Acylation of 1,4 -dimethoxynaphthalene with acetic anhydride...

Acylation of 1,4 -dimethoxynaphthalene with acetic anhydride (1.2 equiv) and $\mathrm{AlCl}_{3}$ (2.2 equiv) in dichloroethane at $60^{\circ} \mathrm{C}$ leads to two products, as shown below. Suggest a rationalization for the formation of these two products. What might account for the demethylation observed in product 13-B?

Key Concepts

Demethylation Mechanism

Demethylation in the context of aromatic compounds often arises when a methoxy substituent undergoes cleavage of its methyl group under the influence of a strong Lewis acid. In this scenario, the Lewis acid not only catalyzes the formation of the electrophile but also destabilizes the methoxy group by coordinating to the oxygen atom. This coordination can encourage a nucleophilic attack or fragmentation that results in the loss of the methyl group, giving rise to products with altered substitution patterns.

Lewis Acid Catalysis

Lewis acid catalysis involves the use of a compound such as aluminum chloride (AlCl3) that can accept electron pairs. In acylation reactions, the Lewis acid assists in generating the potent electrophile (the acylium ion) from the acylating agent. Additionally, strong Lewis acids can activate adjacent functional groups, thereby facilitating side reactions such as demethylation in electron-rich systems.

Friedel-Crafts Acylation

Friedel-Crafts acylation is a classic electrophilic aromatic substitution reaction where an acylium ion is generated from an acylating agent (such as an acetic anhydride) in the presence of a Lewis acid catalyst. The generated acylium ion then attacks the aromatic ring, substituting a hydrogen atom and forming a ketone functional group on the aromatic system. This mechanism elucidates the formation of acylated products in substituted aromatic compounds.

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a reaction mechanism in which an electron-rich aromatic ring reacts with an electrophile, leading to substitution of a hydrogen atom. The reaction's regioselectivity is influenced by the substituents already present on the aromatic system which can activate or deactivate particular positions. Understanding the directing effects of substituents is crucial for predicting the formation of multiple products in polysubstituted aromatic reactions.

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