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18.2 : Reactions at the Benzylic Position: Oxidation and Reduction

The benzylic position describes the position of a carbon atom attached directly to a benzene ring. Benzene by itself does not undergo oxidation. In contrast, the benzylic carbon is quite reactive in the presence of strong oxidizing agents such as KMnO4 or H2CrO4. Therefore, alkylbenzenes are readily oxidized to benzoic acid, irrespective of the type of alkyl groups.

Figure1

Halogen and nitro substituents on a benzene ring remain unaffected by these oxidizing agents.

Figure2

When more than one alkyl side chain is attached to the benzene ring, each gets oxidized to carboxyl groups.

Figure3

The only condition required for benzylic oxidation is to have at least one hydrogen at the benzylic position. This means that tert-butylbenzene, which lacks benzylic hydrogen, does not undergo oxidation.

Figure4

Benzene is also unreactive towards catalytic hydrogenation. A typical alkene double bond attached to a benzene ring can be selectively reduced under specific conditions without affecting the other functional groups. For example, in the selective reduction of 4-phenyl-3-buten-2-one to 4-phenyl-2-butanone, the benzene ring and ketone group remain unaffected.

Figure5

Tags
Benzylic PositionOxidationReductionAlkylbenzenesBenzoic AcidHalogenNitro SubstituentsCatalytic HydrogenationAlkene Double Bond4 phenyl 3 buten 2 one4 phenyl 2 butanone

From Chapter 18:

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18.2 : Reactions at the Benzylic Position: Oxidation and Reduction

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18.1 : NMR Spectroscopy of Benzene Derivatives

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18.3 : Reactions at the Benzylic Position: Halogenation

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18.5 : Electrophilic Aromatic Substitution: Chlorination and Bromination of Benzene

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18.6 : Electrophilic Aromatic Substitution: Fluorination and Iodination of Benzene

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18.15 : <em>ortho</em>&ndash;<em>para</em>-Directing Deactivators: Halogens

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