Physical Properties of Amines Preparation and Properties
→ PhysIcal State and Odour : Aliphatic amines are gases which smell like fish. Primary amines with three or more carbon atoms are liquid amines and all higher members are solids. Aniline and other arylamines are usually colourless but get coloured on storage due to atmospheric oxidation. These are easily absorbed by skin and aryl amine are toxic.
→ Solubility : Lower aliphatic amines are soluble in water because they can form hydrogen bonds with water molecules. However, solubility decreases with increase in molar mass of amines due to increase in size of the hydrophobic alkyl part.
→ Higher amines are essentially insoluble in water. The solubility of alcohols in water is more than that of amine because oxygen atom of alcohol is more electronegative than nitrogen atom of amine and form more stronger hydrogen bond with water molecule. Aromatic amines are insoluble in water but are soluble in organic solvent.
→ Boiling Points : Besides of tertiary amines, primary and secondary amines are involved in intermolecular association due to hýdrogen bonding between N—H bond. This intermolecular association is maximum in primary amines than in secondary amines as there are two hydrogen atoms available for hydrogen bond formation in it.
→ Tertiary amines do not have intermolecular association because of the absence of hydrogen atom for hydrogen bond formation. Therefore, the order of boiling points of isomeric amine is as follows:
Primary > Secondary > Tertiary
The intermolecular hydrogen bonding in primary amine is shown as follows: