Basic Properties of Amines

Basic Properties of Amines

Amines are weak bases. It is possible for the lone pair of electrons located on the nitrogen atom to act like a proton acceptor. Although the strength of amines varies, they all react with strong acids to produce alkylammonium salts.

Reaction of amines with acids

Amines react with acids in the same way as ammonia does.

Amine Reactions
Primary R1-NH2 + HCl ? R1-NH3Cl2R1-NH2 + H2SO4 ? (R1-NH3)2SO4
Secondary R1R2-NH + HCl ? R1R2-NH2Cl2R1R2-NH + H2SO4 ? (R1R2-NH2)2SO4
Tertiary R1R2R3-N + H2SO4 ? R1R2R3-NHCl2R1R2R3-N + H2SO4 ? (R1R2R3-NH)2SO4

Comparison of base strength of different amines

The availability of the lone pair of electrons located on the nitrogen dictates the basicity of ammonia and amines: the more available the lone pair is the stronger the base is.

How available the lone pair is depends on the electron density of the nitrogen atom.

Primary amines have one alkly group attached to the nitrogen atom. This means that the hydrogen atom is a little more electropositive than the carbon atom. Therefore, the nitrogen atom can attract the slight surplus of electrons which the carbon atom possesses. This is called the positive inductive effect and means that the N atom has an electron density excess.

The larger the N atom’s electron density the higher the probability that the lone pair will be released for bonding. Therefore:

  • primary amines are stronger bases than ammonia because they possess one alkyl group
  • secondary amines are stronger bases than primary amines because they possess two alkyl groups
  • tertiary amines are stronger bases than secondary amines because they possess three alkyl groups

Reaction of alkylammonium salts with bases

If alkalis are added to alkylammonium salts they will be converted back into amines because they are slightly acidic.

Ammonium salt Reaction
Primary R1-NH3Cl + NaOH ? R1NH2 + NaCl +H2O
Secondary R1R2-NH2Cl + NaOH ? R1R2-NH + NaCl +H2O
Tertiary R1R2R3-NHCl + NaOH ? R1R2R3-N + NaCl +H2O

As the N atom is not bonded to any hydrogen atoms quaternary ammonium salts are not acidic.


Uses of Quaternary Ammonium Salts

The main use of quaternary salts is as cationic surfactants. The purpose of a surfactant is to reduce the surface tension of a liquid. Ammonium salts with long-chains are used primarily for this.

They can also be added to solids. Solids, like glass and fibres, tend to attract negative charges. A cation helps to cancel this charge out and, therefore, acts like an anti-static agent.