Electrophiles with substituted benzenes

Kinetic Aspects of Ar-SE Reactions Reactivity and Regioselectivity in Reactions of Electrophiles with Substituted Benzenes... 

Electrophilic addition reactions. See also Addition reactions with butylenes, 4 405-408 of maleic anhydride, 75 490 with methacrylic acid/derivatives, 76 236-237 of propylene, 20 774 Electrophilic aromatic substitution, benzene, 3 599-601 Electrophilic attack, at nitrogen and carbon, 27 98... 

When the nucleophile is an electron-rich molecule, RC60+ can be reduced via single electron transfer, producing a dimer (47). Thus, electrophilic aromatic substitution normally occurs with substituted benzenes (Figure 22, [A]), but the mode of the reaction is switched if the benzene is strongly activated (Figure 22, [B]). 

In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, seienophene[233,234], and cyclobutadiene iron carbonyl complex[235] react with alkenes to give vinyl heterocycles. The ease of the reaction of styrene with substituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. 

Application of the Hammett pa equation to the related reaction of ethoxycarbonylmethylene with substituted benzenes indicates that here too the carbene behaves as an electrophile of low selectivity (Baldwin and Smith, 1967). 

Limitations on electrophilic substitution reactions with substituted benzenes... 

Limitations on Electrophilic Substitution Reactions with Substituted Benzenes... 

Now let s consider electrophilic aromatic substitution reactions with substituted benzene rings. In order to explore this topic, we must first review important terminology that we will use frequently throughout the remainder of this chapter. The various positions on a monosubstituted benzene ring are referred to in the following way ... 

Although more stable than a typical alkyl carbocation because of resonance, the intermediate in electrophilic aromatic substitution is nevertheless much less stable than the starting benzene ring itself, with its 150 kj/mol of aromatic stability. Thus, the reaction of an electrophile with a benzene ring is endergonic, has a substantial activation energy, and is rather slow. 

With this as background let us now examine each of the electrophilic aromatic substitution reactions presented m Table 12 1 m more detail especially with respect to the electrophile that attacks benzene... 

Figure 12 3 adapts the general mechanism of electrophilic aromatic substitution to the nitration of benzene The first step is rate determining m it benzene reacts with nitro mum ion to give the cyclohexadienyl cation intermediate In the second step the aro maticity of the ring is restored by loss of a proton from the cyclohexadienyl cation... 

Complexation of bromine with iron(III) bromide makes bromine more elec trophilic and it attacks benzene to give a cyclohexadienyl intermediate as shown m step 1 of the mechanism (Figure 12 6) In step 2 as m nitration and sulfonation loss of a proton from the cyclohexadienyl cation is rapid and gives the product of electrophilic aromatic substitution... 

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... 

Partial rate factor (Section 12 10) In electrophilic aromatic substitution a number that compares the rate of attack at a particular nng carbon with the rate of attack at a single po sition of benzene... 

The table below gives first-order rate constants for reaction of substituted benzenes with w-nitrobenzenesulfonyl peroxide. From these data, calculate the overall relative reactivity and partial rate factors. Does this reaction fit the pattern of an electrophilic aromatic substitution If so, does the active electrophile exhibit low, moderate, or high substrate and position selectivity ... 

Because of Us high polarity and low nucleophilicity, a trifluoroacetic acid medium is usually used for the investigation of such carbocationic processes as solvolysis, protonation of alkenes, skeletal rearrangements, and hydride shifts [22-24] It also has been used for several synthetically useful reachons, such as electrophilic aromatic substitution [25], reductions [26, 27], and oxidations [28] Trifluoroacetic acid is a good medium for the nitration of aromatic compounds Nitration of benzene or toluene with sodium nitrate in trifluoroacetic acid is almost quantitative after 4 h at room temperature [25] Under these conditions, toluene gives the usual mixture of mononitrotoluenes in an o m p ratio of 61 6 2 6 35 8 A trifluoroacetic acid medium can be used for the reduction of acids, ketones, and alcohols with sodium borohydnde [26] or triethylsilane [27] Diary Iketones are smoothly reduced by sodium borohydnde in trifluoroacetic acid to diarylmethanes (equation 13)... 

Section 12.1 On reaction with electrophilic reagents, compounds that contain a benzene ring undergo electrophilic aromatic substitution. Table 12.1 in Section 12.1 and Table 12.3 in this summai-y give exanples. 

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