Halogenation of benzene

Benzene reacts with bromine and chlorine in the presence of Lewis acids to give halogenated substitution products in good yield. 

The Lewis acids typically used are aluminum chloride (AICI3) and iron chloride (FeCl3) for chlorination, and iron bromide (FeBr3) for bromination. The purpose of the Lewis acid is to make the halogen a stronger electrophile. A mechanism for electrophilic aromatic bromination is shown here. 

Bromine combines with FeBrjto form a complex. 

Step 2 Electrophilic attack and fcamation of the sigma complex. H 

Comparison with Alkenes Benzene is not as reactive as alkenes, which react rapidly with bromine at room temperature to give addition products (Section 8-8). For example, cyclohexene reacts to give rrans-l,2-dibromocyclohexane. This reaction is exothermic by about 121 kJ/mol (29 kcal/mol). 

The analogous addition of bromine to benzene is endothermic because it requires the loss of aromatic stability. The addition is not seen under normal circumstances. 

The energy diagram for the bromination of benzene shows that the first step is endothermic and rate-limiting and the second step is strongly exothermic. 

The substitution of bromine for a hydrogen atom gives an aromatic product. The substitution is exothermic, but it requires a Lewis acid catalyst to convert bromine to a stronger electrophile. 

The reaction works equally well with AICI3, AlBrj, FeClj, FeBrj, and a number of other Lewis bases. Some catalysts can also be generated through reactions like 2 FeBr2(s) + Br2G) 2 FeBr3(s). 

The mechanism begins with the attack on the chlorine molecule by aluminum chloride. (This step would be the same if ironGlf) chloride were the catalyst.) The CL ion attracts a pair of electrons from the benzene to form an intermediate species. The presence of resonance in this intermediate stabilizes it and helps the reaction along. 

These resonance forms and similar forms are important to all the electrophilic substitution mechanisms in this chapter. 

As noted in the earlier section Basics of Electrophilic Substitution Reactions, the loss of the hydrogen ion (H ) requires the presence of a strong base. The chloride ion (CL) is a base, but it isn t strong enough to accomplish this task. However, as shown in the mechanism, the tetrachloro-aluminate ion (A1C1 ) is a sufficiently strong base. This process also regenerates the catalyst so that it s available to continue the process. 

According to the usual procedure for preparing bromobenzene, bromine is added to benzene in the presence of metallic iron (customarily a few carpet tacks) and the reaction mixture is heated. 

Step 1 Sulfur trioxide attacks benzene in the rate-determining step 

Benzene and sulfur trioxide Cyclohexadienyl cation intermediate 

Step 2 A proton is lost from the sp hybridized carbon of the intermediate to restore the aromaticity of the ring. The species shown that abstracts the proton is a hydrogen sulfate ion formed by ionization of sulfuric acid. 

Bromine, although it adds rapidly to alkenes, is too weak an electrophile to react at an appreciable rate with benzene. A catalyst that increases the electrophilic properties of bromine must be present. Somehow carpet tacks can do this. How  

The active catalyst is not iron itself but iron(III) bromide, formed by reaction of iron and bromine. 

Iron(lll) bromide, a weak Lewis acid, combines with bromine to form a Lewis acid/Lewis base complex. 

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Halogenation. Depending on the conditions either substitution or addition products can be obtained by the halogenation of benzene. 

This reaction set may be regarded as parallel reactions with respect to consumption of species B and as a series reaction with respect to species A, V, and W. Common examples include the nitration and halogenation of benzene and other organic compounds to form polysubstituted compounds. To characterize the qualitative behavior of such systems, it is useful to consider reactions 9.3.3 and 9.3.4 as mechanistic equations and to analyze the effects of different contacting patterns on the yield of species V. We shall follow the treatment of Levenspiel (7). 

The usual halogenation of benzene takes place in the presence of a Lewis acid, such as FeBrs (Unit 10, Class XII), which polarises the halogen molecule. In case of phenol, the polarisation of bromine molecule takes place even in the absence of Lewis acid. It is due to the highly activating effect of -OH group attached to the benzene ring. 

Over recent years, the electrochlorination of a wide range of substrates has been reported. The direct halogenation of benzene has been studied at Pt and Rh electrodes and the in situ spectro-electrochemical monitoring of the process with confocal microprobe Raman methods allowed optimization of the reaction conditions [80]. Toluene has been chlorinated at DSAs and the kinetics of this process have been explored in detail [81]. The electrochlorination of 1,4-dimethoxy-2-tert-butylbenzene has been reported in CCI4 and in acetonitrile environments [82]. A difference in mechanism has been proposed to explain the observation of l,4-dimethoxy-2-tert-butyl-5-chlorobenzene and l,4-dimethoxy-2-tert-butyl-6-chlorobenzene, respectively, as the main products. Succinimide is electrochlo-rinated to give A -chlorosuccinimide at platinum electrodes, but the process has been reported to be relatively inefficient due to side reactions (when compared... 

The two most common methods of preparing aryl halides are by direct halogenation of benzene and via diazonium salt reactions. 

The halogenation of benzene to synthesize aryl halides is the oldest method known. 

Halogenation of alkanes requires conditions under which halogen atoms are formed, that is, high temperature or light. Halogenation of benzene, on the other hand, involves transfer of positive halogen, which is promoted by acid catalysts like ferric chloride. 

In the halogenation of benzene, an iron catalyst is used. The iron is converted to iron(lll) halide (FeXs). Iron(lll) halide reacts further to polarize the X2 molecule. 

Although a stable dibromide cannot be isolated for a similar study of the bromination of benzene, the kinetics of the iodine catalyzed bromina-tion of benzene is identical with that of phenanthrene.26 Consequently it is very probable that the mechanism of the halogenation of benzene is the same as that proposed for phenanthrene. The kinetics of chlorination,27 bromination,28 and iodination by iodine chloride29 are also in agreement with this interpretation. The halogenation of phenols, however, appears to be a different, more complex process. 0... 

Sulfonation and halogenation of benzene are described in other chapters of this book. The initial discussion Of phenol production will start with sodium benzenesulfonate and chlorobenzene as raw materials. 

In recovering the hydrogen chloride from the hydrolytic step, it is essential to use only a limited quantity of water in the washing operations so that a 17 per cent solution is delivered to the acid evaporators. This is suitable for the subsequent step involving the halogenation of benzene. Operating in a closed cycle, the hydrochloric acid picks up impurities so that part of it must be purged continuously. This loss and that involved in the production of polychlorobenzenes constitute the replacement requirements of acid. 

High temperature or light is necessary. The halogenation of benzene involves transfer of a positive halogen (X+) (which is protonated by acid catalysts). This means that the position of attack (whether on the benzene or on the side chain) will depend on the nature of the attacking species (an ion or a radical). Since the conditions in this problem are those that favor the production of bromine radicals, halogenation occurs exclusively to the n-propyl side chain. Hence,... 

By contrast, a more direct and atomically efficient route for the production of aromatic amines would be to eliminate the need for halogenation of benzene. This can be achieved by a class of reaction illustrated in Figure 2 which is known as... 

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