How to Distinguish Primary, Secondary and Tertiary Amines (All Tests)
Chemistry · Amines · NEET
To tell apart 1°, 2° and 3° amines, use two main tests. The Hinsberg test (benzenesulphonyl chloride) sorts all three: 1° gives a solid that dissolves in alkali, 2° gives a solid that stays undissolved, and 3° does not react. The carbylamine test is a special check for 1° amines only: they make a very bad-smelling isocyanide. Memory hook: "1° dissolves, 2° stays solid, 3° does nothing."
Hinsberg test flow: 1° amine keeps an N-H so its solid dissolves in alkali; 2° amine has no N-H so its solid stays undissolved; 3° amine gives no reaction. The carbylamine test (foul smell) is a separate confirmatory test for primary amines only.
Your doubts, answered
What is the easiest single test that separates all three amines at once?
The Hinsberg test does all three in one go. You add benzenesulphonyl chloride (C6H5SO2Cl), then add alkali (NaOH). Primary amine → makes a solid that DISSOLVES in alkali. Secondary amine → makes a solid that stays UNDISSOLVED. Tertiary amine → NO reaction at all (nothing happens). Because the three results are all different, this one test tells you which amine you have. NEET loves this test.
Why does the primary amine product dissolve in alkali but the secondary one does not?
With a primary amine, the product is C6H5SO2-NH-R. It still has one N-H bond. The strong -SO2- group pulls electrons and makes this N-H acidic, so alkali (NaOH) removes it and the solid dissolves as a salt. With a secondary amine the product is C6H5SO2-NR2. Here nitrogen has NO H left, so there is nothing acidic for alkali to remove, and the solid stays undissolved. This N-H difference is the whole idea.
Why does the tertiary amine not react with Hinsberg reagent at all?
A tertiary amine (R3N) has NO N-H bond. Hinsberg reagent needs an N-H to form a bond (a sulphonamide). With no N-H, the tertiary amine cannot form the product, so nothing happens. In the old lab version it stays as an insoluble oily liquid that reacts only after acid is added. For NEET, remember: tertiary amine = no reaction with Hinsberg reagent.
Which amines give the carbylamine test and which do not?
ONLY primary amines (both aliphatic and aromatic) give the carbylamine test. You heat the amine with chloroform (CHCl3) and alcoholic KOH. A primary amine forms an isocyanide (R-NC), which has a very foul, unpleasant smell. Secondary and tertiary amines do NOT give this test. So a bad smell = a primary amine is present. This is a quick confirmatory test for 1° amines.
How does the nitrous acid (HNO2) test tell the amines apart?
With cold HNO2 (NaNO2 + HCl): a primary aliphatic amine gives nitrogen gas (bubbles) because it forms an unstable diazonium salt that breaks down at once. A primary aromatic amine (like aniline) forms a stable diazonium salt only at 0-5°C. A secondary amine gives a yellow oily nitrosamine. A tertiary amine either does not react clearly or just forms a salt. So bubbles of N2 = primary aliphatic amine.
Do I have to memorise all three tests for NEET?
Focus most on the Hinsberg test and carbylamine test — these two appear again and again in NEET (2019, 2020, 2021). Know the exact Hinsberg result for each amine (dissolves / stays solid / no reaction) and that carbylamine is for primary amines only. The nitrous acid test is good extra support but is asked less often. Learn the small results table below by heart.
⚠️ The NEET trap ✗ Thinking the Hinsberg product from a primary amine is the one that stays UNDISSOLVED (insoluble) in alkali. ✓ The PRIMARY amine product dissolves in alkali (it still has an acidic N-H). It is the SECONDARY amine product that stays undissolved (no N-H left). 🧠 1° still has N-H → acidic → dissolves. 2° has no N-H → stays solid. Match the H count, not your gut.
Real NEET questions
NEET 2021
Identify the compound that will react with Hinsberg's reagent to give a solid which dissolves in alkali.
A · (CH3)2CH-NH2 (isopropylamine, a primary amine) ✓
B · (CH3)2N-CH3 (an N-methyl tertiary amine)
C · (CH3)2CH-NO2 (2-nitropropane)
D · (CH3)3N (trimethylamine, a tertiary amine)
Solution: Hinsberg's reagent is benzenesulphonyl chloride, C6H5SO2Cl. A PRIMARY amine forms C6H5SO2-NH-CH(CH3)2, which still has one N-H. The -SO2- group makes this N-H acidic, so the solid dissolves in alkali (NaOH) as a salt. A secondary amine would give C6H5SO2-NR2 (no N-H) = a solid that stays undissolved. Tertiary amines (B, D) have no N-H, so they give no sulphonamide. The nitro compound (C) is not an amine. So only the primary amine (A) gives a solid that dissolves in alkali.
NEET 2020
Which of the following amines will give the carbylamine test?
A · C6H5N(CH3)2 (N,N-dimethylaniline)
B · C6H5NHC2H5 (N-ethylaniline)
C · C6H5NH2 (aniline) ✓
D · C6H5NHCH3 (N-methylaniline)
Solution: The carbylamine (isocyanide) test is given ONLY by primary amines. On heating with chloroform and alcoholic KOH, a primary amine gives a foul-smelling isocyanide: C6H5NH2 + CHCl3 + 3KOH → C6H5NC + 3KCl + 3H2O. Aniline (C) is a primary amine (-NH2) and gives the test. N-methylaniline (D) and N-ethylaniline (B) are secondary amines; N,N-dimethylaniline (A) is tertiary. None of these has an -NH2 group, so they fail. Answer: (C).
NEET 2019 Odisha
The amine that reacts with Hinsberg's reagent to give an alkali-insoluble product is:
A · (CH3)2CH-NH-CH(CH3)2 (diisopropylamine, a secondary amine) ✓
B · (C2H5)3N (triethylamine, a tertiary amine)
C · CH3-C(NH2)(CH3)-CH2CH2CH3 (a primary amine)
D · (CH3)3C-CH(NH2)CH3 (a primary amine)
Solution: With Hinsberg reagent, a SECONDARY amine forms an N,N-disubstituted sulphonamide C6H5SO2-NR2. This product has NO N-H bond, so alkali cannot remove any proton and the solid stays undissolved (alkali-insoluble). A primary amine would give a product with an acidic N-H that dissolves in alkali, and a tertiary amine gives no sulphonamide. So the alkali-insoluble solid comes from the secondary amine, diisopropylamine (A).
Solved Amines NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the fastest way to identify a primary amine?
Use the carbylamine test. Heat the amine with chloroform (CHCl3) and alcoholic KOH. If you get a very bad-smelling gas (an isocyanide), a primary amine is present. Only primary amines give this test, so a foul smell confirms a 1° amine quickly.
What are the Hinsberg test results for 1°, 2° and 3° amines?
Primary amine → solid that dissolves in alkali. Secondary amine → solid that stays undissolved in alkali. Tertiary amine → no reaction (no product forms). These three different results let one test separate all three amines.
What is Hinsberg's reagent?
Hinsberg's reagent is benzenesulphonyl chloride, C6H5SO2Cl. It reacts with primary and secondary amines to form sulphonamides but does not react with tertiary amines because they have no N-H bond.
Can the nitrous acid test tell apart aliphatic and aromatic primary amines?
Yes. A primary aliphatic amine with HNO2 gives nitrogen gas (bubbles) because its diazonium salt is unstable. A primary aromatic amine (like aniline) forms a stable diazonium salt at 0-5°C, which does not release N2 unless warmed. So bubbles at once = aliphatic 1° amine.
Why does the carbylamine test fail for secondary and tertiary amines?
The carbylamine reaction needs a free -NH2 group (two N-H bonds) to form the isocyanide. Secondary amines have only one N-H and tertiary amines have none, so neither can make the isocyanide. That is why only primary amines give the foul smell.
