6.1.6

• directing and withdrawing groups

  
• The directing effect is how a functional group attached directly to an aromatic ring affects which carbon atoms are more likely to undergo substitution.

  • Phenol will readily undergo electrophilic substitutions with a variety of reagents without the presence of a catalyst.

  → This enhanced reactivity is due to extra electrons from the oxygen on the hydroxyl group being donated to the pi-system of the aromatic ring
• The directing effect is how a functional group attached directly to an aromatic ring affects which carbon atoms are more likely to undergo substitution.

  • Phenol will readily undergo electrophilic substitutions with a variety of reagents without the presence of a catalyst

  → This enhanced reactivity is due to extra electrons from the oxygen on the hydroxyl group being donated to the pi-system of the aromatic ring
• Bromination

  Phenol will undergo a triple substitution reaction with bromine water at room temperature. The balanced chemical equation for this reaction is: C6H5OH + 3Br2 → C6H2Br3OH + 3НВг

  • The resulting product is a white precipitate of 2,4,6-tribromophenol, which smells of antiseptic.
  • Phenol will undergo a single substitution reaction with dilute nitric acid (HNO3) at room temperature

  → This reaction forms a mixture of 2-nitrophenol and 4-nitrophenol

  The balanced chemical equation for this reaction is: C6H5OH + HNO3 → C6H4(NO2)OH + H2O
• Bromination 2

  • Unlike nitration with benzene, this reaction does not require concentrated nitric acid or a sulfuric acid catalyst

  → However, if concentrated nitric acid is used, a triple substitution reaction occurs forming 2,4,6-trinitrophenol
• Position of Substitution

  • In phenol, the hydroxyl group pushes additional electrons into the pi-system. This makes substitution reactions mainly occur on the 2 and 4 positions of the aromatic ring. The hydroxyl group activates these carbon atoms so that their rate of substitution is faster than the other positions.

  → This is known as the 2- and 4-directing effect. This effect is more pronounced in aromatic compounds with an NH, group directly attached to the aromatic ring
• Position of Substitution 2

  • When -NO2 groups are directly attached to the aromatic ring, a 3-directing effect is seen.

  → The nitro group withdraws electrons from the -system and makes the rate of substitution highest on the third carbon atom

  • For organic synthesis it is important that a reaction pathway can be designed to maximise the desired product.
  • By considering the electron donating or withdrawing effects of a directly-attached functional group, predictions can be made as to the position(s) in which substitution will take place.