Physical properties of aliphatic amines

i) Physical state and smell

Lower members of aliphatic amines are gases while higher members are liquids. Among, aryl amines, lower members are liquids but higher members are solids.

Methyl amine and ethyl amine have ammoniacal smell but higher amines have fishy smell.

Most of the aromatic amines are colorless in pure form. However, they become colored on keeping due to oxidation in air. Aromatic amines are generally toxic. They are easily absorbed through the skin.

ii) Boiling points

Amines are polar compounds and have higher boiling points than non-polar compounds such as hydrocarbons of some molecular mass.

All amines can form hydrogen bonds as protons acceptors by coordination with the lone pair of electrons on the -N atom. But only primary and secondary amines can donate proton in hydrogen bonding. Thus 1^o and 2^o amines because of intermolecular hydrogen bonding have higher boiling points than tertiary amines.

However amines have lower boiling points than those of corresponding alcohols or carboxylic acids. This is due to the reason that O - H bond is more polar than N-H bond and hence hydrogen bonds in alcohols and carboxylic acids are stronger than hydrogen bonds in amines.

iii) Solubility

Smaller amines are soluble in water but as the hydrophobic part of the amine becomes larger than six carbons, their solubility in water decreases and still larger amines are essentially insoluble in water.

As the hydrogen bonds in amines is weaker than in alcohols, the solubility of amines in water is less than that of alcohols.

The solubility of lower aliphatic amines in water is due to their capability to form hydrogen bonds with water molecules

Amines are soluble in organic solvents like ether, benzene, alcohol etc.