Electrophilic aromatic substitution Nitration Sulfonation

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... 

Electrophilic aromatic substitution (Sec tion 22 14) Arylamines are very reac tive toward electrophilic aromatic sub stitution It IS customary to protect arylamines as their N acyl derivatives before carrying out ring nitration chio rination bromination sulfonation or Friedel-Crafts reactions... 

The hydroxyl group is a strongly activating, ortho- and para-directing substituent in electrophilic aromatic substitution reactions (Section 16.4). As a result, phenols are highly reactive substrates for electrophilic halogenation, nitration, sulfonation, and lTiedel-Crafts reactions. 

Equipped with these reference trends for steric and electronic effects, one is prepared to survey more general classes of electrophilic aromatic substitution on benzocycloalkenes. Such reactions include nitration, halogenation, sulfonation, and alkylation. Each has its own mechanistic peculiarities, but their product distributions can be rationalized by consideration of the appropriate reference. 

These equations show the general theoretical basis for the empirical order of rate constants given earlier for electrophilic attack on an aromatic ligand L, its metal complex ML, and its protonated form HL, one finds kt > n > hl. Conflicting reports in the literature state that coordination can both accelerate electrophilic aromatic substitution (30) and slow it down enormously (2). In the first case the rates of nitration of the diprotonated form of 0-phenanthroline and its Co(III) and Fe(III) complexes were compared. Here coordination prevents protonation in the mixed acid medium used for nitration and kML > h2l. In the second case the phenolate form of 8-hydroxyquinoline-5-sulfonic acid and its metal chelates were compared. The complexes underwent iodination much more slowly, if at all, and kL > kML ... 

Aromatic compounds react mainly by electrophilic aromatic substitution, in which one or more ring hydrogens are replaced by various electrophiles. Typical reactions are chlorination, bromination, nitration, sulfonation, alkylation, and acylation (the last two are Friedel-Crafts reactions). The mechanism involves two steps addition of the electrophile to a ring carbon, to produce an intermediate benzenonium ion, followed by proton loss to again achieve the (now substituted) aromatic system. 

Know the meaning of electrophilic aromatic substitution, halogenation, nitration, sulfonation, alkylation, acylation, Friedel-Crafts reaction. 

Predict products and propose mechanisms for the common electrophilic aromatic substitutions halogenation, nitration, sulfonation, and Friedel-Crafts alkylation and acylation. Problems 17-44, 47, 48, 51, 59, G4, and 70... 

In practical terms, it is usually possible to get high yields of para products from these reactions. Both nitration and sulfonation of bromobenzene give enough material to make the synthesis worthwhile. Though mixtures of products are always bad in a synthesis, electrophilic aromatic substitution is usually simple to carry out on a large enough scale to make separation of the major product a workable method. 

These steps illustrate how to generate the electrophile E for nitration and sulfonation, the process that begins any mechanism for electrophilic aromatic substitution. To complete either of these mechanisms, you must replace the electrophile by either or S03H in the general mechanism (Mechanism 18.1). Thus, the two-step sequence that replaces H by E is the same r ardless of E. This is shown in Sample Problem 18.1 u.sing the reaction of benzene with the nitronium ion. 

Instead, these heterocycles and their derivatives most commonly undergo electrophilic substitution nitration, sulfonation, halogenation. Friedel-Crafts acylation, even the Reimer-Tiemann reaction and coupling with diazonium salts. Heats of combustion indicate resonance stabilization to the extent of 22-28 kcal/ mole somewhat less than the resonance energy of benzene (36 kcal/mde), but much greater than that of most conjugateci dienes (about Tlccal/mole). On the basis of these properties, pyrrole, furan, and thiophene must be considered aromatic. Clearly, formulas I, II, and III do not adequately represent the structures of these compounds. 

The pyridine ring undergoes electrophilic aromatic substitution reactions with great difficulty. Halogenation and sulfonation can be carried out under drastic conditions, but nitration occurs in veiy low yield, and Friedel-Crafts reactions are not successful. Reactions usually give the 3-substituted product. 

In contrast to aromatic nitration and other electrophilic aromatic substitutions aromatic sulfonation is reversible. Sulfonation takes place in strong acidic conditions and desulfonation is the mode of action in a hot aqueous acid. 

The characteristic reaction of benzene and its derivatives is electrophilic aromatic substitution. In these reactions, a hydrogen on the benzene ring is replaced by a chlorine (chlorination), a bromine (bromination), an alkyl or acyl group (Friedel-Crafts alkylation or acylation), a nitro group (nitration), or a sulfonic acid group (sulfonation). 

Benzene s aromaticity causes it to undergo electrophilic aromatic substitution reactions. The electrophilic addition reactions characteristic of alkenes and dienes would lead to much less stable nonaromatic addition products. The most common electrophilic aromatic substitution reactions are halogenation, nitration, sulfonation, and Friedel-Crafts acylation and alkylation. Once the electrophile is generated, all electrophilic aromatic substitution reactions take place by the same two-step mechanism (1) The aromatic compound reacts with an electrophile, forming a carbocation intermediate and (2) a base pulls off a proton from the carbon that... 

Like benzene, substituted benzenes undergo the five electrophilic aromatic substitution reactions discussed in Chapter 15 and listed in Section 16.2 halogenation, nitration, sulfonation, alkylation, and acylation. Now we need to find out whether a substituted benzene is more reactive or less reactive than benzene itself. The answer... 

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