Identification of Primary, Secondary and Tertiary Amines
Amines are organic compounds derived from ammonia (NH3) in which one or more hydrogen atoms are replaced by alkyl or aryl groups. Based on the number of carbon groups attached to nitrogen, amines are classified as primary, secondary, and tertiary amines. This topic is very important for CBSE Class 12 Chemistry as well as competitive examinations.
1. Classification of Amines
Primary Amine (1°)
In primary amines, nitrogen is attached to one alkyl or aryl group and contains two hydrogen atoms.
General Formula: R–NH2
Example: CH3NH2 (Methyl amine)
Secondary Amine (2°)
In secondary amines, nitrogen is attached to two carbon groups and contains one hydrogen atom.
General Formula: R–NH–R'
Example: (CH3)2NH (Dimethyl amine)
Tertiary Amine (3°)
In tertiary amines, nitrogen is attached to three carbon groups and has no hydrogen atom.
General Formula: R–N(R')–R''
Example: (CH3)3N (Trimethyl amine)
2. Laboratory Methods to Distinguish Amines
A. Hinsberg Test
This is the most important test for distinguishing primary, secondary, and tertiary amines.
Reagent Used: Benzenesulphonyl chloride (C6H5SO2Cl) and NaOH
Reaction Diagram:
Primary Amine:
R–NH2 + C6H5SO2Cl → R–NH–SO2–C6H5 (Soluble in alkali)
Secondary Amine:
R2NH + C6H5SO2Cl → R2N–SO2–C6H5 (Insoluble)
Tertiary Amine:
No reaction with benzenesulphonyl chloride
Observation Table:
| Amine Type | Observation |
|---|---|
| Primary | Forms soluble sulphonamide |
| Secondary | Forms insoluble sulphonamide |
| Tertiary | No reaction |
B. Carbylamine Test (Isocyanide Test)
This test is specific only for primary amines.
Reagents: Chloroform (CHCl3) and alcoholic KOH
Reaction Diagram:
R–NH2 + CHCl3 + 3KOH → R–NC + 3KCl + 3H2O
The formation of a foul-smelling isocyanide confirms the presence of a primary amine.
C. Nitrous Acid Test
Nitrous acid is prepared in situ by reacting sodium nitrite with dilute hydrochloric acid.
Primary Aliphatic Amine:
R–NH2 + HNO2 → R–OH + N2↑ + H2O
Secondary Amine:
Forms yellow oily nitrosoamine
Tertiary Amine:
No gas evolved, forms salt
3. CBSE Practical Style Experiment
Aim:
To distinguish between primary, secondary, and tertiary amines using Hinsberg test.
Apparatus and Chemicals:
Test tubes, dropper, NaOH solution, benzenesulphonyl chloride, given amine sample
Procedure:
- Take a small amount of given amine in a test tube.
- Add aqueous NaOH solution.
- Add few drops of benzenesulphonyl chloride.
- Shake the mixture well.
- Observe solubility or formation of precipitate.
Observation:
If the product dissolves in alkali, the amine is primary. If it is insoluble, the amine is secondary. If no reaction occurs, the amine is tertiary.
Result:
The given organic compound is identified based on the observation.
Precautions:
- Use freshly prepared reagents.
- Avoid inhaling fumes.
- Handle chemicals carefully.
4. Multiple Choice Questions (MCQs)
Q1. Which test is used to distinguish all three types of amines?
A. Carbylamine test
B. Nitrous acid test
C. Hinsberg test
D. Lassaigne test
Q2. Carbylamine test is given by:
A. Primary amine only
B. Secondary amine only
C. Tertiary amine only
D. All amines
Q3. Which amine does not react with benzenesulphonyl chloride?
A. Methyl amine
B. Dimethyl amine
C. Trimethyl amine
D. Ethyl amine
Q4. Yellow oily compound is formed when secondary amine reacts with:
A. NaOH
B. Nitrous acid
C. Chloroform
D. HCl
Answers:
Q1. C
Q2. A
Q3. C
Q4. B
Conclusion
Primary, secondary, and tertiary amines can be easily distinguished using chemical tests such as Hinsberg test, Carbylamine test, and Nitrous acid test. These tests form an essential part of CBSE chemistry practicals and conceptual understanding of organic chemistry.
