We introduced you to the 10-electron aromatic system of naphthalene. As you would expect, it undergoes electrophilic aromatic substitution with the same reagents you met, but the regioselectivity of its reactions is of a different type to the ortho, meta, para selectivity we have been talking about. Naphthalene has 10 carbons: two form the ring junction, and aren’t available for substitution reactions, and the other eight are of just two types α (the 1-position, next to the ring junction) and β (the 2-position). Electrophilic substitution on naphthalene normally occurs at a site next to the ring junction (α). This is because the HOMO has its largest coefficient at this atom, but you can rationalize the result by looking at the long, linear delocalization in the resulting cation, which can be represented by a single train of arrows. This extended conjugation makes naphthalene more nucleophilic than benzene. So, bromination occurs at the α-position in good yield even without a Lewis acid.

Reaction at the other position (β) is less favourable as the intermediate cation is cross conjugated. The cation delocalizes into both rings, but no long linear chain of arrows is possible.

If the reaction is irreversible, the α-product is usually formed. But if the reaction is reversible, as is the case with sulfonation, the position of substitution may be determined by temperature. Sulfonation at low temperatures gives the α-product by kinetic control, while sulfonation at high temperatures gives the β-product by thermodynamic control. The β-product is formed more slowly but it is more stable as there is less steric hindrance between the large sulfonic acid group and the orange hydrogen on the other ring. Under conditions allowing reversible sulfonation, eventually all the product ends up β.

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مواضيع ذات صلة

Alkylation of nitroalkanes

Alkylation of nitriles

Some important considerations that affect all alkylations

Conjugate addition

Electrophilic attack on conjugated dienes

Regiospecific preparation of allylic chlorides

Nucleophilic attack on allylic compounds

Radical abstraction

Radical addition

Electrophilic attack on alkenes

Regioselectivity of intramolecular reactions

Regiocontrol by choice of route