Oxidation Of Aromatic Alkanes With KMnO4 To Give Carboxylic Acids

Description: Treatment of an alkylbenzene with potassium permanganate results in oxidation to give the benzoic acid.

Notes: The position directly adjacent to an aromatic group is called the “benzylic” position.

The reaction only works if there is a hydrogen attached to the carbon.

Examples:

Notes: Note that in example 2 the extra carbons are cleaved to give the same product as in example 1. And in example 3, two benzoic acids are formed. Finally, when no hydrogens are present on the benzylic carbon, no reaction occurs (example 4).

Mechanism: For the purposes of Org 1/ Org 2 the mechanism isn’t considered that important. Manganese acts in mysterious ways. It’s thought that the first step is removal of a hydrogen by one of the oxygens on MnO4(–) in a free-radical reaction. Beyond that, it gets complicated.

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(Advanced) References and Further Reading

For some examples “in the wild”, see the following.

• Mechanistic studies supporting an initial C-H abstraction. J. Am. Chem. Soc. 1963, 85, 3487.
• Another mechanistic study with tertiary benzylic carbon. J. Am. Chem. Soc. 1970 92 329
• Oxidation of various hydrocarbons with KMnO4 using trifluoroacetic acid: Can. J. Chem. 1978, 56, 1273

Real-Life Examples:

Org. Synth. 1927, 7, 18

DOI Link: 10.15227/orgsyn.007.0018 Click to Flip

Org. Synth. 1922, 2, 53

DOI Link: 10.15227/orgsyn.002.0053 Click to Flip

Org. Synth. 1930, 10, 20

DOI Link: 10.15227/orgsyn.010.0020 Click to Flip

Org. Synth. 1940, 20, 79

DOI Link: 10.15227/orgsyn.020.0079 Click to Flip