Q.Which of the following species are involved in the carbylamine test?
(Two or more options may be correct.)

The carbylamine test is a specific test for primary amines. The reaction involves heating the amine with chloroform and alcoholic potassium hydroxide.

Key idea: The test relies on the formation of an isocyanide (carbylamine), which has a foul smell.

Reasoning:

1. The primary amine ($R-NH_2$) reacts with chloroform ($CHCl_3$) in the presence of a strong base ($KOH$).
2. The reaction proceeds via a dichlorocarbene intermediate ($:CCl_2$), generated from $CHCl_3$ and base.
3. The final product is an isocyanide ($R-NC$), which is the species detected. …

The carbylamine test is a classic qualitative test for primary amines (aliphatic or aromatic). It relies on the ability of a primary amine to react with chloroform in the presence of a strong base to form an isocyanide (also called a carbylamine). The isocyanide has a characteristic foul smell, which is how the test is identified.

Why does this work? The reaction proceeds via a dichlorocarbene intermediate ($:CCl_2$), generated from chloroform and base. This highly reactive carbene inserts into the N–H bond of the primary amine, eventually yielding the isocyanide. Secondary and tertiary amines do not have the required N–H group, so they give no reaction — making the test specific for primary amines.

Now, let’s examine each option:

1. Option (i): $R{-}NC$

   This is an isocyanide — the product of the carbylamine test. It is formed when the reaction is successful. So yes, it is involved (as the end product).

2. Option (ii): $CHCl_3$

   Chloroform is one of the essential reagents. Without it, no dichlorocarbene is generated, and no isocyanide forms. So this is definitely involved.

3. Option (iii): $COCl_2$

   Phosgene is not used in the carbylamine test. It might be confused because it also contains chlorine and carbonyl, but the actual reagent is chloroform, not phosgene. So this is not involved.

4. Option (iv): $NaNO_2 + HCl$ …

Concept: Carbylamine Reaction (Isocyanide Test)

The carbylamine test is a confirmatory test for primary amines ($R-NH_2$). It detects the presence of a primary amino group by forming a foul-smelling isocyanide (carbylamine).

Method: Reagent Identification in Carbylamine Test

Step 1 — Recall the reaction conditions

The test is performed by heating the primary amine with:

• Chloroform ($CHCl_3$)
• Alcoholic potassium hydroxide (alc. $KOH$)

Step 2 — Write the general reaction

Step 3 — Identify the species involved

From the reaction:

• The reactants are: primary amine, $CHCl_3$, and $KOH$ (base).
• The product formed is: $R-NC$ (alkyl isocyanide).

Step 4 — Match with given options

• (A) $R{-}NC$ → This is the product of the test. …

Common Mistakes in the Carbylamine Test Question

Mistake 1: Confusing the Reagents with Those of Other Tests

The mistake: Students often select $NaNO_2 + HCl$ (Option D) because they associate it with amine reactions. This reagent is used in the diazotisation test (for primary aromatic amines), not the carbylamine test.

Why it happens: Both tests involve primary amines, but the reagents are completely different. The carbylamine test is specific to primary aliphatic and aromatic amines.

How to avoid: Memorise the exact reagent set for the carbylamine test:

• $CHCl_3$ (chloroform)
• $KOH$ (alcoholic, not aqueous)
• The product is an isocyanide ($R{-}NC$)

So the correct options here are (A) $R{-}NC$ (the product) and (B) $CHCl_3$ (the reagent).

Mistake 2: Thinking $COCl_2$ (Phosgene) Is Involved

The mistake: Selecting $COCl_2$ (Option C) because it contains chlorine and looks similar to chloroform.

Why it happens: Students confuse $CHCl_3$ with $COCl_2$ due to similar formulas.

How to avoid: Remember the reaction mechanism:

1. Dichlorocarbene ($:CCl_2$) is generated in situ from $CHCl_3$ and base.
2. This carbene attacks the amine to form the isocyanide.

$COCl_2$ is not used — it’s a toxic gas used in other organic syntheses (e.g., making urethanes), not in the carbylamine test.

Mistake 3: Forgetting That $R{-}NC$ Is the Product, Not a Reagent

The mistake: Some students think $R{-}NC$ is a reagent and mark it incorrectly.

Why it happens: The question asks for “species involved” — this includes both reactants and products. Students often only look for reagents.

How to avoid: Read the question carefully: “involved in” means any species that appears in the reaction — starting materials, intermediates, or products. The isocyanide ($R{-}NC$) is the characteristic product of the test.

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