Carbylamine and Hinsberg Tests for Amines Explained

The carbylamine test and the Hinsberg test are the two classic qualitative tests that identify amines and classify them as primary (1°), secondary (2°) or tertiary (3°). Together they answer "is it an amine, and which kind?" in minutes, with no instruments.

They matter because amines are everywhere — drugs, dyes, proteins — and the three classes behave very differently: primary amines form diazonium salts, secondary amines form oily nitrosamines, tertiary amines do neither. Knowing the class predicts the chemistry.

Key Concepts

• Carbylamine (isocyanide) test: a primary amine heated with chloroform and alcoholic KOH gives an isocyanide (R–N≡C) with an unbearably foul smell — positive only for 1° amines (aliphatic and aromatic).
• Hinsberg reagent: benzenesulphonyl chloride (C6H5SO2Cl), which reacts differently with each amine class.
• Sulphonamide: the product R–NH–SO2C6H5 from a primary amine; it still has an acidic N–H, so it dissolves in NaOH.
• Solubility logic: 1° amine → sulphonamide soluble in alkali; 2° amine → sulphonamide with no N–H, insoluble in alkali; 3° amine → no reaction at all (forms a salt soluble in acid instead).

How It Works

Carbylamine test. Warm the amine with CHCl3 and alcoholic KOH: R–NH2 + CHCl3 + 3 KOH → R–NC + 3 KCl + 3 H2O. Dichlorocarbene (:CCl2) generated from chloroform is attacked by the primary amine, and successive dehydrohalogenations give the isocyanide. The offensive odour is unmistakable — secondary and tertiary amines give no reaction. (Work in a fume hood: isocyanides are toxic.)

Hinsberg test. Shake the amine with benzenesulphonyl chloride in NaOH, then acidify:

1. Primary amine: forms N-alkylbenzenesulphonamide, whose remaining N–H is acidic enough to dissolve in NaOH; adding acid reprecipitates it. Observation: dissolves in base, precipitate returns on acidification.
2. Secondary amine: forms N,N-dialkylbenzenesulphonamide with no N–H — insoluble in NaOH throughout. Observation: solid stays undissolved.
3. Tertiary amine: does not react (no N–H to lose); it simply dissolves in the acidic work-up as its salt. Observation: no sulphonamide forms.

Worked Example

Methylamine (CH3NH2, 31 g/mol) reacts with benzenesulphonyl chloride (176.5 g/mol) in the Hinsberg test. What mass of sulphonamide forms from 62 g of methylamine (complete reaction)?

1. 62 g of methylamine = 62 ÷ 31 = 2.00 mol.
2. Equation: CH3NH2 + C6H5SO2Cl → C6H5SO2NHCH3 + HCl (1:1).
3. Molar mass of the sulphonamide (C7H9NO2S) = 84 + 9 + 14 + 32 + 32 = 171 g/mol.
4. 2.00 mol × 171 g/mol = 342 g of sulphonamide.
5. This product dissolves in NaOH (it retains one acidic N–H) — confirming the amine was primary.

Common Misconceptions

• "Secondary amines give the carbylamine test." Wrong. Only primary amines form isocyanides; 2° and 3° amines give no foul smell at all.
• "Hinsberg distinguishes amines by colour." Wrong. It distinguishes by solubility — whether the sulphonamide dissolves in alkali and reprecipitates on acidification.
• "Aromatic primary amines fail the carbylamine test." Wrong. Aniline and other aromatic 1° amines give the isocyanide smell too; the test works for both aliphatic and aromatic primary amines.

Real-World Applications

• Qualitative analysis: the standard textbook sequence to prove an unknown is a 1°, 2° or 3° amine.
• Drug quality control: confirming the amine class of pharmaceutical intermediates, since the class controls downstream reactivity.
• Forensic chemistry: rapid field classification of amine-containing substances before instrumental analysis.

To recap: the carbylamine test flags primary amines by their foul-smelling isocyanide, and the Hinsberg test sorts all three classes by sulphonamide solubility — primary dissolves in base and reprecipitates on acid, secondary stays insoluble, tertiary never reacts. Two quick tests, and the amine's identity class is settled.