Which compound forms benzoic acid when oxidized with acidified KMnO₄ or K₂Cr₂O₇?

Oxidation of alkylbenzenes is an important reaction in organic chemistry. When an alkyl group attached to a benzene ring is oxidized using strong oxidizing agents such as acidified potassium permanganate (KMnO₄) or potassium dichromate (K₂Cr₂O₇), it is converted into a carboxylic acid, specifically benzoic acid, regardless of the length of the alkyl chain.

For example, toluene (methylbenzene) has a methyl group attached to the benzene ring. Oxidation of the methyl group yields benzoic acid. Ethylbenzene, with an ethyl group, also undergoes oxidation, converting the terminal carbon of the alkyl chain into a carboxyl group to form benzoic acid. Similarly, n-propylbenzene undergoes oxidation at the benzylic position, resulting in benzoic acid as the final product.

The reaction occurs because the benzylic hydrogen atoms (hydrogens attached to the carbon adjacent to the benzene ring) are reactive and can be oxidized by strong oxidizing agents. The process is independent of the length of the alkyl chain; only the benzylic carbon is oxidized. This property is widely used in organic synthesis to prepare aromatic carboxylic acids from various alkylbenzenes.

Potassium permanganate (KMnO₄) acts as a strong oxidizing agent and produces MnO₂ as a byproduct, while acidified potassium dichromate (K₂Cr₂O₇) produces Cr³⁺ ions. Both agents achieve the same transformation.

This reaction demonstrates the stability of the benzene ring during oxidation and the reactivity of the benzylic position. It is a fundamental reaction in aromatic chemistry, important for preparing derivatives of benzene such as benzoic acid for industrial and laboratory applications.

The key point is that all alkylbenzenes with a benzylic hydrogen, regardless of chain length, will form benzoic acid under these conditions.