Aromatic Hydrocarbons
In the nitration of toluene, the –CH₃ group directs the incoming –NO₂ group predominantly to the ortho and para positions because –CH₃ is:
Select the correct option:
Solution
An electron-donating group by hyperconjugation
The –CH₃ group in toluene is an ortho/para director and ring activator. It donates electron density into the ring primarily through hyperconjugation: the C–H sigma bonds of the methyl group overlap with the empty/partially filled π system of the ring, delocalising electrons and increasing electron density at the ortho and para positions. (Pure resonance donation is negligible for –CH₃ because carbon does not have lone pairs for resonance; this distinguishes it from –OH or –NH₂ which donate by resonance). The increased electron density at ortho and para positions makes those sites more susceptible to electrophilic attack by the NO₂⁺ (nitronium ion). The –CH₃ group is mildly electron-donating overall, so the ring is slightly activated relative to benzene. It is not electron-withdrawing, not a resonance donor, and not a meta director.
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About This Question
- Subject
- chemistry
- Chapter
- hydrocarbons
- Topic
- aromatic hydrocarbons
- Difficulty
- Hard
- Year
- 2025
Solution
Correct Answer:
An electron-donating group by hyperconjugation
The –CH₃ group in toluene is an ortho/para director and ring activator. It donates electron density into the ring primarily through hyperconjugation: the C–H sigma bonds of the methyl group overlap with the empty/partially filled π system of the ring, delocalising electrons and increasing electron density at the ortho and para positions. (Pure resonance donation is negligible for –CH₃ because carbon does not have lone pairs for resonance; this distinguishes it from –OH or –NH₂ which donate by resonance). The increased electron density at ortho and para positions makes those sites more susceptible to electrophilic attack by the NO₂⁺ (nitronium ion). The –CH₃ group is mildly electron-donating overall, so the ring is slightly activated relative to benzene. It is not electron-withdrawing, not a resonance donor, and not a meta director.
This hard difficulty chemistry question is from the chapter hydrocarbons, covering the topic of aromatic hydrocarbons. It appeared in the 2025 exam.
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