Question

Describe the method of identification of primary, secondary and tertiary amines. Classify the following compounds into primary, secondary and tertiary amines: (i) C₆H₅NHCH₃ (ii) CH₃(CH₂)₂NH₂ (iii) (CH₃CH₂)₂NCH₃.

Hint:

Use the Hinsberg benzenesulphonyl chloride test (solubility in KOH) plus the carbylamine test for primary amines, then count the carbon groups on nitrogen to label each structure.

Answer 1

Step 1: Basis of identification.
Amines are classed by how many carbon groups are joined to nitrogen: one in a primary amine, two in a secondary amine, three in a tertiary amine. The number of N-H bonds falls from two to one to zero, and the tests below use exactly this difference.
Step 2: Hinsberg test.
Each amine is shaken with benzenesulphonyl chloride (Hinsberg reagent, C₆H₅SO₂Cl) and then treated with aqueous KOH.
Primary amine: forms an N-substituted sulphonamide that still has one acidic N-H, so it dissolves in KOH to give a clear solution.
Secondary amine: forms a sulphonamide with no N-H, so it stays as an insoluble solid in KOH.
Tertiary amine: has no N-H and does not react.
Step 3: Carbylamine test.
On warming with chloroform and alcoholic KOH, only primary amines (aliphatic or aromatic) give an isocyanide (carbylamine) with a very unpleasant smell. Secondary and tertiary amines give no such smell, so the test is specific for primary amines.
R-NH₂ + CHCl₃ + 3KOH → R-NC + 3KCl + 3H₂O
Step 4: Classify the given compounds.
(i) C₆H₅NHCH₃ (N-methylaniline): nitrogen holds a phenyl group and a methyl group, that is two carbon groups, so it is a secondary amine.
(ii) CH₃(CH₂)₂NH₂ (propan-1-amine): nitrogen holds only one carbon chain, so it is a primary amine.
(iii) (CH₃CH₂)₂NCH₃ (N-ethyl-N-methylethanamine): nitrogen holds three carbon groups (two ethyl and one methyl), so it is a tertiary amine.