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In the salt-and-organic practical, the carbylamine test is the quickest way to say "this is a primary amine". In theory papers it appears as an equation to complete, a "which of these gives a positive test" question, or an isomer count. This post explains what the reagents do, why the test works only for primary amines, and how to handle the question types, with a worked example and a practice routine.
Where is this in the syllabus?
The JEE (Main) 2026 syllabus and the NEET (UG) 2026 syllabus list "identification of primary, secondary and tertiary amines" and, in the practical unit, detection of amino groups. The CBSE Chemistry curriculum for 2026-27 includes tests for "amino (Primary)" groups among Class 12 practicals. The reaction is in section 9.6 of the NCERT Class 12 unit on amines (Reprint 2026-27), which says: "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."
What is the balanced equation?
RNH2 + CHCl3 + 3KOH → R–N≡C + 3KCl + 3H2O
Check it atom by atom: 3 Cl, 3 K and 3 O on each side. Hydrogen: 2 from the amine, 1 from chloroform and 3 from KOH make 6, which is exactly the 6 in three water molecules. The carbon from chloroform ends up as the terminal carbon of the isocyanide group.
With aniline: C6H5NH2 + CHCl3 + 3KOH → C6H5NC + 3KCl + 3H2O, giving phenyl isocyanide.
Why do only primary amines react?
Look at what the nitrogen has to do. In R–N≡C the nitrogen is bonded to R and to the new carbon by a triple bond, and carries no hydrogen. Starting from RNH2, it loses both of its hydrogens. A secondary amine, R2NH, has only one hydrogen to lose and already carries two carbon groups. A tertiary amine has none. Neither can form R–N≡C.
Where does the new carbon come from? Strong base removes HCl from chloroform to give dichlorocarbene, :CCl2. Jack Hine (1950) proposed this "carbon dichloride" as the intermediate when chloroform reacts with base. The amine's lone pair attacks the carbene carbon, and two more HCl are removed by the base. The same carbene puts the –CHO group onto phenol in the Reimer-Tiemann reaction.
The reaction is also a real preparative method, not only a test. *Organic Syntheses*, which publishes checked laboratory procedures, carries a phase-transfer Hofmann carbylamine reaction for making tert-butyl isocyanide.
Worked example: which isomers give a positive test?
Part 1. NCERT Intext Question 9.2 asks for all the amines of formula C4H11N. How many are there, and how many give a positive carbylamine test?
Primary (positive): butan-1-amine, butan-2-amine, 2-methylpropan-1-amine, 2-methylpropan-2-amine. That is 4.
Secondary (negative): N-methylpropan-1-amine, N-methylpropan-2-amine, N-ethylethanamine. That is 3.
Tertiary (negative): N,N-dimethylethanamine. That is 1.
Total 8 amines, 4 positive. Note that 2-methylpropan-2-amine is a primary amine on a tertiary carbon. The test cares about the nitrogen, not the carbon.
Part 2. Among C7H9N compounds with a benzene ring, which give the test? o-, m- and p-toluidine and benzylamine are primary, so all four are positive. N-methylaniline is secondary, so it is negative.
Part 3. For 0.93 g of aniline (93.13 g/mol, 0.0100 mol), the equation needs 0.0100 mol of CHCl3 (1.19 g) and 0.0300 mol of KOH (1.68 g), and at most 0.0100 mol of phenyl isocyanide (103.12 g/mol, 1.03 g) can form. In the lab, the test uses only a few drops; this is a stoichiometry exercise.
Common mistakes and how to avoid them
Thinking aromatic amines fail. NCERT says aliphatic and aromatic primary amines both react.
Calling a secondary amine positive. It has only one N–H. The test is negative.
Writing 1 KOH. Three HCl are removed overall, so you need 3KOH.
Using it to separate secondary from tertiary. The carbylamine test only flags primary amines. For the other two, use Hinsberg's reagent or nitrous acid, as in our guide to amines and diazonium salts.
Safety note: chloroform and isocyanides are harmful, so the test is done on a few drops, in a fume cupboard, with your teacher present.
If isomer-counting questions keep catching you, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning, so you can check your list against a full solution.
A practice routine
Write the general equation and the aniline equation from memory and balance them.
Do NCERT Intext Question 9.8 for C3H9N, then mark which isomers give the carbylamine test (you should find four amines, two of them primary).
Draw R–N≡C and R–C≡N side by side and label them.
Make a three-row card: carbylamine, Hinsberg, nitrous acid, with the result for 1°, 2° and 3° amines.
Before your next practical, write the reagents and the "primary only" rule on the first page of your lab notebook.
