution reaction in which CL serves as the electrophile. The reaction occurs in the presence of a Lewis acid catalyst such as FeCls. The products are a mixture of mono- and dichlorobenzenes. The ortho- and the para-dichlorobenzenes are more common than meta-dichlorobenzene. The ratio of the mono-chloro to dichloro products essentially depends on the benzene/chlorine ratio and the residence time. The ratio of the dichloro-isomers (0- to p- to m-dichlorobenzenes) mainly depends on the reaction temperature and residence time  

Chemicals Based on Benzene, Toluene, and Xylenes 277 Table 10-2 

Typical liquid-phase reaction conditions for the chlorination of benzene using FeCls catalyst are 80-100°C and atmospheric pressure. When a high benzene/Cl2 ratio is used, the product mixture is approximately 80% monochlorobenzene, 15% p-dichlorobenzene and 5% o-dichlorobenzene. 

Continuous chlorination processes permit the removal of mono-chlorohenzene as it is formed, resulting in lower yields of higher chlorinated benzene. 

Monochlorohenzene is also produced in a vapor-phase process at approximately 300°C. The hy-product HCl goes into a regenerative oxychlorination reactor. The catalyst is a promoted copper oxide on a silica carrier  

The chlorination of benzene produces monochlorobenzene, dichlorobenzene, and trichlorobenzene through the successive reactions 

There is no liquid or vapor hold-up in the reflux condenser, i.e., no dynamics are involved. 

The system operates under isothermal and isobaric conditions. 

There is negligible mass transfer resistance between the gaseous CI2 and the CI2 in solution, i.e., the CI2 goes immediately into solution up to the solubility limit. 

Usually what is required is to know the time for (1) maximizing monochlorobenzene, (2) maximizing dichlorobenzene, and (3) maximizing trichlorobenzene. 

---

BHC is manufactured by chlorination of benzene in the presence of ultra-violet light. The gamma-isomer is obtained from the crude mixture by selective crystallization, and forms colourless crystals, m.p. I13" C. U.S. production 1980 400 tonnes. 

The o- and p-isomers are manufactured by the direct chlorination of benzene in the presence of iron as a catalyst, the resulting mixture being separated by fractional distillation. The w-isomer may be obtained by isomerization of the 0- or p-compound in the presence of a catalyst. 

Continuous chlorination of benzene at 30—50°C in the presence of a Lewis acid typically yields 85% monochlorobenzene. Temperatures in the range of 150—190°C favor production of the dichlorobenzene products. The para isomer is produced in a ratio of 2—3 to 1 of the ortho isomer. Other methods of aromatic ring chlorination include use of a mixture of hydrogen chloride and air in the presence of a copper—salt catalyst, or sulfuryl chloride in the presence of aluminum chloride at ambient temperatures. Free-radical chlorination of toluene successively yields benzyl chloride, benzal chloride, and benzotrichloride. Related chlorination agents include sulfuryl chloride, tert-huty hypochlorite, and /V-ch1orosuccinimide which yield benzyl chloride under the influence of light, heat, or radical initiators. 

The chlorination of benzene can theoretically produce 12 different chlorobenzenes. With the exception of 1,3-dichlorobenzene, 1,3,5-trichlorobenzene, and 1,2,3,5-tetrachlorobenzene, all of the compounds are produced readily by chlorinating benzene in the presence of a Friedel-Crafts catalyst (see Friedel-CRAFTS reactions). The usual catalyst is ferric chloride either as such or generated in situ by exposing a large surface of iron to the Hquid being chlorinated. With the exception of hexachlorobenzene, each compound can be further chlorinated therefore, the finished product is always a mixture of chlorobenzenes. Refined products are obtained by distillation and crystallization. 

Chlorobenzenes were first synthesized around the middle of the nineteenth century the first direct chlorination of benzene was reported in 1905 (1). Commercial production was begun in 1909 by the former United Alkali Co. in England (2). In 1915, the Hooker Electrochemical Co. at Niagara EaUs, New York, brought on stream its first chlorobenzenes plant in the United States with a capacity of about 8200 metric tons per year. 

All of the chlorobenzenes are now produced by chlorination of benzene in the Hquid phase. Ferric chloride is the most common catalyst. Although precautions are taken to keep water out of the system, it is possible that the FeCl3H20 complex catalyst is present in most operations owing to traces of moisture in benzene entering the reactor. This FeCl3H20 complex is probably the most effective catalyst (13). 

The hquid-phase chlorination of benzene is an ideal example of a set of sequential reactions with varying rates from the single-chlorinated molecule to the completely chlorinated molecule containing six chlorines. Classical papers have modeled the chlorination of benzene through the dichlorobenzenes (14,15). A reactor system may be simulated with the relative rate equations and flow equation. The batch reactor gives the minimum ratio of... 

Commercial chlorination of benzene today is carried out as a three-product process (monochlorobenzene and 0- and -dichlorobenzenes). The most economical operation is achieved with a typical product spHt of about 85% monochlorobenzene and a minimum of 15% dichlorobenzenes. Typically, about two parts of -dichlorobenzene are formed for each part of (9-isomer. It is not economical to eliminate the coproduction of the dichlorobenzenes. To maximize monochlorobenzene production (90% monochlorobenzene and 10% dichlorobenzene), benzene is lightly chlorinated the density of the reaction mixture is monitored to minimize polychlorobenzene production and the unreacted benzene is recycled. 

For preparative purposes, a Lewis acid such as AICI3 or FeCl3 is often used to catalyze chlorination. Chlorination of benzene by AICI3 is overall third-order. ... 

Wilkes and co-workers have investigated the chlorination of benzene in both acidic and basic chloroaluminate(III) ionic liquids [66]. In the acidic ionic liquid [EMIM]C1/A1C13 (X(A1C13) > 0.5), the chlorination reaction initially gave chlorobenzene, which in turn reacted with a second molecule of chlorine to give dichlorobenzenes. In the basic ionic liquid, the reaction was more complex. In addition to the... 

Scheme 5.1-38 The chlorination of benzene in acidic and basic chloroaluminate ionic liquids.

Similar to the alkylation and the chlorination of benzene, the nitration reaction is an electrophilic substitution of a benzene hydrogen (a proton) with a nitronium ion (NO ). The liquid-phase reaction occurs in presence of both concentrated nitric and sulfuric acids at approximately 50°C. Concentrated sulfuric acid has two functions it reacts with nitric acid to form the nitronium ion, and it absorbs the water formed during the reaction, which shifts the equilibrium to the formation of nitrobenzene ... 

It follows from the above that, in the reactions of fairly unreactive aromatics, the formation of Cl+ (either from HOC1 or H2OCl+) will be relatively fast compared with the subsequent reaction of this ion with the aromatic so that the kinetics will be governed mainly by the third term in equation (94). Hence de la Mare et al.204 found the rate of chlorination of benzene and toluene by acidified hypochlorous acid to depend on the concentration and nature of the aromatic and to increase with hydrogen ion concentration though (as in the case of positive... 

Mason256 has measured the second-order rate coefficients and Arrhenius parameters for the chlorination of benzene, biphenyl, naphthalene, and phe-nanthrene in acetic acid (containing 0.05 % water) and these are given in Table 62. 

The rates of chlorination of benzene, biphenyl, and diphenylmethane by chlorine acetate in 98 % aqueous acetic acid at 25 °C have also been determined and the second-order rate coefficients are 0.00118, 0.0364, and 0.0311, respective-]y209 , 270 jjje varjation in rate with change in water content of the acetic acid was the same as that previously observed209 for toluene, and thus in ca. 75 % aqueous acid the coefficients were 0.00073, 0.027 and 0.0241 however, elsewhere in ref. 209a a 4-fold decrease in rate coefficient for diphenylmethane was claimed to accompany the same increase in water content of the medium. 

Trifluoroacetic acid has been examined as a solvent and chlorination of benzene in this is first-order in aromatic and chlorine, but for benzene a higher activation energy (11.4, determined from data at 25.0 and 45.4 °C) was obtained than for chlorination in carbon tetrachloride this unexpected result was attributed to an increase in desolvation energy of the reactants273. 

Design a plant to produce 20,000 tonnes/year of monochlorobenzene together with not less than 2000 tonnes/year of dichlorobenzene, by the direct chlorination of benzene. 

Consider the following chlorination of benzene reaction system of elementary reactions. 

Various industrial pilot plants and full-scale operations, using radiation-chemical processing have been reported, with production rates -50 to -1000 tons per year (Spinks and Woods, 1990 Chutny and Kucera, 1974). Production rates less than -50 tons per year are not considered viable. These operations are or have been conducted in countries such as the United States, the former U.S.S.R., Japan, and France. However, some operations have also been reported in the former Czechoslovakia and Romania, especially in connection with petroleum industry. In the United States, chlorination of benzene to gammexane (hexachlorocyclohexane) was hotly pursued at one time by radiation or photoinitiation. Since the early seventies the activity has dwindled, presumably due to lack of demand and environmental considerations. 

Dichlorobenzene is produced by the chlorination of benzene or chlorobenzene in the presence of a catalyst (typically ferric oxide) followed by either fiactional distillation or crystallization of the resulting mixture of chlorinated benzenes to yield 1,4-dichlorobenzene (HSDB 1998 IRPTC 1985). 

Chlorination of benzene gives an addition product that is a mixture of stereoisomers known collectively as hexachlorocyclohexane (HCH). At one time, this was incorrectly termed benzene hexachloride. The mixtnre has insecticidal activity, though activity was found to reside in only one isomer, the so-called gamma isomer, y-HCH. y-HCH, sometimes under its generic name lindane, has been a mainstay insecticide for many years, and is about the only example of the chlorinated hydrocarbons that has not been banned and is still available for general use. Although chlorinated hydrocarbons have proved very effective insecticides, they are not readily degraded in the environment, they accumulate and persist in animal tissues, and have proved toxic to many bird and animal species. 

The role of the Lewis acid AICI3 in the chlorination of benzene is illustrated below we can consider the electrophilic species as C1+. 

The chlorination of PtCl2(vp) in benzene gives as by-product hexa-chlorocyclohexane, which is known to be formed by the radical chlorination of benzene. This hcis been regarded [19) as evidence for the intermediacy of radicals in this oxidation. This is supported by the possibility... 

T can be synthesized easily. Chlorination of benzene gives 1,2,4,5-tetrachlorobenzene (why this isomer ) which reacts with caustic to give 2,4,5-trichlorophenol. A conversion similar to the preceding one yields the phenoxyacetic acid 2,4,5-T. 

Three or More Reactions. Analysis of three or more reactions can be made by procedures analogous to those presented. Of course, the mathematics becomes more involved however, much of the extra labor can be avoided by selecting experimental conditions in which only two reactions need be considered at any time. Figure 8.15 shows product distribution curves for one such reaction set, the progressive chlorination of benzene. 

Figure 8.15 Product distribution in the progressive chlorination of benzene ...

---