16.8 Oxidation of Aromatic Compounds

Oxidation of Alkylbenzenes

Oxidation of alkylbenzenes converts alkyl side chains into carboxylic acids while leaving the aromatic ring untouched. The ring's aromatic stability protects it from the oxidizing agents, so all the action happens on the side chain. This section also covers side-chain bromination using NBS, which relies on the special stability of benzylic radicals.

Oxidation of Aromatic Side Chains

Strong oxidizing agents like potassium permanganate ($KMnO_4$) and chromic acid ($H_2CrO_4$) can oxidize the alkyl side chain of an alkylbenzene all the way to a carboxylic acid. The aromatic ring stays intact because its delocalized electron system is too stable for these reagents to disrupt.

The key requirement: the side chain must have at least one benzylic C–H bond (a hydrogen on the carbon directly attached to the ring). Without that hydrogen, oxidation won't proceed. That's why tert-butylbenzene resists oxidation under these conditions.

A few important patterns to remember:

• Any alkyl group with a benzylic C–H gets oxidized down to a $-COOH$ group directly on the ring. Toluene ($C_6H_5CH_3$), ethylbenzene ($C_6H_5CH_2CH_3$), and propylbenzene all give benzoic acid ($C_6H_5COOH$). The extra carbons beyond the benzylic position are cleaved off as $CO_2$.
• This means the carbon count of the side chain does not stay the same. A three-carbon propyl group becomes a one-carbon carboxylic acid.
• If the ring has multiple alkyl substituents, each one with a benzylic C–H gets oxidized independently. For example, para-xylene gives terephthalic acid (benzene-1,4-dicarboxylic acid), which is used to make PET plastics.

Side-Chain Bromination Mechanism

N-Bromosuccinimide (NBS) selectively brominates the benzylic position of alkylbenzenes through a radical chain mechanism. NBS is preferred over $Br_2$ because it maintains a low, steady concentration of $Br \cdot$ radicals, which favors substitution at the most stable radical site rather than addition to the ring.

The reaction requires a radical initiator (benzoyl peroxide, AIBN, or UV light) to get started. Here's how the chain mechanism works:

1. Initiation: The initiator decomposes to form radicals, which abstract a bromine atom from NBS, generating a $Br \cdot$ radical.
2. Propagation step 1: The $Br \cdot$ radical abstracts a hydrogen atom from the benzylic position, forming HBr and a benzylic radical. This step is selective because the benzylic radical is resonance-stabilized (see below).
3. Propagation step 2: The benzylic radical reacts with another molecule of NBS, abstracting a bromine atom. This gives the benzylic bromide product and a succinimidyl radical, which goes on to generate another $Br \cdot$ and continue the chain.
4. Termination: Any two radicals combine, ending the chain.

The selectivity for the benzylic position comes entirely from the stability of the benzylic radical intermediate. Because forming that radical has a lower activation energy than forming a non-benzylic radical, NBS brominates almost exclusively at the benzylic carbon.

Stability of Benzyl Radicals

Benzylic radicals are unusually stable because the unpaired electron delocalizes into the aromatic ring through resonance. You can draw the lone electron spread across the ortho and para positions of the ring, giving a total of five resonance contributors (one with the radical on the benzylic carbon, two on the ortho carbons, and two on the para carbon). This extensive delocalization lowers the radical's energy significantly.

Compare this to other radical types:

• Alkyl radicals (methyl, ethyl, isopropyl, tert-butyl) are stabilized only by hyperconjugation and inductive effects. Their stability order is: tertiary > secondary > primary > methyl. None of them benefit from resonance.
• Allyl radicals are resonance-stabilized across the adjacent $C=C$ double bond, giving three resonance structures. This makes them more stable than simple alkyl radicals, but less stable than benzylic radicals (3 resonance structures vs. 5).

The practical consequence: benzylic radicals form more easily and persist longer than other carbon radicals. That's exactly why NBS bromination and $KMnO_4$ oxidation both target the benzylic position so selectively.

Free Radical Pathways in Aromatic Oxidation

Many aromatic oxidation reactions proceed through radical intermediates at the benzylic position. Even the $KMnO_4$ oxidation of toluene is thought to involve initial hydrogen abstraction at the benzylic carbon, generating a benzylic radical that is then further oxidized by the metal reagent. The resonance stabilization of this radical is what makes the benzylic position the consistent point of attack across different oxidation methods, whether the reagent is a strong metal oxidant or a halogen radical source like NBS.