Problem 2 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

A compound ' \(\mathrm{X}^{\prime}\) on treatment with \(\mathrm{Br}_{2} / \mathrm{NaOH}\), provided \(\mathrm{C}_{3} \mathrm{H}_{9} \mathrm{~N}\), which gives positive carbylamine test. Compound ' \(X\) ' is :

Step-by-Step Solution

Answer

Compound 'X' is butanamide (C3H7CONH2).

Step 1: Understand the Reaction Conditions

The reaction condition given is Br2/NaOH, which is used for Hofmann rearrangement. This reaction typically converts primary amides into primary amines with one fewer carbon atom.

Step 2: Parse the Product Information

The product of the reaction, C3H9N, is found to give a positive carbylamine test, indicating that the product is a primary amine. The carbylamine test is specific for primary amines, which forms an isocyanide when reacted with chloroform and a base.

Step 3: Identify Characteristics of Compound 'X'

Since the product is a primary amine with the formula C3H9N, it must be derived from a primary amide having one more carbon atom, implying the original compound was C4H8ON (butanoic acid amide or butanamide, C3H7CONH2).

Step 4: Deduce Compound 'X'

Compound 'X' is a primary amide that undergoes Hofmann rearrangement to produce the primary amine C3H9N. Therefore, compound 'X' must be butanamide (C3H7CONH2), as the reaction removes one carbon due to decarboxylation during the process.

Key Concepts

Primary Amines

Primary amines are organic compounds characterized by an amino group \((\text{-NH}_2)\), connected to an alkyl or aromatic group. They are often denoted by the formula \(\text{R-NH}_2\), where \(\text{R}\) represents the organic substituent.
The presence of a single alkyl chain differentiates primary amines from secondary and tertiary amines, which have two and three alkyl or aromatic groups, respectively.
Primary amines are basic in nature and engage in a variety of chemical reactions due to the nucleophilic nitrogen atom.
Some common properties of primary amines include:

• Basicity: The lone pair of electrons on nitrogen makes them act as weak bases.
• Solubility: Smaller primary amines are soluble in water due to hydrogen bonding.
• Boiling Points: Higher than comparable hydrocarbons due to hydrogen bonding.

In the context of Hofmann rearrangement, a primary amine is the product obtained by the rearrangement of a primary amide. This most interestingly results in the loss of one carbon atom from the structure, leading to the formation of a primary amine from the respective starting amide.

Carbylamine Test

The carbylamine test is a classic chemical test used to detect the presence of primary amines in a compound.
It is highly selective and does not give positive results with secondary or tertiary amines.
To perform the carbylamine test, a primary amine is heated with chloroform \(\text{CHCl}_3\) and an alcoholic base, like \(\text{KOH}\).
During the reaction, an isocyanide (also known as carbylamine) is formed, which typically has a strong, unpleasant odor.
This is a clear indication of the presence of a primary amine.Some key points about the carbylamine reaction are:

• Reaction: Primary amine + \(\text{CHCl}_3\) + base \(\rightarrow\) Isocyanide + other products.
• Specificity: Only primary amines give a positive result.
• Odor: The presence of a foul odor confirms the reaction took place.

This test highlights primary amines' capability to undergo specific transformations, and in the problem, serves to identify the presence of a primary amine as the product of Hofmann rearrangement.

Primary Amides

Primary amides are organic compounds containing the functional group \(\text{-CONH}_2\), where a carbonyl group is attached to an amino group.
They can be derived from carboxylic acids by substituting the hydroxyl group with an amino group.
In their structure, amides have a carbon atom double-bonded to oxygen (C=O) and single-bonded to nitrogen.Key characteristics of primary amides include:

• Polarity: Highly polar due to the carbonyl and amino groups.
• Hydrogen Bonding: Strong hydrogen bonding abilities, enhancing boiling points.
• Solubility: Generally soluble in water, particularly the lower molecular weight amides.

In the context of Hofmann rearrangement, primary amides serve as the starting materials. They are transformed into primary amines through a fascinating pathway involving migration and decarboxylation. For instance, in the original exercise, butanamide undergoes this reaction pathway, losing one carbon atom, which results in the amino product with one carbon less than the original amide.