Química Orgánica 2 · Efectos de los sustituyentes y orientación
Nitration of 4-chlorophenol ortho to OH
Condiciones: dilute HNO3
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The ring attacks the nitronium ion ortho to OH
Phenols are so activated that dilute nitric acid is enough; a small amount of nitronium ion forms from nitric acid itself (drawn already formed). With the para position blocked by Cl, attack occurs ortho to OH, putting the cation on the OH-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 OH oxygen stabilizes the cation
An OH lone pair forms a C=O+ bond so every atom has an octet. Chlorine could also donate, but its 3p lone pair overlaps poorly and it withdraws strongly by induction, so OH is by far the stronger director.
Las flechas curvas
- The OH oxygen lone pair forms a C=O+ π bond to the cationic carbon, giving the octet-complete oxonium contributor.
OH versus Cl
The stronger donor controls the position.
Water removes the ring proton
Water, the solvent and strongest base available in dilute nitric acid, removes the hydrogen on the sp3 carbon; the electrons flow back to oxygen and the ring is aromatic again.
Las flechas curvas
- A water lone pair takes the hydrogen on the sp3 ring carbon (water is assumed as the base).
- 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-chloro-2-nitrophenol
The product is 4-chloro-2-nitrophenol (and hydronium): the nitro group goes ortho to OH, the strongest activator present.
Qué vas a practicar
Draw nitration of 4-chlorophenol ortho to the OH group and show why OH, not Cl, directs.
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