Aromatic chemistry: Oxidation and reduction

OXIDATION AND REDUCTION

Key Notes

Oxidation

Aromatic rings are resistant to oxidation but alkyl chains attached to the ring are not. Alkyl substituents containing a benzylic hydrogen are oxidized to a  carboxylic acid.

Reduction

The aromatic ring is difficult to reduce with hydrogen and requires vigor- ous reaction conditions using high pressure and heat, or strong catalysts such as rhodium. Cyclohexane products are obtained. The resistance of the aromatic ring to reduction allows the selective reduction of substituents such as ketones and nitro groups without affecting the aromatic ring itself.

Oxidation

Aromatic rings are remarkably stable to oxidation and are resistant to oxidizing agents such as potassium permanganate or sodium dichromate. However, alkyl substituents on aromatic ring are surprisingly susceptible to oxidation. This can be put to good use in the synthesis of aromatic compounds since it is possible to oxidize an alkyl chain to a carboxylic acid without oxidizing the aromatic ring. The mechanism of this reaction is not fully understood, but it is known that a benzylic hydrogen has to be present (i.e. the carbon directly attached to the ring must have a hydrogen). Alkyl groups lacking a benzylic hydrogen are not oxidized.

Reduction

Aromatic rings can be hydrogenated to cycloalkanes, but the reduction has to be carried out under strong conditions using a nickel catalyst, high temperature and high pressure (Fig. 2) – much stronger conditions than would be required for the  is  because  of  the  inherent  stability  of reduction  of  alkenes  .  Thisaromatic rings. The reduction can also be carried out using hydrogen and a platinum catalyst under high pressure, or with hydrogen and a rhodium/carbon catalyst. The latter is a more powerful catalyst and the reaction can be done at room temperature and at atmospheric pressure.

The resistance of the aromatic ring to reduction is useful since it is possible to reduce functional groups which might be attached to the ring without reducing the aromatic ring itself. For example, the carbonyl group of an aromatic ketone can be reduced with hydrogen over a palladium catalyst without affecting the aromatic ring (Fig. 3). This allows the synthesis of primary alkylbenzenes which cannot be synthesized directly by the Friedel–Crafts alkylation. It is worth noting that the aromatic ring makes the ketone group more reactive to reduction than would normally be the case. Aliphatic ketones would not be reduced under these conditions. Nitro groups can also be reduced to amino groups under these conditions without affecting the aromatic ring.