For chlorination of benzene

An interesting method for chlorination of benzene consists in mixing benzene with chlorine and passing the solution through a contact column filled with iron turnings (Poma-Cesano-Maderno). 

Two examples of nucleophilic aromatic substitution for hydrogen reactions were described from which we have proposed two new atomically efficient processes for the manufacturing of commercially relevant aromatic amines. Our mechanistic studies have revealed that the direct oxidation of a-complex intermediates by either nitro groups or O2 can eliminate the need for chlorination of benzene as a starting point for the manufacturing of aromatic amines. Accordingly, these reactions demonstrate the key objective of alternate chemical design which is not to make the waste in the first place. 

Treatment of benzene with bromine in the presence of ferric chloride or aluminum chloride gives bromobenzene and HBr. The mechanism for this reaction is the same as that for chlorination of benzene. 

FIGURE 4.5 Structures of stationary points for chlorination of benzene in aqueous solution optimizied at the PCM-M06-2X/6-31 +G(d,p) level. Bond lengths in angstroms and bond angles in degrees. 

The mechanism for chlorination of benzene is the same as that for bromination. 

The heats of formation of Tt-complexes are small thus, — A//2soc for complexes of benzene and mesitylene with iodine in carbon tetrachloride are 5-5 and i2-o kj mol , respectively. Although substituent effects which increase the rates of electrophilic substitutions also increase the stabilities of the 7r-complexes, these effects are very much weaker in the latter circumstances than in the former the heats of formation just quoted should be compared with the relative rates of chlorination and bromination of benzene and mesitylene (i 3 o6 x 10 and i a-Sq x 10 , respectively, in acetic acid at 25 °C). 

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. ... 

The complex kinetic expression for chlorination of anisole by hypochlorous acid (p. 577) becomes simpler for both less reactive and more reactive substrates. For benzene, the expression is... 

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. 

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. 

Second-order rate coefficients have been obtained for chlorination of alkyl-benzenes in acetic acid solutions (containing up to 27.6 M of water) at temperatures between 0 and 35 °C, and enthalpies and entropies of activation (determined over 25 °C range) are given in Table 63 for the substitution at the position indicated266. 

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. 

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 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... 

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. 

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)...

In contrast with chlorination, bromination of thiophene always gives substitution products exclusively, and no addition products have been isolated under a variety of experimental conditions.82 Lauer83 first studied the kinetics of the reaction of thiophene with molecular bromine however, the reported value for the rate relative to benzene (2 x 104) is not reliable, because of the uncorrected value of k for bromination of benzene (for a discussion of this point, see Marino72). Later, the rate of bromination of thiophene relative to benzene in acetic acid was determined as 1.9 x 109, by comparing the times necessary to achieve 10% reaction in the bromination of thiophene and mesitylene, under the same conditions.72... 

Chloramines. This reagent chlorinates primary and secondary amines, but is most useful for chlorination of hindered amines. The reaction is carried out in water or benzene-water. The co-product, cyanuric acid, is removed by filtration. 

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. 

The introduction of NCS by Tuleen and Stevens, as a reagent for sulfide chlorination led to a major improvement in the preparation of a-chlorosulfides. This crystalline reagent is easily handled and its reactivity is such that chlorination can be controlled to afford the monochlorination product selectively. Furthermore, NCS can be used for chlorination of acid-sensitive substrates. NCS is soluble in carbon tetrachloride at room temperamre at ordinary concentrations, whereas its conjugate product, succinimide, is not. Solutions of a-chlorosulfides are therefore often prepared with NCS in CCI4 and simply filtered prior to use without further purification. Other nonpolar solvents that have been used with NCS include chloroform, dichloromethane and benzene. Some of the very many examples of the use of NCS for sulfide chlorination from the recent literature are summarized in Table 2. For several of the entries yields were not recorded. This is almost always due to the fact that a-chlorosulfides are produced and treated as unstable reaction intermediates en route to more stable products. 

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 ... 

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