Question

The total number of compounds from below when treated with hot KMnO4 giving benzoic acid is:

• A. 6
• B. 3
• C. 5
• D. 4

Hint:

When using hot KMnO4, remember that it will oxidize alkyl groups on aromatic rings to carboxylic acids, making it useful for converting side chains into carboxyl groups.

Answer 1

The Correct Option is C

The reaction of organic compounds with hot potassium permanganate (KMnO₄) is an example of oxidative cleavage. Aromatic hydrocarbons with side chains, when oxidized, can be converted to benzoic acid, provided that there is at least one benzylic hydrogen present. The process can be summarized as follows: 

When a compound containing the structure of an alkyl benzene is treated with hot KMnO₄, the side chain is oxidized all the way to a carboxyl group (-COOH), resulting in the formation of benzoic acid if any hydrocarbon chains containing benzylic hydrogens (i.e., C-H bonds directly attached to the carbon adjacent to the benzene ring) are present.

Now, consider the given options: we need to find out which compounds, when treated with hot KMnO₄, will give benzoic acid:

1. Toluene or any compound with alkyl (methyl, ethyl, etc.) groups attached directly to the benzene ring.
2. Xylene (symmetrical with alkyl group on different positions), if at least one alkyl group has a benzylic hydrogen.
3. Anything that already has benzoic acid won't undergo further conversion to benzoic acid under these conditions.

Thus, out of the provided compounds in the figure, only those containing alkyl groups directly bonded to benzene with benzylic hydrogens will yield benzoic acid after reaction with hot KMnO₄.

In conclusion, the total number of compounds that form benzoic acid when treated with hot KMnO₄ is 5.