Question

Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, E⁺

1. Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene 
2. Toluene, p-H₃C-C₆H₄-NO₂, p-O₂N-C₆H₄-NO₂.

Answer 1

General principle:

The reactivity of an aromatic compound towards an electrophile (E⁺) depends on the electron density of the benzene ring.

• Electron-donating groups activate the ring.
• Electron-withdrawing groups deactivate the ring.
• More and stronger electron-withdrawing groups → lower reactivity.

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(a) Chlorobenzene, p-Nitrochlorobenzene, 2,4-Dinitrochlorobenzene

–Cl is deactivating (−I effect) but donates electrons by resonance (+R).
–NO₂ is a strong electron-withdrawing group (−I, −R).

Comparison:

• Chlorobenzene: Only one deactivating group (Cl)
• p-Nitrochlorobenzene: One Cl and one strongly deactivating NO₂
• 2,4-Dinitrochlorobenzene: One Cl and two strongly deactivating NO₂ groups

Decreasing order of reactivity:

Chlorobenzene > p-Nitrochlorobenzene > 2,4-Dinitrochlorobenzene

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(b) Toluene, p-CH₃-C₆H₄-NO₂, p-O₂N-C₆H₄-NO₂

–CH₃ is an electron-donating group (+I, hyperconjugation).
–NO₂ is a strong electron-withdrawing group.

Comparison:

• Toluene: Activated by CH₃
• p-Nitrotoluene: One activating (CH₃) and one deactivating (NO₂) group
• p-Dinitrobenzene: Two strongly deactivating NO₂ groups

Decreasing order of reactivity:

Toluene > p-Nitrotoluene > p-Dinitrobenzene

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Final Answers:

(a) Chlorobenzene > p-Nitrochlorobenzene > 2,4-Dinitrochlorobenzene
(b) Toluene > p-CH₃-C₆H₄-NO₂ > p-O₂N-C₆H₄-NO₂