Question
What is the carbylamine reaction? Write the reaction for aniline with chloroform and alcoholic KOH. Why does this test work only for primary amines and not for secondary or tertiary amines?
(NCERT Class 12 — asked directly in CBSE boards and frequently in NEET)
Solution — Step by Step
The carbylamine reaction (also called isocyanide test) occurs when a primary amine is heated with chloroform () and alcoholic :
The product is an isocyanide (carbylamine), which has an extremely foul smell. That smell is the positive test.
For aniline ():
The product phenyl isocyanide has a terrible, penetrating odour — unmistakable in the lab.
The mechanism requires two N–H bonds on the nitrogen. Here’s why:
- The intermediate dichlorocarbene () generated from attacks the nitrogen of the amine
- Two successive eliminations of HCl occur from the N–H bonds, forming the triple bond
- Secondary amines () have only one N–H bond — they can’t complete the second elimination
- Tertiary amines () have no N–H bonds at all — no reaction
So the test is specific to primary amines: both aliphatic () and aromatic ().
Why This Works
The real reactive species is dichlorocarbene (), formed when KOH dehydrohalogenates chloroform:
This carbene inserts into the N–H bond of the primary amine. Two subsequent base-assisted HCl eliminations convert the single bond into a triple bond (but with the lone pair on carbon — hence isocyanide, not cyanide).
The foul smell is characteristic enough that even a tiny amount of isocyanide is detectable. That’s what makes it such a reliable bench test.
Alternative Method — Distinguishing Primary, Secondary, Tertiary
If the exam asks you to distinguish between all three types of amines:
| Test | Primary | Secondary | Tertiary |
|---|---|---|---|
| Carbylamine test | Foul smell (positive) | No reaction | No reaction |
| Hinsberg test () | Dissolves in NaOH | Insoluble product | No reaction with reagent |
| (nitrous acid) | Aliphatic: gas. Aromatic: diazonium salt | Oily nitrosamine | Tertiary aromatic: p-nitroso compound |
For NEET: the carbylamine test is a one-mark guaranteed question almost every other year. Just remember the three reagents — primary amine + + alc. KOH → foul-smelling isocyanide. Don’t confuse isocyanide () with cyanide () — the connectivity is different.
Common Mistake
Students often write the product as (cyanide) instead of (isocyanide). These are different compounds. In isocyanide, the carbon is bonded to nitrogen with the lone pair on carbon (). In cyanide, nitrogen carries the lone pair (). The carbylamine reaction specifically produces the isocyanide. Writing CN instead of NC will cost you marks in CBSE boards.