How Benzene Reacts

Aromatic compounds such as benzene undergo electrophilic aromatic substitution reactions an electrophile substitutes for one of the hydrogens attached to the benzene ring. 

This description should remind you of the first step in an electrophilic addition reaction of an alkene the nucleophilic alkene reacts with an electrophile and forms a carbocation intermediate (Section 6.0). In the second step of the reaction, the carbocation reacts with a nucleophile (Z ) to form an addition product. 

Reaction of benzene with an electrophile. Because of the greater stability of the aromatic product, the reaction procedes by an electrophilic substitution reaction (path b) rather than by an electrophilic addition reaction (path a). 

Reaction coordinate diagrams for electrophilic aromatic substitution and electrophilic addition. 

If electrophilic addition to benzene is overall an endergonic reaction, how can electrophilic addition to an alkene be overall an exergonic reaction  

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A What is the usual type of reaction undergone by the aromatic benzene ring, substitution or addition Show how benzene reacts with a mixture of concentrated nitric and sulfuric acids. (2)... 

CharactenshcaUy the reagents that react with the aromatic nng of benzene and its derivatives are electrophiles We already have some experience with electrophihc reagents particularly with respect to how they react with alkenes Electrophihc reagents add to alkenes... 

Aromatic compounds such as benzene react with alkyl chlorides in Ihe presence of AlCl i catalyst to yield alkylbenzenes. The reaction occurs through a carbocation intermediate, formed by reaction of the alkyl chloride with AICI3 (R—Cl + A1CI 1 - U+ + AICl4 ). How can you explain the observaiion that reaction of benzene with 1-chloropropane yields isopropylbenzene as the major product ... 

How is the course of halogen substitution in the benzene nucleus to be explained It is not at all probable that direct replacement of hydrogen occurs, such as we must assume in the formation of benzyl chloride and in the reaction between methane and chlorine, since the hydrogen attached to the doubly bound carbon atom of olefines exhibits no special reactivity. However, various facts which will be considered later (p. 164) indicate that benzene reacts with halogen in fundamentally the same way as does ethylene. The behaviour of ethylene towards bromine is the subject of the next preparation. 

How does benzene react with l-chloro-2-fluoropropane under the catalysis by strong Lewis acids such as boron trifluoride or aluminum chloride What is compound D ... 

How does benzene react with 2-chloro-1,1,1-trifluoropropane in the Friedel-Crafts reaction, and what is the product E ... 

Benzene is a reactant in the BTX system. There are two possible avenues for how it may be converted into a product. The first occurs when benzene reacts with ethylene to form toluene by reaction 1. The rate of benzene conversion in this reaction is thus written with respect to both reactants, which is given by... 

In order to force a reaction to occur, we introduced a Lewis acid into the reaction mixture, which generated a better electrophile (Br" is a better electrophile than Br2). In fact, all of the reactions we have explored thus far have been examples of benzene reacting with powerful electrophiles (Cl, N02, alkyl", acyl" ", and SO3). Now, we will turn our attention to the nucleophile—how can we modify the reactivity of the aromatic ring ... 

Benzene reacts with mercuric acetate to give phenylmercuric acetate using perchloric acid (HCIO ) as a catalyst. What is the electrophile How does it form ... 

Polycyclic aromatic hydrocarbons undergo electrophilic aromatic substitution when treated with the same reagents that react with benzene In general polycyclic aromatic hydrocarbons are more reactive than benzene Most lack the symmetry of benzene how ever and mixtures of products may be formed even on monosubstitution Among poly cyclic aromatic hydrocarbons we will discuss only naphthalene and that only briefly Two sites are available for substitution m naphthalene C 1 and C 2 C 1 being normally the preferred site of electrophilic attack... 

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 ... 

Figure 21.2 shows how in the calculation results benzene is transported through the aquifer. The pulse of benzene migrates at the rate of groundwater flow, traversing the aquifer in ten years. As a result of biodegradation by the natural microbial consortium, however, the benzene concentration decreases markedly with time, compared to the non-reacting case. 

Figure 4.11 shows an example of how ZSM-5 is applied as a catalyst for xylene production. The zeolite has two channel types - vertical and horizontal - which form a zigzag 3D connected structure [62,63]. Methanol and toluene react in the presence of the Bronsted acid sites, giving a mixture of xylenes inside the zeolite cages. However, while benzene, toluene, and p-xylene can easily diffuse in and out of the channels, the bulkier m- and o-xylene remain trapped inside the cages, and eventually isomerize (the disproportionation of o-xylene to trimethylbenzene and toluene involves a bulky biaryl transition structure, which does not fit in the zeolite cage). For more information on zeolite studies using computer simulations, see Chapter 6. 

The ferf-butyl cations liberated from compounds Ar—tert- Bu, upon ipso reaction of a proton, may also react again with the aromatic compound from which they stemmed. If this course is taken, the te/7-butyl groups are ultimately bound to the aromatic nucleus with a regioselectivity that is dictated by thermodynamic control. Figure 5.4 shows how in this way 1,2,4-tri-ferf-butylbenzene is smoothly isomerized to give 1,3,5-tri-fcrt-buty 1-benzene. 

All of the electrophilic aromatic substitution reactions follow this same general mechanism. The only difference is the structure of the electrophile and how it is generated. Let s look at a specific example, the nitration of benzene. This reaction is accomplished by reacting benzene with nitric acid in the presence of sulfuric acid ... 

With this in mind, how would you expect fluorobenzene to react Most election density is removed first from the ortho positions by induction, then from the meta positions, and then from the para position. Any conjugation of the lone pairs on fluorine with the tc system would increase the electron density in the ortho and para positions. Both effects favour the para position and this is where most substitution occurs. But is the ring more or less reactive than benzene This is hard to say and the honest answer is that sometimes fluorobenzene is more reactive in the para position than benzene (for example, in proton exchange and in acetylation—see later) and sometimes it is less reactive than benzene (for example, in nitration). In all cases, fluorobenzene is significantly more reactive than the other halobenzenes. We appreciate that this is a rather surprising conclusion, but the evidence supports it. 

Vitamin E and BHT are radical inhibitors, so they terminate radical chain mechanisms by reacting with radicals. How do they trap radicals Both vitamin E and BHT use a hydroxy group bonded to a benzene ring—a general structure called a phenol. 

Explain why triphenylene resembles benzene in that it does not undergo addition reactions with Br2, but phenanthrene reacts with Br2 to yield the addition product drawn. (Hint Draw resonance structures for both triphenylene and phenanthrene, and use them to determine how delocalized each iz bond is.)... 

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