Experiment: tests for the amino group

Written by TopicLamp Editorial Team Checked by TopicLamp Review Team Updated 8 Nov 2026 11 min read

Beyond NCERT In the JEE Main and NEET syllabus; the Class 12 book has no section for it.

Quick answer

An amine dissolves in dilute HCl as a water-soluble salt, which the Lab Manual reads as the presence of an amine; strictly, it shows a basic compound, not uniquely an amine. A primary amine heated with chloroform and alcoholic KOH gives an isocyanide with an obnoxious smell (the carbylamine test); it is toxic, so the vapour must not be deliberately smelled. An aromatic primary amine, diazotised at 0–5 °C and added to alkaline β-naphthol, gives a scarlet red azo dye.

How do you test for the amino group?

Tests for the amino group

An amine dissolves in dilute hydrochloric acid, because it forms a water-soluble salt; strictly, this shows a base, not uniquely an amine (reasoned). A primary amine, aliphatic or aromatic, heated with chloroform and alcoholic potassium hydroxide gives an isocyanide with an obnoxious smell (the carbylamine test); the isocyanide is highly toxic, so it is never sniffed deliberately. An aromatic primary amine, diazotised in ice and added to alkaline β-naphthol, gives a scarlet red dye (the azo dye test) (NCERT Laboratory Manual, Unit 8, pages 97–98). The carbylamine test rests on these NCERT lines:

Aliphatic and aromatic primary amines on heating with chloroform and ethanolic potassium hydroxide form isocyanides or carbylamines which are foul smelling substances. Secondary and tertiary amines do not show this reaction.

NCERT Chemistry Part II (Class 12), Section 9.6, page 271

The NCERT Class 12 textbook explains the chemistry behind all three tests: salt formation (NCERT Section 9.6, pages 266–267), the carbylamine reaction (page 271), diazotisation (Section 9.7, page 274) and coupling (Section 9.9, page 276, with phenol and aniline). It does not describe the method, and it does not mention β-naphthol, which the Lab Manual uses. The method below follows the NCERT Laboratory Manual (Chemistry, Class 12), Unit 8, Experiment 8.1, part VI (pages 97–99), the same experiment as the tests for hydroxyl groups.

In plain words: the Lab Manual reads dissolving in acid as an amine, the carbylamine smell as a primary amine, and the scarlet dye as an aromatic primary amine. Interpret the tests together: acid solubility strictly shows a base, and the azo test needs working diazotisation and coupling. Use known positive controls and a reagent blank; no single colour or solubility result identifies a unique compound (reasoned).

Theory

Compounds with an amino group are basic, so they react easily with acids to form salts, which are soluble in water (Lab Manual, page 97; NCERT pages 266–267).
Primary amines have two hydrogen atoms on nitrogen (–NH₂), secondary amines one (–NH–) and tertiary amines none (Lab Manual, page 97).
Only primary amines, aliphatic or aromatic, give the carbylamine test (Lab Manual, page 97; NCERT page 271).
An aromatic primary amine such as aniline reacts with nitrous acid, made in the tube from sodium nitrite and HCl at 0–5 °C, to give a diazonium salt, which couples with β-naphthol to give a scarlet red dye, sparingly soluble in water (Lab Manual, page 97). 0–5 °C is 273–278 K, the range in NCERT (page 274).

Amino group tests

C₆H₅NH₂ + HCl → C₆H₅NH₃⁺Cl⁻

R–NH₂ + CHCl₃ + 3KOH → R–NC + 3KCl + 3H₂O (heat)

C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O (0–5 °C)

C₆H₅N₂⁺Cl⁻ + C₁₀H₇OH + NaOH → C₆H₅N=NC₁₀H₆OH + NaCl + H₂O

The first two equations are the Lab Manual’s (pages 97–98). The third is NCERT’s diazotisation equation (page 274), which the Lab Manual draws as a scheme with NaNO₂ + HCl over the arrow (page 97). The fourth is ours, balanced (reasoned): the Lab Manual’s scheme on page 97 draws the coupling at the carbon next to the –OH of β-naphthol and leaves out the NaOH, NaCl and water.

Solubility in dilute hydrochloric acid

Procedure. Take 1 mL of the compound in a test tube, add a few drops of dilute HCl and shake well. If the compound dissolves, the Lab Manual takes it to show “the presence of an amine” (page 98); strictly, it shows a base, so compare with a water control (reasoned).
Acid amount matters: beyond the NCERT text, 1 mL of aniline is about 1.02 g (NIOSH, via PubChem), or 0.011 mol, requiring about 0.011 mol HCl for full salt formation. “A few drops” without an acid concentration does not specify enough acid. In a supervised test, use the approved sample size and acid concentration; incomplete dissolution with insufficient acid is not a reliable negative result.
Aniline, for example, gives anilinium chloride, which is soluble in water. Aniline itself is not soluble in water, as NCERT puts it (Exercise 9.3 (ii), page 278); beyond the NCERT text, a little dissolves, about 36 g per litre at 25 °C (HSDB via PubChem).
The test points to an amine only for a compound that does not dissolve in water by itself, such as aniline at concentrations above its solubility. Lower aliphatic amines dissolve in water anyway (NCERT page 265), and so do non-basic compounds such as the lower alcohols (NCERT Unit 7, page 204) and propanone (NCERT Unit 8, page 235), so shake a little of the compound with water first (reasoned).
Amine salts give back the parent amine with a base such as NaOH (NCERT page 267), so NaOH regenerates the free base; a separate layer or precipitate appears only if its solubility is exceeded (reasoned).

Carbylamine test

Procedure. Take 2–3 drops of the compound, add 2–3 drops of chloroform, then an equal volume of 0.5 M alcoholic potassium hydroxide, and heat gently. An obnoxious smell of carbylamine (isocyanide) “confirms the presence of primary amino group” (Lab Manual, page 98); the Lab Manual also says “Do not inhale the vapours”, so the smell is noticed, never sniffed deliberately.
Caution. The carbylamine is highly toxic. The Lab Manual destroys it at once: cool the tube, carefully add an excess of concentrated HCl, and flush it into the sink (pages 97–98). Our safety advice: let the instructor do this in a fume hood and collect the treated mixture, which still contains chloroform, as the laboratory’s waste procedure directs.
Secondary and tertiary amines do not give this test (NCERT page 271).

Azo dye test

Procedure.
1. Dissolve about 0.2 g of the compound in 2 mL of dilute HCl and cool the tube in ice.
2. Add 2 mL of cold 2.5% aqueous sodium nitrite.
3. In another tube, dissolve 0.2 g of β-naphthol in dilute sodium hydroxide.
4. Add the diazonium chloride solution slowly, with shaking, to the cold β-naphthol solution.
The formation of a scarlet red dye “confirms the presence of aromatic primary amine” (Lab Manual, page 98). Strictly, scarlet red is the colour the Lab Manual reports for aniline here; other arylamines can give other azo colours, so use a known control (reasoned).
Precautions. Keep the mixture below 5 °C during diazotisation, because diazonium chloride is unstable at higher temperatures, and always add the diazonium solution to the alkaline β-naphthol, not the other way round (Lab Manual, page 99). Our safety advice: use the cold solution promptly, and never isolate or dry the diazonium chloride, which “decomposes easily in the dry state” (NCERT page 275).
Aliphatic primary amines give no dye: their diazonium salts are highly unstable (NCERT page 274), so there is nothing to couple (reasoned).

Results at a glance

Test	Positive result	Shows
dilute HCl	dissolves (not in water alone)	an amine (Lab Manual); strictly, a base
carbylamine	obnoxious smell of isocyanide (do not inhale)	primary amine
azo dye	scarlet red dye (for aniline)	aromatic primary amine

Safety

The Lab Manual’s margin symbols (page 98) mark aniline as toxic, potassium hydroxide as corrosive, and sodium nitrite as an oxidiser and toxic, and chloroform and β-naphthol carry the St Andrew’s cross. Beyond the NCERT text: aniline is also toxic in contact with skin (GHS hazard statement H311, ECHA notifications via PubChem). Our safety advice: work under a supervised protocol with a fume hood, eye protection and suitable gloves, and check the current safety data sheet for each reagent.
Do not inhale the vapours, and destroy the isocyanide as soon as the test is over (Lab Manual, pages 98–99). Chloroform vapour itself depresses the central nervous system when breathed in (NCERT Unit 6, page 187).

Solved examples

Example 1: Butan-1-amine or aniline?

Two liquids both dissolve in dilute HCl and both give the carbylamine test. One is butan-1-amine, the other aniline. Which test tells them apart?

Answer: The azo dye test. Aniline, an aromatic primary amine, gives a scarlet red dye with nitrous acid and then alkaline β-naphthol; butan-1-amine gives none, because its diazonium salt is highly unstable (Lab Manual, page 98; NCERT page 274; compare Lab Manual Discussion Question (xiii), page 99).

Example 2: Mass of anilinium chloride

What mass of anilinium chloride forms when 1.86 g of aniline dissolves in excess dilute HCl? (C = 12, H = 1, N = 14, Cl = 35.5.)

Answer: M(C₆H₅NH₂) = 72 + 7 + 14 = 93 g mol⁻¹, so 1.86/93 = 0.0200 mol. M(C₆H₅NH₃Cl) = 72 + 8 + 14 + 35.5 = 129.5 g mol⁻¹: 0.0200 × 129.5 = 2.59 g (reasoned from the Lab Manual’s equation, page 98).

Common mistakes to avoid

Common mistake

Reading the carbylamine test as a test for any amine

Only primary amines give it. Secondary and tertiary amines dissolve in acid but give no isocyanide.

Common mistake

Pouring β-naphthol into the diazonium solution

The diazonium chloride solution goes into the alkaline β-naphthol, not the reverse, and everything stays below 5 °C (Lab Manual, page 99).

Quick revision summary

Dilute HCl: an amine dissolves as its salt; C₆H₅NH₂ + HCl → C₆H₅NH₃⁺Cl⁻.
Carbylamine: 2–3 drops of compound + 2–3 drops of CHCl₃ + alcoholic KOH, heat gently; an obnoxious smell (noticed, never sniffed) means a primary amine (Lab Manual). Destroy the product at once with excess conc. HCl (Lab Manual), under the instructor’s procedure.
Azo dye: 0.2 g in 2 mL dilute HCl, iced; 2 mL of 2.5% NaNO₂; into 0.2 g of β-naphthol in NaOH; a scarlet red dye means an aromatic primary amine.

Formula sheet

C_₆H_₅NH_₂ + H^⁺ → C_₆H_₅NH_₃^⁺ (in dilute HCl)
C_₆H_₅NH_₂ + CHCl_₃ + 3KOH → C_₆H_₅NC + 3KCl + 3H_₂O
ArNH_₂ + NaNO_₂ + 2HCl → ArN_₂^⁺Cl^⁻ + NaCl + 2H_₂O (0–5 °C)

Questions students ask

Which compounds give the azo dye test?

The azo dye test is given by aromatic primary amines, those with an –NH₂ group directly on an aryl carbon, such as aniline, o-toluidine and p-bromoaniline (NCERT Table 9.1, page 261), because only their diazonium salts last long enough in ice-cold solution to couple with β-naphthol (NCERT page 274; Lab Manual, page 97). Secondary and tertiary amines give no scarlet dye, since nitrous acid does not turn them into diazonium salts (reasoned from NCERT pages 271 and 274).

Does phenol give the azo dye test?

Phenol does not give the azo dye test for amines, because it has no –NH₂ group to diazotise; in the test it could only stand in for β-naphthol, the compound the diazonium ion couples with (reasoned from Lab Manual, page 97). A diazonium salt made from aniline does couple with phenol to give an azo dye (NCERT page 276; coupling reactions), so a phenol can be detected the other way round (reasoned). β-Naphthol suits the amine test because the scarlet red dye it gives is sparingly soluble in water (Lab Manual, page 97), so a positive result shows plainly (reasoned).

What is the scarlet red dye formed in the azo dye test?

With aniline, the scarlet red dye of the azo dye test is 1-phenylazo-2-naphthol, C₆H₅N=NC₁₀H₆OH, formed when the diazonium ion couples at the carbon next to the –OH of β-naphthol, as the NCERT Laboratory Manual draws it (page 97). Beyond the NCERT text, this compound is the dye Sudan I, which forms brick-red crystals, is practically insoluble in water but dissolves in ethanol and benzene, and is used to colour waxes, solvents and polishes (PubChem; Wikipedia, “Sudan I”). Like other azo dyes it is coloured because its –N=N– bond joins two aromatic ring systems into one extended conjugated system (NCERT page 276).

Practice questions

Try each question first, then open the answer to check your working.

Q1. Which of these give the carbylamine test: ethanamine, N-methylethanamine, aniline, N,N-dimethylaniline?

Show answerHide answer

Ethanamine and aniline, the two primary amines. N-Methylethanamine (secondary) and N,N-dimethylaniline (tertiary) do not (NCERT page 271).
Q2. A compound dissolves in dilute HCl and gives the carbylamine test, but no dye in the azo dye test. What kind of compound is it?

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An aliphatic primary amine: it is basic, it is primary, and it is not an aromatic primary amine (reasoned from Lab Manual, pages 97–98), provided the azo test’s own controls worked.