Physical Properties of Amines

1. Physical state and smell: among aliphatic amines, the lower members are gases while higher members are liquids. Among arylamines the lower members are liquids while higher members are solids.

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

Most of the aromatic amines are colourless in pure form. However, they become coloured on keeping, due to oxidation on air. Aromatic amines are generally toxic. They are easily absorbed through the skin.
    
2. Boiling points: primary and secondary amines are capable of forming intermolecular hydrogen bonds between N atom of one molecule and H-atom of the other due to the presence of polar N –H bonds.

The H-bonding causes intermolecular association of amines as a result of this, amines has higher boiling points than the non-polar compounds of comparable molar masses. 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 therefore, intermolecular hydrogen bonds in alcohols and carboxylic acids are relatively stronger than those in amines.



Among isomeric amines, the primary amines have higher boiling points than corresponding secondary amines. This is due to more extensive H-bonding in primary amines because of two N-H bonds in the molecule.

Tertiary amines do not have any N – H bond in their molecules. Therefore, they are unable to form H-bonds and consequently, have lowest boiling points.

For example,


    
3. Solubility: aliphatic amines of lower molar mass are soluble in water. With increase in molar mass the hydrocarbon part of the molecules become larger and consequently, the solubility in water gradually decreases; the borderline being reached at about six carbon atoms. The higher amines are more soluble in less polar solvents such as ethers, alcohols and benzene. The solubility of lower aliphatic amines in water is due to their capability to form hydrogen bonds with water molecules.

Aromatic amines are insoluble in water but soluble in ether, alcohols and benzene. Their inability to dissolve in water is due to more dominant aryl part which is non-polar and decreases tendency of H-bonding with water molecules.

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