Química Orgánica 2 · Efectos de los sustituyentes y orientación
Meta nitration of the trimethylanilinium ion
Condiciones: HNO3, H2SO4
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El mecanismo, paso a paso
Siguiente reproduce las flechas curvas de cada paso y las explica. Los pasos escritos están abajo.
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The ring attacks nitronium meta to N+
The nitronium ion, O=N+=O, is formed from nitric acid by sulfuric acid (protonation of the N–OH, loss of water); it is drawn here already formed. The –N(CH3)3+ group has no lone pair and pulls electron density strongly through the σ bond, so the ring is very deactivated. The π pair at a meta carbon forms the C–N bond.
Las flechas curvas
- The ring π pair forms the new C–N bond to the nitronium nitrogen.
- One N=O π pair moves onto oxygen so nitrogen does not exceed four bonds; the new group is a neutral nitro group.
Keep the charge away from the ipso carbon
After meta attack the positive charge is shared by the two carbons ortho and the one para to the attack site, none of which is the carbon bearing N+. Ortho or para attack would give a contributor with + directly next to N+, a severe charge–charge repulsion.
Las flechas curvas
- The ring π pair shifts toward the cation, moving the positive charge to the carbon ortho to the ammonium group, but never onto the ipso carbon.
A purely inductive meta director
There is no resonance donation; only charge repulsion in the σ complex matters.
Hydrogen sulfate removes the ring proton
Hydrogen sulfate, the strongest base in mixed acid, removes the hydrogen on the sp3 ring carbon; the electrons flow back around the ring and aromaticity is restored.
Las flechas curvas
- A hydrogen sulfate oxygen lone pair takes the hydrogen on the sp3 ring carbon.
- The C–H bond pair becomes a ring π bond, starting to restore the aromatic sextet.
- A ring π pair shifts one bond along to make room for the new π bond.
3-nitro-N,N,N-trimethylanilinium
Nitration gives almost exclusively the meta product, with sulfuric acid regenerated. A positively charged substituent is a purely inductive, strongly deactivating meta director.
Qué vas a practicar
Draw nitration of the trimethylanilinium ion at the meta position and show that its σ complex never puts positive charge next to N+.
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