Química Orgánica 2 · Efectos de los sustituyentes y orientación
Nitration of 4-methylanisole ortho to methoxy
Condiciones: HNO3, H2SO4
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Arrastra desde los electrones que se mueven hasta donde van, o toca uno y luego el otro. Cada flecha se revisa; Pista te ayuda cuando te atascas. No necesitas cuenta.
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 ortho to methoxy
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 two directors point at different carbons: OCH3 directs to C2/C6 (its ortho positions) and CH3 to C3/C5, and each para position is already taken, so the directors compete and the stronger donor, OCH3, wins: attack ortho to OCH3 puts the cation right on the methoxy-bearing carbon.
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.
The methoxy oxygen stabilizes the cation
The oxygen lone pair forms a C=O+ bond, giving a σ complex in which every atom has an octet. A methyl group can only help by hyperconjugation, which is much weaker, so OCH3 controls the regiochemistry.
Las flechas curvas
- A methoxy oxygen lone pair forms a C=O+ π bond to the cationic carbon, giving the octet-complete oxonium contributor.
Which director wins?
When two activators compete, the stronger π donor decides.
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.
- The π pair to the positive heteroatom moves back onto it as a lone pair, neutralizing it.
4-methyl-2-nitroanisole
The product is 4-methyl-2-nitroanisole: the nitro group sits ortho to the stronger activator, OCH3, and sulfuric acid is regenerated.
Qué vas a practicar
Draw nitration of 4-methylanisole at the carbon ortho to the methoxy group and show why that site wins.
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