The chlorination of benzene

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. 

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... 

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.

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. 

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 ... 

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. 

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). 

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

Fig. 7. Reaction scheme for the chlorination of benzene by the PtCI/ ion. [Based on Garnett (29) and Sanders ( 4).]...

Fig. 4. Typical industrial reactor for the preparation of chlorobenzene by the chlorination of benzene. The Dow Chemical Company)...

Further chlorination reactions can also take place, but since they involve insignificant amounts of reactants they have been considered to be negligible. The kinetics of the process were studied by McMullin (1948), who showed that the chlorination of benzene (A), monochlorobenzene (B) and dichlorobenzene (C) is in all cases first-order and irreversible. 

Chlorobenzene is produced commercially by the chlorination of benzene in the presence of a catalyst (e.g., ferric chloride, aluminum chloride, or stannic chloride). This process yields a mixture of chlorobenzene, dichlorobenzenes, and higher analogs which are distilled and crystallized to obtain pure products (EPA 1985a Hughes et al. 1983). 

Another herbicide, 2,4,5-trichlorophenoxyacetic acid, is synthesized by starting with the chlorination of benzene to give 1,2,4,5-tetrachloroben-zene, which reacts with caustic to give 2,4,5-trichlorophenol. Conversion to the sodium salt followed by reaction with sodium chloroacetate and acidification gives 2,4,5-trichlorophenoxyacetic acid. Agent Orange is a 1-to-l mixture of the butyl esters of 2,4,5-trichlorophenoxyacetic acid and 2,4,-dichlorophenoxyacetic acid. 

Industrial chemical reactions are often more complex than the earlier types of reaction kinetics. Complex reactions can be a combination of consecutive and parallel reactions, sometimes with individual steps being reversible. An example is the chlorination of a mixture of benzene and toluene. An example of consecutive reactions is the chlorination of methane to methyl chloride and subsequent chlorination to yield carbon tetrachloride. A further example involves the chlorination of benzene to monochlorobenzene, and subsequent chlorination... 

The chlorination of benzene can be conducted in a steel tower with a head of steel and ceramic rings, lined with acidproof tiles. The upper (expanded) part of the tower is a mist extractor the lower part of the apparatus receives benzene and gaseous chlorine. 

Chlorobenzenes. Of the 12 different chlorobenzenes that can result from the chlorination of benzene, three are of most commercial importance monochlorobenzene (MCB), o-dichlorobenzene (ODCB), and /7-dichlorobenzene (PDCB). Chlorination of benzene can be done either batchwise or continuously in the presence of a catalyst such as ferric chloride, aluminum chloride, or stannic chloride. It is usually run as a three-product process the current product distribution is about 52 percent to MCB, 17 percent ODCB and 31 percent PDCB. The pure compounds are separated from the crude by distillation and crystallization. 

Figure. 13.62 to 13.65 provide examples of chlorination reactions. In the first example, the commonly used agent FeCl3/Cl2 is employed for the chlorination of benzene and naphthalene rings. This method is not practical for the chlorination of anthraquinone. In this case the most important reaction is the tetrachlorination process shown in Fig. 13.63. 

ProblGin 18.3 Draw a detailed mechanism for the chlorination of benzene using CIg and FeCIg. 

For chlorination in the liquid phase, chlorine gas is led into the liquid and is dispersed in the form of fine bubbles. In most chlorina-tions heat and a catalyst are employed. Phosphorus and its halides, sulfur and its halides, iron, aluminum chloride, antimony chloride, and iodine are among the common catalysts used for chlorinations. For laboratory use red phosphorus, iron, and thionyl chloride give good results. Aluminum chloride (or aluminum-mercury couple), although a very efficient catalyst, clogs the disperser. The use of actinic light is sometimes used to promote chlorination in the liquid phase, and is extensively used in the vapor phase. The action of the catalysts is assumed to activate a few chlorine molecules which initiate chain reactions. For example, in the chlorination of benzene the reaction begins with the dissociation of a few molecules of chlorine to atoms ... 

Accordingly, the iodoamidation of cyclohexene in acetonitrile was shown to result in quantitative yields. The chlorination of benzene derivatives [167] as well as the io-dination of anisole (mixture of the three isomers [168]) was also reported. However, these substitution reactions required very positive potentials, suggesting that the organic substrate has to be concomitantly oxidized. 

Raschig phenol process. Commercial process for the production of phenol by the hydrolysis of chlorobenzene, produced by the chlorination of benzene with hydrochloric acid and air. 

The second approach involves a change of products or their proportionate conversions and may be illustrated by chlorination of benzene. Although monochlorobenzene was the product commercially needed, some o- and p-dichlorobenzene were always formed in the process. These were stored by the wise manufacturer until excellent markets for these products were developed. At present certain manufacturers conduct the chlorination of benzene to make the maximum amount of the dichloro derivatives. Here the unit consumption factor of chlorine, for example, varies with the proportion of the poly-chloro derivatives made. When benzene is chlorinated in this fashion, there will be not one conversion factor but two series, giving for each the conversion factor of benzene or of chlorine to chlorobenzene, p-dichlorobenzene, and o-dichlorobenzene. For this multiple-product manufacture the over-all picture can be obtained only by the summation of the individual product conversion factors. 

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