Amines

• naming amines

  carbon chain -yl amine

  e.g. methylamine

  when the amino group (NH2) is not atttached to first carbon we call it aminoalkane

  e.g. 2-aminopropane
• amines are classified as primary, secondary, or tertiary amines depending on the number of hydrogens substituted on the amino group
• preparation of primary amines

  • halogenoalkanes with excess ammonia by nucleophilic substitution
  • reduction of nitriles (RCN) with hydrogen and nickel catalyst
  • reduction of nitro (RNO2) derivatives of benzene with tin or iron catalyst and concentrated HCl
• amines can behave as either bases, nucleophiles or ligands (never at the same time). these properties depend on the ability of the nitrogen atom in the amine to donate its pair of electrons
• amine acting as a base - donates the lone pair to a proton (H+ ion)
• amine acting as a nucleophile - amine donates lone pair to a partially positive carbon
• amine acting as a ligand - amine donates lone pair to a transition metal ion
• amines are Brønsted–Lowry bases and are proton acceptors. Form coordinate bonds with H+ ions
• in water, ammonia solution is alkaline

  RNH2 + H2O --> RNH3+ + OH-
• in dilute acid, amines dissolve due to the formation of a soluble ionic salt.

  RNH2 + HCl --> RNH4+ + Cl-

  the amine can be reformed by adding NaOH (purifying amines)

  RNH4Cl + NaOH --> RNH2 + NaCl + H2O
• the strength of amines as bases depends on the availability of the lone pair of electrons on the nitrogen atom
• primary aliphatic (chain) amines are stronger bases than ammonia due to the alkyl group, which is electron-releasing, causing the positive inductive effect. this makes the lone pair on the nitrogen more available.
• primary aliphatic amines become stonger bases as the chain length increases
• aromatic amines are weak bases as the lone pair on the nitrogen interacts with the delocalised electrons in the ring, making the lone pair less available
• aromatic amines become stronger bases as the amino group gets further away from the ring as the lone pair on the nitrogen is less attracted to the delocalised electrons in the ring and more available
• excess halogenoalkane reacts with primary amines to form secondary amines, with secondary amines to form tertiary amines, and with tertiary amines to form quaternary ammonium salts. ammonium salts can not act as nucleophiles as they do not have lone pairs
• uses of amines

  • quaternary ammonium salts are used as cationic surfactants in fabric conditioners and hair products
  • aromatic amines are used as synthetic dyes