Aromatic Hydrocarbons

Question

In the nitration of toluene, the –CH₃ group directs the incoming –NO₂ group predominantly to the ortho and para positions because –CH₃ is:

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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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Aromatic Hydrocarbons

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About This Question

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