Number of compounds from the following which cannot undergo Friedel-Crafts reactions is :____
toluene, nitrobenzene, xylene, cumene, aniline, chlorobenzene, m-nitroaniline, m-dinitrobenzene
toluene, nitrobenzene, xylene, cumene, aniline, chlorobenzene, m-nitroaniline, m-dinitrobenzene
Correct Answer: 4
Approach Solution - 1
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).
Approach Solution -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)
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