Question

Number of compounds from the following which cannot undergo Friedel-Crafts reactions is :____
toluene, nitrobenzene, xylene, cumene, aniline, chlorobenzene, m-nitroaniline, m-dinitrobenzene

Answer 1

Correct Answer: 4

The Friedel-Crafts reaction is a key method used in organic chemistry to attach substituents to an aromatic ring. For this reaction to occur, the aromatic compound must not contain strongly deactivating groups such as -NO₂ or -NH₂ which reduce the electron density on the ring required for electrophilic aromatic substitution.

Let's examine each compound one by one:

• Toluene: Can undergo Friedel-Crafts reactions as the methyl group is activating.
• Nitrobenzene: Cannot undergo Friedel-Crafts reactions due to the strongly deactivating -NO₂ group.
• Xylene: Can undergo Friedel-Crafts reactions as it has alkyl groups which are activating.
• Cumene: Can undergo Friedel-Crafts reactions due to its activating isopropyl group.
• Aniline: Cannot undergo Friedel-Crafts reactions because the -NH₂ group forms complexes with the catalyst, a Lewis acid, thus inhibiting the reaction.
• Chlorobenzene: Typically does not undergo Friedel-Crafts reactions efficiently because chlorine is a weakly deactivating group, though less so than -NO₂ or -NH₂.
• m-Nitroaniline: Cannot undergo Friedel-Crafts reactions due to deactivating -NO₂ and complex-forming -NH₂ groups.
• m-Dinitrobenzene: Cannot undergo Friedel-Crafts reactions as the multiple -NO₂ groups are strongly deactivating.

Counting the compounds that cannot undergo Friedel-Crafts reactions, we have: nitrobenzene, aniline, m-nitroaniline, and m-dinitrobenzene, totaling 4 compounds.

Thus, the number of compounds that cannot undergo Friedel-Crafts reactions, which is 4, falls within the given range (4,4).

Answer 2

Friedel-Crafts reactions require an aromatic compound and an electrophile, facilitated by a Lewis acid catalyst (e.g., AlCl$_3$). However, 
certain compounds cannot undergo Friedel-Crafts reactions due to deactivating groups or coordination issues with the catalyst.
 Toluene, xylene, and cumene: These are activated aromatic compounds and can undergo Friedel-Crafts reactions.
 Chlorobenzene: Chlorine is an electron-withdrawing group but is ortho/para-directing; hence it can still undergo Friedel-Crafts reactions.
 Nitrobenzene, m-nitroaniline, m-dinitrobenzene: Nitro groups are strongly deactivating, making the aromatic ring unreactive for Friedel-Crafts reactions.
 Aniline: The amino group ($-\text{NH}_2$) coordinates with the Lewis acid catalyst (AlCl$_3$), deactivating the ring.
Compounds that cannot undergo Friedel-Crafts reactions:
Nitrobenzene, aniline, m-nitroaniline, m-dinitrobenzene (4 compounds).
Final Answer: (4)