8.8 Oxidation and Reduction of Aromatic Compounds

Oxidation of Alkyl Side Chains

Despite its unsaturation, the benzene ring is inert to strong oxidizing agents such as KMnO₄, which will cleave alkene carbon–carbon bonds. It turns out, however, that the presence of the aromatic ring has a dramatic effect on the reactivity of alkyl side chains. These side chains react rapidly with oxidizing agents and are converted into carboxyl groups, –CO₂H.The net effect is conversion of an alkylbenzene into a benzoic acid, Ar–R → Ar– CO₂H. Butylbenzene is oxidized by aqueous KMnO₄ to give benzoic acid, for instance.

A similar oxidation is employed industrially for the preparation of the terephthalic acid used in the production of polyester fibers. Worldwide, approximately 118 million tons per year of terephthalic acid is produced by oxidation of p-xylene, using air as the oxidant and Co(III) salts as catalyst.

The mechanism of side-chain oxidation is complex and involves reaction of C–H bonds at the position next to the aromatic ring to form intermediate benzylic radicals. tert-Butylbenzene has no benzylic hydrogens, however, and is therefore inert.

Analogous side-chain oxidations occur in various biosynthetic pathways. The neurotransmitter norepinephrine, for instance, is biosynthesized from dopamine by a benzylic hydroxylation reaction. The process is catalyzed by the copper-containing enzyme dopamine β-monooxygenase and occurs by a radical mechanism. A copper–oxygen species in the enzyme first abstracts the pro-R benzylic hydrogen to give a radical, and a hydroxyl is then transferred from copper to carbon.

To hydrogenate an aromatic ring, it’s necessary either to use a platinum catalyst with hydrogen gas at a pressure of several hundred atmospheres or to use a more effective catalyst such as rhodium on carbon. Under these conditions, aromatic rings are converted into cyclohexanes. For example, o-xylene yields 1,2-dimethylcyclohexane, and 4-tert– butylphenol gives 4-tert-butylcyclohexanol.

Problem 8.13
What aromatic products would you obtain from the KMnO₄ oxidation of the following substances?

(a)

(b)