Problem 100 - A Complete Resource Book in Chemistry for JEE Main

Question

A positive carbylamine test is given by (1) \(\mathrm{N}, \mathrm{N}\)-dimethylaniline (2) 2,4 -dimethylaniline (3) N-methyl-o-methylaniline (4) p-methylbenzylamine (a) 2,4 (b) 2,3 (c) \(1,2,4\) (d) \(2,3,4\)

Step-by-Step Solution

Answer

The answer is (a) 2,4.

Step 1: Understand the Carbylamine Reaction

The carbylamine reaction, also known as the Hofmann isocyanide synthesis, occurs when a primary amine reacts with chloroform in the presence of alcoholic KOH to produce an isocyanide, which has a foul odor. This reaction is specific to primary amines.

Step 2: Evaluate Each Compound for Primary Amines

We need to determine which compounds have primary amine groups (\(-\mathrm{NH}_2\)). 1. \(\mathrm{N}, \mathrm{N}\)-dimethylaniline has a secondary amine group; it's not a primary amine.2. 2,4-dimethylaniline has a primary amine group at the aniline nitrogen.3. \(\mathrm{N}\)-methyl-o-methylaniline has a secondary amine group; it's not a primary amine.4. p-methylbenzylamine has a primary amine group at the benzyl position.

Step 3: Identify Which Options Have Primary Amines

From the evaluation, we see that compounds 2 and 4 have primary amines and can give a positive carbylamine test.

Step 4: Compare with the Options Provided

Now, we need to match our findings with the provided options: (a) 2,4 — Matches our findings. (b) 2,3 — Compound 3 is not a primary amine. (c) 1,2,4 — Compound 1 is not a primary amine. (d) 2,3,4 — Compound 3 is not a primary amine.

Key Concepts

Hofmann isocyanide synthesis

The Hofmann isocyanide synthesis, also known as the carbylamine reaction, is a chemical reaction that produces isocyanides or carbylamines. Isocyanides are recognizable by their distinctive, unpleasant odor. This reaction specifically involves primary amines. When a primary amine reacts with chloroform \((\text{CHCl}_3)\), in the presence of a strong base like alcoholic potassium hydroxide (KOH), it forms the isocyanide compound.

What makes this reaction special is its selectivity for primary amines. Secondary and tertiary amines do not undergo this reaction. Thus, this reaction serves as a useful test for the presence of primary amines in a compound.

• Primary amine + CHCl\(_3\) + alcoholic KOH → Isocyanide (Carbylamine) with a foul smell

The Hofmann isocyanide synthesis is a cornerstone reaction in organic chemistry, often utilized in synthetic and analytical chemistry to identify compounds with primary amine functionalities.

primary amines

Primary amines are crucial in organic chemistry due to their unique reactivity patterns. Structurally, a primary amine contains one alkyl or aryl group attached to the nitrogen atom, with the general formula R-NH\(_2\). This makes them significantly different from secondary and tertiary amines, where the nitrogen atom is bonded to two and three organic groups, respectively.

In the context of the Hofmann isocyanide synthesis or the carbylamine reaction, primary amines are the only class of amines that engage in the reaction to form isocyanides. Their ability to participate in this reaction hinges on the presence of the hydrogen atoms bonded directly to the nitrogen atom.

Primary amines are often foundational in numerous chemical syntheses and are found in many biologically active molecules and pharmaceuticals.

chloroform and alcoholic KOH

Chloroform and alcoholic KOH are the essential reagents in the carbylamine reaction. Chloroform \(\text{CHCl}_3\) is utilized for its role as a halogenated solvent that effectively interacts with the amine and alcoholic KOH.

The use of potassium hydroxide (KOH) in an alcoholic solution (often ethanol) turns KOH into a potent base. This strong base assists in deprotonating the amine, enabling the production of a carbanion intermediate from chloroform.

• KOH (alcoholic form) turns into a strong base
• Facilitates the reaction to yield isocyanide

The combination of chloroform and alcoholic KOH creates a highly reactive environment needed for the conversion of primary amines to the unpleasant-smelling isocyanides, showcasing the specific application of these reagents in organic synthesis and analysis tasks.