This is part of the HSC Chemistry course under the topic Reactions of Organic Acids and Bases.

HSC Chemistry Syllabus

  • investigate the structural formulae, properties and functional group including: amines

  • explain the properties within and between the homologous series of amines with reference to the intermolecular and intramolecular bonding present

  • investigate the differences between an organic acid and organic base

Organic Bases: Structure & Properties of Amines

This video discusses the structure and properties of amines.


Structure of Amines 

An amine is a functional group where a nitrogen atom is connected to carbon and/or hydrogen atoms. Depending on the number of carbon atom(s) the nitrogen is connected to, an amine is further classified as primary (one carbon atom), secondary (two carbon atoms) and tertiary (three carbon atoms).


Nitrogen atom in an amine occupies a trigonal pyramidal shape as the negative electron lone pair repels the three adjacent covalent bonds, preventing the nitrogen from adopting a trigonal planar shape.



Nomenclature of Amines

  • Identify the longest carbon chain that contains the amine or amide functional group. This is used in conjunction with the functional group’s suffix to name the main part of the molecule.
  • Remaining alkyl groups attached to the amine or amide functional group are considered as substituents. Their positions are denoted by ‘N’.
  • Suffix -‘amine’
  • Prefix -‘amino’

Nomenclature Priority

  • Amines have lower priority than most functional groups but hydrocarbons.


Properties of Amines

Amines are organic bases

Amines are organic Brønsted-Lowry bases. The nitrogen atom’s electron lone pair allows amines to accept protons (H+) from water or an acid to form its positively charged conjugate acid.

All organic bases are weak bases that partially ionise with water to form an equilibrium system (example shown below).


    Boiling and Melting Points

    Primary amines (1º) have higher boiling and melting points than secondary amines (2º) of the same molecular mass. This is because:
    • primary amines can form more hydrogen bonds between molecules than secondary amines
    • secondary amines have a smaller (weaker) molecular dipole due to the presence of nitrogen within the carbon chain as opposed to at the end in primary amine.


      Hydrogen bonding between primary amine molecules

      Hydrogen bonding between secondary amine molecules

      Table: tertiary amines have much lower boiling (and melting) points than primary and secondary amines of the same molar mass. Secondary amines have lower boiling points than primary amines.



      Molar mass 

      (g mol–1)

      Boiling point (ºC)






















      • BP & MP of amine and amide compounds increase with the number of carbon atom increases because the strength of dispersion forces increases with molecular mass.
      • Amines and amides have permanent dipoles due to the presence of nitrogen in their functional groups.
      • Amines and amides have higher boiling and melting points than hydrocarbons of similar molecular weight.
      • Tertiary amines and amides cannot form hydrogen bonds. Thus, they have much lower boiling and melting points than primary and secondary counterparts.


      Tertiary amines do not contain hydrogens that can partake in hydrogen bonding

      • Amines are usually compared with alcohols while amides are compared with carboxylic acids. This is because each pair of functional groups shares structural similarities.
      • Boiling point of amines vs alcohol: amines generally have lower BP & MP than alcohols of similar molecular mass.
        • Nitrogen is less electronegative than oxygen. As a result, hydrogen bonds formed between amine molecules are weaker than those between alcohol molecules.
        • Tertiary amines have much lower boiling and melting points than alcohols of similar molecular mass because tertiary amines do not have a hydrogen atom bound to a nitrogen, oxygen or fluorine atom. Thus, they cannot form hydrogen bonds.

      Solubility in Water

      • Small amines are polar molecules and therefore they are soluble in water.
      • All types of amines  can form hydrogen bonds with water.
        • Primary and secondary amines  are more soluble due to their ability to donate and accept hydrogen bonds.
        • Tertiary amines  are less soluble as they can only accept hydrogen bonds from water molecules.


      Methanamine forms hydrogen bonds with H2O

      Generic representation of hydrogen bonds between amide molecules and H2O.


      • Like other functional groups, solubility of amines  decreases as the number of carbon atoms increases. This is because molecules become less polar with the addition of alkyl groups.
      • Amines are more soluble than hydrocarbons as they are non-polar.
      • Solubility of amines vs alcohols
        • Primary amines are similar in solubility compared to corresponding alcohols. While primary amines are able to donate more hydrogen bonds than alcohols, alcohols can form stronger dipole-dipole forces and hydrogen bonds with water. Comparison of solubility between these two functional groups is therefore quite difficult.
        • Secondary and tertiary amines are less soluble than corresponding alcohols. Tertiary amines can only accept hydrogen bonds from water, so they much lower solubility than alcohols.