Carbylamine Reaction (Isocyanide Test): Mechanism, Which Amines Pass and Exam Traps

ChemistryClass 12, AminesFor JEE and NEET aspirants

The carbylamine reaction turns a primary amine into an isocyanide (also called a carbylamine), R–NC. The amine is warmed with chloroform and ethanolic potassium hydroxide, and the isocyanide is recognised by its very unpleasant smell. Secondary and tertiary amines do not show this reaction, so it is the isocyanide test for primary amines, aliphatic or aromatic. It is in the Class 12 NCERT chapter on amines, and JEE Main and NEET ask which amines give the test and what the product is.

At a glance

General:
$RN H_{2} + CHC l_{3} + 3KOH heat RNC + 3KCl + 3 H_{2} O$
Aniline:
$C_{6} H_{5} N H_{2} + CHC l_{3} + 3KOH heat C_{6} H_{5} NC + 3KCl + 3 H_{2} O$
Positive: primary amines, both aliphatic ( $RN H_{2}$ ) and aromatic ( $ArN H_{2}$ ).
Negative: secondary and tertiary amines.
Electrophile: dichlorocarbene, $: CC l_{2}$ , made from chloroform by the base.
Product: the isocyanide R–N≡C (better written $R - N^{+} \equiv C^{-}$ ), an isomer of the nitrile R–C≡N.

1. Mechanism

The base first turns chloroform into dichlorocarbene, the same electrophile as in the Reimer-Tiemann reaction: $O H^{-}$ removes the H of $CHC l_{3}$ , and $CC l_{3}^{-}$ loses $C l^{-}$ . The lone pair of the amine nitrogen then attacks the carbene carbon, which has only six electrons.

Figure 1: The amine's N adds to dichlorocarbene; the proton shift and two eliminations of HCl remove both hydrogens that were on N. That is why only a primary amine gives the isocyanide.

Carbene. $CHC l_{3}$ + $O H^{-}$ gives $: CC l_{2}$ , $C l^{-}$ and water.
Addition. The amine N bonds to the carbene C; a proton shift gives $RNH - CHC l_{2}$ .
First elimination. The base removes the proton from N and chloride leaves from C: $R - N = CHCl$ .
Second elimination. The base removes the proton from C and the last chloride leaves, giving the isocyanide $R - N^{+} \equiv C^{-}$ .

Exam Trick

"Two H on N, two HCl, one smell." The reaction needs two N–H bonds to lose two molecules of HCl, and only a primary amine has two. A secondary amine has one N–H, a tertiary amine none.

2. The isocyanide test

Amine	Class	Carbylamine test
Ethanamine, $C_{2} H_{5} N H_{2}$	1°, aliphatic	Positive: ethyl isocyanide
Aniline, $C_{6} H_{5} N H_{2}$	1°, aromatic	Positive: phenyl isocyanide
Benzylamine, $C_{6} H_{5} C H_{2} N H_{2}$	1°, aliphatic ( $N H_{2}$ on $C H_{2}$ , not on the ring)	Positive: benzyl isocyanide
Hexane-1,6-diamine	1° (two $N H_{2}$ )	Positive at both ends
Diethylamine, $(C_{2} H_{5})_{2} NH$	2°	Negative
N-Methylaniline, $C_{6} H_{5} NHC H_{3}$	2°	Negative
Triethylamine, $(C_{2} H_{5})_{3} N$	3°	Negative

The test separates primary amines from the other two classes, but it cannot tell a secondary amine from a tertiary one; Hinsberg's reagent (benzenesulphonyl chloride) does that.

Isocyanides and nitriles. An isocyanide, R–N≡C, is joined to R through nitrogen; a nitrile, R–C≡N, through carbon. Alkyl halides give mainly isocyanides with AgCN and nitriles with KCN. Their reactions differ too:

RNC + 2 H_{2} O H^{+} RN H_{2} + HCOOH

RNC + 4 [H] LiAl H_{4} or H_{2} /Ni RNHC H_{3}

Acid hydrolysis of an isocyanide gives a primary amine and methanoic acid (a nitrile, R–C≡N, gives the acid RCOOH instead); reduction gives a secondary amine with an N-methyl group.

JEE Advanced

The isocyanide carbon. In $R - N^{+} \equiv C^{-}$ the terminal carbon has a lone pair and a formal negative charge, like the carbon of carbon monoxide. That is the carbon the carbene brought in (the chloroform carbon), and it ends with a lone pair, just as the carbene had.

One carbene, two named reactions. Dichlorocarbene from chloroform and base reacts with a phenoxide ion at the ring (Reimer-Tiemann, aqueous NaOH) and with a primary amine at the nitrogen (carbylamine, alcoholic KOH).

Reading a sequence backwards. A foul-smelling product made with $CHC l_{3}$ and KOH means the step before it gave a primary amine, for example from a Hoffmann bromamide degradation or from reducing a nitrile.

3. Worked examples

Solved Example 1

Which of these give the carbylamine test: aniline, dimethylamine, methylamine, trimethylamine, N-methylaniline?

Solution:

Aniline and methylamine, the two primary amines. Dimethylamine and N-methylaniline are secondary, and trimethylamine is tertiary.

Solved Example 2

C_{2} H_{5} Cl

with reagent X gives a foul-smelling compound Z. Propanamide with

B r_{2}

/NaOH gives Y, and Y with

CHC l_{3}

and ethanolic KOH gives the same Z. Identify X, Y and Z.

Solution:

Y is ethylamine (the Hoffmann bromamide degradation removes one carbon), and the carbylamine reaction turns it into Z, ethyl isocyanide, $C_{2} H_{5} NC$ . X is AgCN, which gives the isocyanide from $C_{2} H_{5} Cl$ ; KCN would give the nitrile instead.

Solved Example 3

How can ethylamine be told apart from diethylamine?

Solution:

Warm each with $CHC l_{3}$ and alcoholic KOH. Ethylamine, a primary amine, gives the foul smell of ethyl isocyanide; diethylamine, a secondary amine, gives none.

4. Exam traps

Watch out

Only primary amines respond, aromatic ones included; secondary and tertiary amines do not.
The product is an isocyanide, R–NC, not a nitrile, R–CN.
The reagent is chloroform with alcoholic KOH; the reactive species is dichlorocarbene.
AgCN with an alkyl halide also gives an isocyanide, but that is not the carbylamine reaction.
Acid hydrolysis of an isocyanide gives a primary amine and HCOOH; reduction gives a secondary amine.
A compound with no $N H_{2}$ group, such as an N-alkylphthalimide, gives no isocyanide.
The test cannot separate secondary from tertiary amines; use Hinsberg's reagent for that.

Past-paper questions on Carbylamine reaction

Our past papers: JEE Main 2025-2026, JEE Advanced 2022-2026, NEET 2018-2019, 2022-2026. We found 11 questions in them that test this:

JEE Main 7 (2025-2026)
JEE Advanced 3 (2023, 2025)
NEET 1 (2026)

Each question was read and checked by hand (list checked 2026-10-15). A question that shows its structures only as pictures can be missed by our search. Each opens in a new tab, with the question to try and our step-by-step solution.

Learn it with the chapter

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