Physical & Chemical Properties of Amines
- The lower aliphatic amines are gases with fishy odour.
- Primary amines with three or more carbon atoms are liquid and still higher ones are solid.
- Aniline and other arylamines are usually colourless but get coloured on storage due to atmospheric oxidation.
- 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.
- Amines are soluble in organic solvents like alcohol, ether and benzene.
- Boiling Points:-
- Primary and secondary amines are engaged in intermolecular association due to hydrogen bonding between nitrogen of one and hydrogen of another molecule.
- This intermolecular association is more 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 due to the absence of hydrogen atom available for hydrogen bond formation. Therefore, the order of boiling points of isomeric amines is as follows:
Primary > Secondary > Tertiary
- Intermolecular hydrogen bonding in primary amines is as:-
Chemical Properties of Amines
- Amines behave as nucleophiles due to the presence of unshared electron pair.
- Difference in electronegativity between nitrogen and hydrogen atoms and the presence of unshared pair of electrons over the nitrogen atom makes amines reactive.
- Primary, secondary and tertiary amines differ in reactions because of number of hydrogen attached to nitrogen.
Following are the chemical properties of Amines:-
- Basicity of Amines
- They are basic in nature and form salts when react with acids.
- Amine salts on treatment with a base like NaOH, regenerate the parent amine.
- Amine salts are soluble in water but insoluble in organic solvents like ether.
- They form ammonium salts when react with mineral acids.
- Amines have an unshared pair of electrons on nitrogen atom due to which they behave as Lewis base.
Why Amines are basic in nature in terms of Kb and pKb values?
- Larger the value of Kb or smaller the value of pKb, stronger is the base. The pKb values of few amines are given in.
- pKb value of ammonia is 4.75. Aliphatic amines are stronger bases than ammonia due to +I effect of alkyl groups leading to high electron density on the nitrogen atom.
- Their pKb values lie in the range of 3 to 4.22. On the other hand, aromatic amines are weaker bases than ammonia due to the electron withdrawing nature of the aryl group.