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Amine questions often ask you to tell a primary, secondary and tertiary amine apart, or to separate a mixture of all three. One reagent does both jobs, because it gives three different results. This post explains why, walks through a separation and an identification puzzle, and lists the traps.
Where is this in the syllabus?
The NCERT Class 12 chapter Amines (2026-27 reprint) covers it as "Reaction with arylsulphonyl chloride". The JEE (Advanced) 2026 brochure names the Hinsberg test. The JEE (Main) 2026 syllabus and the NEET (UG) 2026 syllabus list the "identification of primary, secondary and tertiary amines", and the CBSE 2026-27 Chemistry curriculum lists the chemical reactions of amines. Check the current year's documents.
What happens with each class of amine?
NCERT calls benzenesulphonyl chloride, C₆H₅SO₂Cl, Hinsberg's reagent, and adds that these days it is replaced by p-toluenesulphonyl chloride. The reagent is named after Oscar Hinsberg, whose 1890 paper dealt with forming acid esters and acid amides in the presence of water and alkali.
Primary amine: RNH₂ + C₆H₅SO₂Cl → C₆H₅SO₂NHR + HCl. NCERT explains that the hydrogen left on nitrogen is strongly acidic because the sulphonyl group withdraws electrons, so the product dissolves in alkali.
Secondary amine: R₂NH gives C₆H₅SO₂NR₂. There is no hydrogen on nitrogen, so the product is not acidic and does not dissolve in alkali.
Tertiary amine: no reaction, because there is no hydrogen on nitrogen to replace.
Worked example 1: separating a mixture
NCERT's examples use ethyl groups, so take a mixture of ethanamine (1°), N-ethylethanamine (2°) and N,N-diethylethanamine (3°). NCERT says the different behaviour is used to separate such a mixture. The logic runs like this:
Treat the mixture with benzenesulphonyl chloride in aqueous alkali.
Ethanamine forms N-ethylbenzenesulphonamide, which dissolves in the alkali as its salt.
N-ethylethanamine forms N,N-diethylbenzenesulphonamide, which does not dissolve and separates out.
N,N-diethylethanamine does not react and stays as the free amine, to be separated from the rest.
Acidifying the alkaline solution brings the primary amine's sulphonamide back out of solution.
Write each step with a structure, and say in words which property moves each compound into which layer. Examiners give marks for the reason as well as the product.
Worked example 2: an identification puzzle
An amine has the formula C₄H₁₁N. There are eight possible amines:
Primary (4): butan-1-amine, butan-2-amine, 2-methylpropan-1-amine, 2-methylpropan-2-amine.
Secondary (3): N-methylpropan-1-amine, N-methylpropan-2-amine, N-ethylethanamine.
Tertiary (1): N,N-dimethylethanamine.
Now read the test:
No reaction with Hinsberg's reagent: only one structure fits, N,N-dimethylethanamine.
A product insoluble in alkali: one of the three secondary amines. You need more information to choose.
A product soluble in alkali: one of the four primary amines. A question may then add a clue, such as the amine being chiral, which leaves only butan-2-amine.
Quantities follow the same 1:1 ratio. 5.91 g of propan-1-amine (C₃H₉N, M = 59.11 g/mol) is 0.100 mol, reacting with 0.100 mol (about 17.7 g) of C₆H₅SO₂Cl to give at most 0.100 × 199.3 ≈ 19.9 g of N-propylbenzenesulphonamide (C₉H₁₃NO₂S).
Which mistakes cost marks?
Saying tertiary amines give an insoluble product. They do not react.
Swapping the solubilities. The primary amine's product dissolves in alkali; the secondary amine's does not.
Calling the primary sulphonamide basic. It dissolves in alkali because its N–H is acidic.
Forgetting aniline is a primary amine. It also gives an alkali-soluble product.
Mixing up the tests. The carbylamine test works only for primary amines, and nitrous acid gives different results again. Our post on amine basicity and diazonium salts covers both, and our guide to functional group tests puts them side by side.
A practice routine
Write the reaction of benzenesulphonyl chloride with methanamine, N-methylmethanamine and N,N-dimethylmethanamine, with product names.
Draw all eight C₄H₁₁N amines and label each 1°, 2° or 3°.
Design a separation for a mixture of aniline, N-methylaniline and N,N-dimethylaniline, with the reason for each step.
Make one card with three columns (carbylamine, Hinsberg, nitrous acid) and three rows (1°, 2°, 3°).
When a separation scheme keeps going wrong, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
Today, draw the three sulphonamide outcomes from memory and mark the one acidic hydrogen in red.
