19. Reactions of Aromatics: EAS and Beyond

Which of the following correctly arranges the compounds $toluene$, $benzene$, and $nitrobenzene$ in order of decreasing reactivity toward electrophilic aromatic substitution?

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

In the general mechanism for electrophilic aromatic substitution, what is the first step?

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.

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

d.

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.

In the context of electrophilic aromatic substitution, which position on a benzene ring substituted with a nitro group (${\mathrm{NO}}_2$) is most likely to act as an electrophilic site?

In the context of electrophilic aromatic substitution, which position on a benzene ring is most susceptible to attack by an electrophile?

What is the major product formed when isopropylbenzene (cumene) undergoes electrophilic aromatic substitution with ${\mathrm{Br}}_2$ in the presence of ${\mathrm{FeBr}}_3$?

In the nitration of benzene, which species acts as the electrophile during the electrophilic aromatic substitution reaction?

In the context of electrophilic aromatic substitution, how many electrophilic centers are present in acetyl chloride (${\mathrm{CH}}_3\mathrm{COCl}$)?

Which of the following species is commonly used as an electrophile in electrophilic aromatic substitution reactions?

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.

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

During the electrophilic aromatic substitution of $\mathrm{isopropylbenzene}$ (cumene) with ${\mathrm{Br}}_2$ in the presence of a Lewis acid catalyst, which position on the benzene ring is most likely to be brominated?

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.