JEE Chemistry 2024 Question with Solution

Given: The compounds are toluene, nitrobenzene, xylene, cumene, aniline, chlorobenzene, m-nitroaniline, and m-dinitrobenzene.

Find: The number of compounds which cannot undergo Friedel-Crafts reactions.

Friedel-Crafts reactions require an aromatic ring that is sufficiently reactive toward electrophilic aromatic substitution. Strongly deactivating groups such as $-\text{NO}_2$ prevent the reaction, and $-\text{NH}_2$ also inhibits the reaction because it coordinates with the Lewis acid catalyst such as $\text{AlCl}_3$.

Now examine each compound:

• Toluene: can undergo Friedel-Crafts reaction because the methyl group is activating.
• Nitrobenzene: cannot undergo Friedel-Crafts reaction due to the strongly deactivating $-\text{NO}_2$ group.
• Xylene: can undergo Friedel-Crafts reaction because alkyl groups are activating.
• Cumene: can undergo Friedel-Crafts reaction due to the activating isopropyl group.
• Aniline: cannot undergo Friedel-Crafts reaction because $-\text{NH}_2$ forms a complex with the Lewis acid catalyst.
• Chlorobenzene: can still undergo Friedel-Crafts reaction according to the solution, since chlorine is weakly deactivating.
• m-Nitroaniline: cannot undergo Friedel-Crafts reaction due to the presence of $-\text{NO}_2$ and $-\text{NH}_2$ groups.
• m-Dinitrobenzene: cannot undergo Friedel-Crafts reaction because two $-\text{NO}_2$ groups strongly deactivate the ring.

Therefore, the compounds that cannot undergo Friedel-Crafts reactions are nitrobenzene, aniline, m-nitroaniline, and m-dinitrobenzene.

So, the required number is $4$.

Given: A list of $8$ aromatic compounds is provided.

Find: How many of them cannot undergo Friedel-Crafts reactions.

The key idea is to identify substituents that either:

1. strongly deactivate the benzene ring, or
2. coordinate with the Lewis acid catalyst and stop the reaction.

From the provided solution:

• Nitrobenzene contains $-\text{NO}_2$, so it is strongly deactivated.
• Aniline contains $-\text{NH}_2$, so it coordinates with $\text{AlCl}_3$ and does not react.
• m-Nitroaniline contains both $-\text{NO}_2$ and $-\text{NH}_2$, so it does not undergo Friedel-Crafts reaction.
• m-Dinitrobenzene contains two $-\text{NO}_2$ groups, making the ring highly deactivated.

The remaining compounds — toluene, xylene, cumene, and chlorobenzene — are treated in the solution as capable of undergoing Friedel-Crafts reactions.

Hence, total compounds that cannot undergo Friedel-Crafts reactions:

Therefore, the answer is $4$.