Amines

• Base properties of amines

  • they have a lone pair of electrons that is readily available for forming dative covalent bonds so accepting a proton.
  • They are weak bases as only a low concentration of hydroxide ions are produced
• Primary aliphatic amines are stronger bases than ammonia as the alkyl groups are electron releasing and push electrons towards the nitrogen atom, so makes the lone pair of nitrogen more readily available
• Secondary are stronger than primary amines because there are more alkyl groups substituted for H atoms

  • more electron density is pushed onto N atoms
• Why doesn't tertiary amines follow trend of strength of bases?

  They are less soluble in water and this makes them less strong than secondary amines
• Aromatic amines

  Do not form basic solutions because the lone pair of electrons on the N atom is delocalised with the ring of electrons in the benzene ring

  • less likely to accepts protons
• Primary aromatic amines do not form basic solutions because the lone pair of electrons on the N atom are delocalised with the ring of electrons in the benzene ring. So N less likely to accepts protons
• Why can't aromatic amines form basic solutions?

  The lone pair on N atom delocalised with the ring of electrons in Benzene, so it's less likely to accept protons
• Order of base strength of amines (from weakest)

  • aromatic amines
  • ammonia
  • alkyl amines
  • 1' amines
  • 3' amines
  • 2' amines
• All amines react as bases with acids to form ammonium salts

  • form ammonium salts
• Ionic salts formed in this reaction are insoluble but every soluble in acid. (bases reacting with acids)
• Addition of NaOH to an ammonium salt will convert it back to amine
• Ionic salts will be solid crystals, if water is evaporated because of strong ionic interactions
• Why is nucleophilic substitution not a good method for making a primary amine?

  Because lone pair of N atom, primary amine can still react in the same way as a nucleophile to form 2', 3' and quaternary salts. So the desire products would have to be separated from the other products
• How can you maximise the amount of primary amines and limit the subsequent reactions?

  Use excess ammonia
• Initial nucleophile for nucleophilic substitution of amines

  ammonia in ethanol
• How to promote the formation of quaternary salt?

  Using excess halogenoalkane
• Using large excess of ammonia maximise the amount of primary amine formed.
• Using excess of halogenoalkane promote the formation of quaternary salt
• Quaternary salts can be used as cationic surfactants

  • reduce surface tension of liquids
  • Positive nitrogen is attracted towards negatively charged surfaces such as glass, hair, fibres and plastics.
  • This helps in the uses as fabric softeners, hair conditioner
• 2 step reaction to prepare amines

  1. Halogenoalkane -> nitrile KCN in aqueous ethanol (heat under reflux)
  2. Reduce nitrile to amine by using LiAlH4
• Disadvantage to preparing amine from nitriles

  2 step reaction so low yield

  KCN is toxic
• Reducing nitroarenes to aromatic amines

  R - Sn and HCl/ Fe and HCl

  C - heating

  M - reduction
• Aliphatic amines and phenylamine can react with acyl chlorides and acid anhydrides to form amides in nucleophilic addition elimination