JEE Chemistry 2025 Question with Solution

Given: A set of aromatic compounds is treated with hot $\mathrm{KMnO_4}$.

Find: The total number of compounds that give benzoic acid.

Hot $\mathrm{KMnO_4}$ oxidizes the side chain attached to a benzene ring to $-\mathrm{COOH}$ only when there is at least one benzylic hydrogen on the carbon directly attached to the ring.

So, we check each compound for the presence of at least one benzylic hydrogen.

From the solution analysis:

1. Ethylbenzene $\left(\mathrm{C_6H_5-CH_2CH_3}\right)$ gives benzoic acid.
2. Isopropylbenzene $\left(\mathrm{C_6H_5-CH(CH_3)_2}\right)$ gives benzoic acid.
3. Tert-butylbenzene $\left(\mathrm{C_6H_5-C(CH_3)_3}\right)$ does not give benzoic acid because there is no benzylic hydrogen.
4. n-Propylbenzene $\left(\mathrm{C_6H_5-CH_2CH_2CH_3}\right)$ gives benzoic acid.
5. Benzyl alcohol $\left(\mathrm{C_6H_5-CH_2OH}\right)$ gives benzoic acid.
6. Benzyl chloride $\left(\mathrm{C_6H_5-CH_2Cl}\right)$ gives benzoic acid.
7. 1,4-Diethylbenzene was listed in the solution, but the counted compounds giving benzoic acid are stated as $1,2,4,5,6$.

Thus, the total number is

Therefore, the correct option is C.

Given: Oxidation with hot $\mathrm{KMnO_4}$.

Find: Which structures are converted to benzoic acid.

Principle: Any alkyl side chain on benzene is oxidized to $-\mathrm{COOH}$ if the benzylic carbon bears at least one hydrogen atom. If the benzylic carbon has no hydrogen, oxidation to benzoic acid does not occur.

Apply the rule:

• If the structure is of the form $\mathrm{C_6H_5-CH(R)_n}$ with at least one benzylic $\mathrm{H}$, it can be oxidized to benzoic acid.
• If the benzylic carbon is quaternary, such as $\mathrm{C_6H_5-C(R)_3}$, it does not react this way.

The solution concludes that compounds numbered $1,2,4,5,6$ satisfy this condition, while the tert-butyl substituted compound does not.

Hence, the number of compounds giving benzoic acid is $5$.

The correct option is C.