Polycyclic Aromatic Compounds

Polycyclic aromatic compounds undergo the same SEAr that benzene does. But there are some new details to be aware of.

Reactions of naphthalene: bromination without catalyst (1-bromonaphthalene, 75%), nitration (1-nitronaphthalene major and 2-nitronaphthalene minor), acylation at position 1 and sulfonation at 1 at 80 °C or at 2 at 160 °C

Naphthalene is more reactive than benzene. For instance, bromination does not need a catalyst and nitration can be performed in acetic acid, a much weaker acid than sulfuric acid.

Position 1 is the most reactive in naphthalene. It can be seen that, except for high-temperature sulfonation, the attacked position is always 1 (alpha).

Describing the possible reaction intermediates with resonance forms easily explains this reactivity trend.

SEAr at position 1 (alpha) of naphthalene

Resonance form of the intermediate of substitution at position 1 of naphthalene with the charge in the attacked ring; the other ring remains aromatic

Another resonance form of the position-1 substitution intermediate that keeps one ring aromatic

Resonance form of the position-1 substitution intermediate with the charge delocalized into the other ring, which loses its aromaticity

The resonance hybrid of this intermediate is quite aromatic and hence less unstable because it has two resonance forms that keep complete aromaticity of at least one ring.

SEAr at position 2 (beta) of naphthalene

The only resonance form of the position-2 substitution intermediate of naphthalene that keeps one ring aromatic

Resonance form of the position-2 substitution intermediate without an intact aromatic ring

The resonance hybrid of this intermediate is less aromatic than the previous one and is thus more unstable because it has only one resonance form where the aromaticity of one ring is kept.

The paradox is that the substitution at position 1 (alpha) yields the most sterically congested product and therefore the least stable one thermodynamically.

Yet, the pathway leading to the 1 (alpha) substitution is the lowest in energy because the corresponding intermediate (see above) is less unstable.

Energy diagram of the substitution of naphthalene: the position-1 pathway is lower (kinetic control), but the position-2 product, free of the clash with the H at position 8, is more stable (thermodynamic control)

Therefore, the substitution product at position 1 (alpha) is formed faster (kinetic control). However, in the case of the reversible sulfonation, if one lets the reaction proceed during a longer time and at a high temperature, the major product is the 2-substituted one (beta; thermodynamic control). Do you see why?

Polycyclic aromatic compounds "can afford" to undergo oxidation and reduction reactions at the expense of the aromaticity of one of their rings.

Oxidation

The strong oxidation of naphthalene leads to the cleavage of one ring.

Its smooth oxidation gives a naphthoquinone instead.

Anthracene oxidation is performed in the central ring where the aromaticity was low from the beginning.

Phenanthrene also oxidizes in the central ring by the same token.

Reduction

Under conditions in which benzene is not reduced at all, naphthalene is reduced by accepting four electrons from sodium.

The resulting tetrahydronaphthalene still preserves one ring's aromaticity.

Anthracene accepts two electrons in the same conditions, the central ring is thus reduced and the aromaticity of two rings is preserved.