Amines

• Amines


  Organic compounds containing a nitrogen atom with a lone pair of electrons
• Primary aliphatic amines


  Amines where the nitrogen is bonded to one alkyl group and two hydrogen atoms
• Reaction of primary aliphatic amines with water

  1. CH3NH2 + H2O ⇌ CH3NH3+ + OH-

  2. NH3 (aq) + H2O (l) ⇌ NH4+ (aq) + OH- (aq)
• Primary aliphatic amines


  • They act as Bronsted-Lowry bases because the lone pair of electrons on the nitrogen is readily available for forming a dative covalent bond with a H+ and so accepting a proton
  • They are weak bases as only a low concentration of hydroxide ions is produced
• Primary aliphatic amines


  • They are stronger bases than ammonia as the alkyl groups are electron releasing and push electrons towards the nitrogen atom and so making the lone pair of electrons on the nitrogen more readily available
• Primary aromatic amines


  Amines where the nitrogen is bonded to an aromatic ring
• Primary aromatic amines


  • They do not form basic solutions because the lone pair of electrons on the nitrogen delocalise with the ring of electrons in the benzene ring, making the nitrogen less able to accept protons
• Base properties of amines

  Amines react with acids to form ammonium salts
• Reactions of amines with acids

  • CH3NH2 (aq) + HCl (aq) → CH3NH3+ Cl- (aq)
• Reaction of ammonium salts with NaOH

  Addition of NaOH to an ammonium salt will convert it back to the amine
• Ammonium salts


  • They are soluble in water because of the ionic interactions
• Secondary amines


  • They are stronger bases than primary amines because they have more alkyl groups that are substituted onto the N atom in place of H atoms, pushing more electron density onto the N atom
• Tertiary amines are not the strongest amine bases
• Order of base strength of amines

  • Aromatic amines < ammonia < primary amines < tertiary amines < secondary amines
• Forming primary amines from halogenoalkanes and ammonia

  CH3CH2Br + 2NH3 → CH3CH2NH2 + NH4Br
• Nucleophilic properties of primary amines

  Primary amines can undergo further nucleophilic substitution reactions to form secondary, tertiary amines and quaternary ammonium salts
• Mechanism for forming primary amines from halogenoalkanes and ammonia


  Nucleophilic attack of ammonia on the halogenoalkane

  2. Removal of a proton by a second ammonia molecule to form the amine
• Forming secondary and tertiary amines from halogenoalkanes and amines


  Nucleophilic attack of the amine on the halogenoalkane

  2. Removal of a proton by ammonia or the amine to form the secondary/tertiary amine
• Forming quaternary ammonium salts from halogenoalkanes and amines


  Only the first step of nucleophilic attack occurs, forming the quaternary ammonium salt
• Overall scheme of amine reactions with halogenoalkanes


  • Primary amine
  • Secondary amine
  • Tertiary amine
  • Quaternary ammonium salt
• Preparing primary amines from nitriles

  Convert halogenoalkane to nitrile using KCN

  2. Reduce nitrile to amine using LiAlH4 or H2/Ni catalyst
• Reducing nitroaromatics to aromatic amines

  Use Sn and HCl or Fe and HCl, heating

  Mechanism: Reduction
• Aromatic amines


  Amines where the nitrogen is bonded to an aromatic ring
• Reactions of aliphatic and aromatic amines with acyl chlorides and acid anhydrides

  Nucleophilic addition-elimination to form amides