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19. Reactions of Aromatics: EAS and Beyond

Electrophilic Aromatic Substitution

19. Reactions of Aromatics: EAS and Beyond

Electrophilic Aromatic Substitution

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Johnny Betancourt

EAS General Mechanism

EAS General Mechanism

Johnny Betancourt

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Textbook Question

Furan undergoes electrophilic aromatic substitution more readily than benzene; mild reagents and conditions are sufficient. For example, furan reacts with bromine to give 2-bromofuran. (b) Explain why furan undergoes bromination (and other electrophilic aromatic substitutions) primarily at the 2-position.

Textbook Question

Furan undergoes electrophilic aromatic substitution more readily than benzene; mild reagents and conditions are sufficient. For example, furan reacts with bromine to give 2-bromofuran. (a) Propose mechanisms for the bromination of furan at the 2-position and at the 3-position. Draw the resonance forms of each sigma complex, and compare their stabilities.

Textbook Question

Styrene (vinylbenzene) undergoes electrophilic aromatic substitution much faster than benzene, and the products are found to be primarily ortho- and para-substituted styrenes. Use resonance forms of the intermediates to explain these results.

Textbook Question

What products would you expect from the following coupling reactions? (b)

Textbook Question

What products would you expect from the following coupling reactions? (a)

Textbook Question

Unlike most other electrophilic aromatic substitutions, sulfonation is often reversible (see Section 17-4). When one sample of toluene is sulfonated at 0 °C and another sample is sulfonated at 100 °C, the following ratios of substitution products result: (c) Because the SO3H group can be added to a benzene ring and removed later, it is sometimes called a blocking group. Show how 2,6-dibromotoluene can be made from toluene using sulfonation and desulfonation as intermediate steps in the synthesis.

Textbook Question

Show how each of the following compounds can be prepared from methyl phenyl ketone: c.

Textbook Question

Indicate how the following compounds can be synthesized from cyclohexanone and any other necessary reagents: d.

Textbook Question

Pyrrole undergoes electrophilic aromatic substitution more readily than benzene, and mild reagents and conditions are sufficient. These reactions normally occur at the 2-position rather than the 3-position, as shown in the following example. <

of reaction> (b) Explain why pyrrole reacts more readily than benzene, and also why substitution occurs primarily at the 2-position rather than the 3-position.

Textbook Question

Pyrrole undergoes electrophilic aromatic substitution more readily than benzene, and mild reagents and conditions are sufficient. These reactions normally occur at the 2-position rather than the 3-position, as shown in the following example. <

of reaction> (a) Propose a mechanism for the acetylation of pyrrole just shown. You may begin with pyrrole and the acylium ion, CH3C≡O+. Be careful to draw all the resonance structures of the intermediate.

Textbook Question

a. Draw resonance contributors to show why pyridine-N-oxide is more reactive than pyridine toward electrophilic aromatic substitution.

Textbook Question

We have considered nucleophilic aromatic substitution of pyridine at the 2-position and 3-position but not at the 4-position. Complete the three possible cases by showing the mechanism for the reaction of methoxide ion with 4-chloropyridine. Show how the intermediate is stabilized by delocalization of the charge onto the nitrogen atom.

Textbook Question

What is the major product of the following reactions? a.

Textbook Question

What is the major product of the following reactions? b.

Textbook Question

Show how the following compounds can be prepared from the given starting material. a.

Textbook Question

When pyrrole is added to a dilute solution of D2SO4 in D2O, 2-deuteriopyrrole is formed. Propose a mechanism to account for the formation of this compound.

Textbook Question

Rank imidazole, pyrrole, and benzene from most reactive to least reactive toward electrophilic aromatic substitution.

Textbook Question

If Kekulé's original hypothesis had been correct and benzene was really an equilibrium between two structures, how many distinct isomers would exist for 1,2-dichlorobenzene?

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Textbook Question

Electrophilic aromatic substitution usually occurs at the 1-position of naphthalene, also called the a position. Predict the major products of the reactions of naphthalene with the following reagents.

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(a) HNO3, H2SO4

(b) Br2, FeBr3

(c) CH3CH2COCl, AlCl3

Textbook Question

Biphenyl is two benzene rings joined by a single bond. The site of substitution for a biphenyl is determined by (1) which phenyl ring is more activated (or less deactivated), and (2) which position on that ring is most reactive, using the fact that a phenyl substituent is activating and ortho, para-directing.

(a) Use resonance forms of a sigma complex to show why a phenyl substituent should be ortho, para-directing.

Textbook Question

Unlike most other electrophilic aromatic substitutions, sulfonation is often reversible (see Section 17-4). When one sample of toluene is sulfonated at 0 °C and another sample is sulfonated at 100 °C, the following ratios of substitution products result:

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(a) Explain the change in the product ratios when the temperature is increased.

(b) Predict what will happen when the product mixture from the reaction at 0 °C is heated to 100 °C.

Textbook Question

In an aqueous solution containing sodium bicarbonate, aniline reacts quickly with bromine to give 2,4,6-tribromoaniline. Nitration of aniline requires very strong conditions, however, and the yields (mostly m-nitroaniline) are poor.

(c) Although nitration of aniline is slow and gives mostly meta substitution, nitration of acetanilide (PhNHCOCH3) goes quickly and gives mostly para substitution. Use resonance forms to explain this difference in reactivity.

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