19. Reactions of Aromatics: EAS and Beyond

What products would you expect from the following coupling reactions?

(a)

What products would you expect from the following coupling reactions?

(b)

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.

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

a.

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:

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.

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.

Sort the following compounds in order of increasing reactivity toward electrophilic aromatic substitution: $t-\mathrm{Bu}-\mathrm{Ph}$ (tert-butylbenzene), $\mathrm{Ph}-{\mathrm{NO}}_2$ (nitrobenzene), $\mathrm{Ph}-{\mathrm{OCH}}_3$ (anisole), and $\mathrm{Ph}$ (benzene). Which sequence is correct?

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?

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.

(a) HNO₃, H₂SO₄

(b) Br₂, FeBr₃

(c) CH₃CH₂COCl, AlCl₃

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.

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

c.

What is the major product of the following reactions?

a.

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.

(vi)

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

What is the major product of the following reactions?

b.