Reduction of Aromatic Compounds

Catalytic hydrogenation of aromatic rings requires forcing conditions (high heat and hydrogen pressure).

Under milder conditions it is possible to reduce the double-bond of an alkene without reducing the aromatic ring.

Notice in the above equation that H₂/Pd does not reduce the keto-carbonyl group. Remember, however, that H₂/Pd will reduce a keto-carbonyl group when it is directly attached to an aromatic ring (see equations 4 and 5 under Carbonyl Reductions).

This reduction of the C=O group next to an aromatic ring is an important synthetic tool. Recall the Friedel-Crafts alkylation from Section 16.3. When attaching larger alkyl groups to arenes there is a possibility of rearrangement of the alkyl group structure.