Electrophilic aromatic substitution usually occurs at the...

Electrophilic aromatic substitution usually occurs at the 1-position of naphthalene, also called the $\alpha$ position. Predict the major products of the reactions of naphthalene with the following reagents. (a) $\mathrm{HNO}_{3}, \mathrm{H}_{2} \mathrm{SO}_{4}$ (b) $\mathrm{Br}_{2}, \mathrm{FeBr}_{3}$ (c) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COCl}, \mathrm{AlCl}_{3}$ (d) isobutylene and HF (e) cyclohexanol and $\mathrm{BF}_{3}$ (f) fuming sulfuric acid

Electrophilic aromatic substitution usually occurs at the 1-position of naphthalene, also called the $\alpha$ position. Predict the major products of the reactions of naphthalene with the following reagents.
(a) $\mathrm{HNO}_{3}, \mathrm{H}_{2} \mathrm{SO}_{4}$
(b) $\mathrm{Br}_{2}, \mathrm{FeBr}_{3}$
(c) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COCl}, \mathrm{AlCl}_{3}$
(d) isobutylene and HF
(e) cyclohexanol and $\mathrm{BF}_{3}$
(f) fuming sulfuric acid

Key Concepts

Electrophilic Aromatic Substitution

This concept involves a reaction mechanism in which an aromatic ring, which is electron-rich, reacts with an electrophile to substitute one of its hydrogen atoms. The process typically proceeds via the formation of a carbocation intermediate known as the sigma complex or arenium ion, which then loses a proton to restore aromaticity.

Regioselectivity in Polycyclic Aromatic Compounds

In polycyclic aromatic compounds like naphthalene, electrophilic aromatic substitution does not occur uniformly at all positions. Instead, the reaction favors positions where the stability of the intermediate is maximized — often the alpha position due to its greater electron density and ability to better delocalize positive charge during the formation of the sigma complex.

Generation of Electrophiles

The formation of a strong electrophile is essential for electrophilic aromatic substitution. Many reagents require activation by acids or Lewis acids to form a species capable of attacking the aromatic ring. For instance, nitration uses a mixture of nitric and sulfuric acid, while halogenation often involves halogens activated by Lewis acids.

Role of Lewis Acid Catalysts in EAS

Lewis acids such as FeBr3 and AlCl3 play a pivotal role in facilitating electrophilic aromatic substitution reactions by activating electrophilic reagents. They coordinate with substrates (like acyl or halogen-containing reagents), rendering them more electrophilic and thus more reactive towards the aromatic ring.

Friedel-Crafts Reactions

Friedel-Crafts reactions are a subset of electrophilic aromatic substitutions that include both alkylation and acylation. In these reactions, the electrophile is often generated in situ by the reaction of a halogenated compound with a Lewis acid, leading to the formation of a stabilized carbocation or acylium ion, which then attacks the aromatic ring.

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