Directing groups

• Further substitution:

  • phenol can undergo an electrophilic substitution reaction with nitric acid to form 2 isomers: 2-nitrophenol and 4-nitrophenol
  • like phenol, many substituted aromatic compounds can undergo a second substitution - disubstitution
  • some of these reactions take place more readily than benzene itself
  • other reactions take place less easily and require extreme conditions
• Activation and deactivation:

  • bromine requires a halogen carrier catalyst to react with benzene, whereas bromine will react rapidly with phenylamine
  • the -NH2 group activated the ring as the aromatic ring reacts more readily with electrophiles
• • nitrobenzene reacts slowly with bromine, requiring both a halogen carrier catalyst and a high temp - the benzene ring in nitrobenzene is less susceptible to electrophilic sub than benzene itself
  • the -NO2 group deactivates the aromatic ring as the ring reacts less readily with electrophiles
• not only does the rate of reaction and extent of substitution differ in the 2 examples but also the position of substitution on the benzene ring is also different:

  • the -NH2 group directs the second substituent to position 2 or 4
  • the -NO2 group directs the second substituent to position 3
• directing effects:

  • there are many different groups that can be attached to a benzene ring
  • different groups can have a directing effect on any second substituent on the benzene ring
  • all 2- and 4- directing groups (ortho and para-directors) are activating with the exception of the halogens
  • all 3- directing groups (meta directors) are deactivating groups
• 2- and 4- directing:

  • -NH2 or -NHR
  • -OH
  • -OR
  • -R or -C6H5
  • -F, -Cl, -Br, -I
• 3-directing:

  • RCOR
  • -COOR
  • -SO3H
  • -CHO
  • -COOH
  • -CN
  • -NO2
  • NR3+
• using directing effects in organic synthesis:

  • the directing effect of substituent groups can be used when planning an organic synthesis
  • when carrying out more than one electrophilic sub reaction on an aromatic compound, may have to consider the order in which the reactions are carried out to ensure the correct sub pattern and that the required product is prepared
• making TNT:

  • an application of directing groups can be seen in the reaction of methylbenzene (toluene) with an excess of conc. nitric and sulfuric acid
  • at 50 degrees toluene forms a monosubstituted product of 2-nitrotoluene
  • the product of this reaction is less reactive than the toluene bc the -NO2 group deactivates the benzene ring. however if temp increased to 70 a disubstituted product 2,4-dinitrotoluene is formed
  • the second -NO2 group deactivated the benzene ring further
  • further reaction will produce 2,4,6-trinitrotoluene - TNT
  • requires extreme conditions - accidental over heating = explode