Q.Identify X in the sequence: m-chloroaniline --CHCl3/KOH--> (Y) --HCl, 300 K--> X + methanoic acid.

The Carbylamine Reaction: A Stink Test for Primary Amines

Imagine you are a chemist in a lab with several unlabelled bottles of amines. You need to quickly tell which ones are primary amines (where the nitrogen is attached to exactly one carbon) and which are secondary or tertiary. You could run a complicated spectrum, or you could just make something that smells terrible. That is the carbylamine reaction.

The reaction is simple in practice: you take your suspected amine, add a little chloroform ($CHCl_3$) and a strong base like alcoholic potassium hydroxide ($KOH$), and warm the mixture. If a primary amine is present, you will produce an isocyanide (also called a carbylamine). Isocyanides have a famously repulsive, penetrating odour — often described as "foul" or "putrid". If you smell that, you have a primary amine.

The Precise Statement

Carbylamine Reaction: A primary amine ($RNH_2$) reacts with chloroform ($CHCl_3$) in the presence of a strong base (usually alcoholic $KOH$) to form an isocyanide ($RNC$) and potassium chloride ($KCl$) and water ($H_2O$).

The general equation is:

The product $RNC$ is the isocyanide — the source of the foul smell.

Why Does It Happen? (The Mechanism)

The reaction proceeds through a dichlorocarbene intermediate. This is the key concept that makes the reaction work.

1. Generation of Dichlorocarbene: The strong base ($KOH$) deprotonates chloroform, which then loses a chloride ion to form a highly reactive species called dichlorocarbene ($:CCl_2$). This carbene is electron-deficient and desperately wants to react.

2. Attack by the Amine: The lone pair on the nitrogen of the primary amine attacks the electron-deficient carbon of the dichlorocarbene. This forms an unstable intermediate.

3. Elimination: The intermediate undergoes a series of dehydrohalogenation steps (loss of $HCl$) driven by the base, ultimately yielding the isocyanide ($RNC$). …