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SolutionsPolycyclic Aromatic Compounds

1. Anthracene

Explain the higher reactivity of the middle position (9) of anthracene towards SEAr.

Acylation of anthracene with acetyl chloride and AlCl₃: the acetyl group enters position 9
Resonance forms of the intermediates of substitution at positions 1 and 9 of anthracene; the aromatic rings preserved are shown in red

The intermediate of substitution at position 1 keeps one benzene ring (a naphthalene) only in some resonance forms. That of position 9 keeps two benzene rings in all its forms: it loses less aromaticity and forms faster.

2. Kinetic and thermodynamic control

Sulfonation of naphthalene at 80 °C gives naphthalene-1-sulfonic acid, and at 160 °C naphthalene-2-sulfonic acid. Explain.

Position 1 reacts faster because its intermediate is more stabilized (kinetic control). But the 1-sulfonic acid is strained by the interaction of SO3H with the H at position 8; since sulfonation is reversible, at high temperature the more stable 2-isomer forms (thermodynamic control).

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