Problem 30 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reacts with Hinsberg reagent (Benzene sulphonyl chloride) to form a product, soluble in \(\mathrm{KOH}\) ? (a) primary amine (b) secondary amine (c) tertiary amine (d) quaternary amine

Step-by-Step Solution

Answer

Primary amine reacts to form a product soluble in KOH.

Step 1: Understand Hinsberg Test

The Hinsberg test is used to distinguish between primary, secondary, and tertiary amines. It involves reacting the amine with benzene sulfonyl chloride.

Step 2: Reaction with Primary Amine

Primary amines react with benzene sulfonyl chloride to form sulfonamide. This product is acidic and can dissolve in \( ext{KOH}\).

Step 3: Reaction with Secondary Amine

Secondary amines also react with benzene sulfonyl chloride to form a sulfonamide. However, the product formed is neutral and does not dissolve in \( ext{KOH}\).

Step 4: Reaction with Tertiary and Quaternary Amines

Tertiary and quaternary amines do not react with benzene sulfonyl chloride under these conditions, thus no product is formed. Without a product, there is nothing to test for solubility in \( ext{KOH}\).

Step 5: Conclusion on Solubility

Since only the sulfonamide from a primary amine is acidic and dissolves in \( ext{KOH}\), the primary amine is the correct answer.

Key Concepts

Primary Amine

A primary amine is an organic compound that has one alkyl or aryl group (represented as R) attached to a nitrogen atom, denoted as \( \mathrm{RNH}_2 \). This structure is crucial as it impacts how primary amines interact with other reagents, such as benzene sulfonyl chloride in the Hinsberg test.

**How Primary Amines React**

• When a primary amine reacts with benzene sulfonyl chloride, it forms a sulfonamide.
• The sulfonamide product is acidic and has the property of being able to dissolve in potassium hydroxide (KOH).

This acidic nature is key, as it distinguishes primary amines from other types of amines in identification. By dissolving in KOH, the sulfonamide reveals that the original compound was a primary amine. This is why primary amines are integral to using the Hinsberg test effectively for identification purposes.

Secondary Amine

Secondary amines are compounds where two organic groups are attached to a nitrogen atom, represented as \( \mathrm{R}_2\mathrm{NH} \). This structure leads to interesting chemical behavior, especially during tests designed to distinguish them from primary and tertiary amines.

**Characteristics of Secondary Amines**

• When reacted with benzene sulfonyl chloride, secondary amines also produce a sulfonamide.
• However, in contrast to those from primary amines, these sulfonamides are neutral.
• As a result, these neutral sulfonamides do not dissolve in potassium hydroxide (KOH).

This means in the context of the Hinsberg test, while secondary amines react with benzene sulfonyl chloride, the lack of solubility in KOH enables chemists to differentiate them from primary amines.

Tertiary Amine

Tertiary amines feature three organic groups attached to a nitrogen atom, formatted as \( \mathrm{R}_3\mathrm{N} \). This rigid structure significantly affects their chemical reactivity and distinguishes them from primary and secondary amines in tests.

**Why Tertiary Amines Differ**

• In the Hinsberg test, tertiary amines do not react with benzene sulfonyl chloride.
• Since there is no reaction, no sulfonamide product is formed.
• Without a product, there's no need to test for solubility in KOH.

Tertiary amines' inability to form a sulfonamide under these conditions stands out as a key identification marker, marking them out from primary and secondary amines during such tests.

Benzene Sulfonyl Chloride

Benzene sulfonyl chloride is a reagent used in the Hinsberg test to differentiate between primary, secondary, and tertiary amines. Its structure and behavior play a crucial role.

**Properties and Role in Testing**

• It reacts with amines to form sulfonamides, but the outcome depends on the type of amine.
• When reacting with primary amines, the resulting sulfonamide is acidic, leading to solubility in KOH.
• For secondary amines, the sulfonamide product is neutral and does not dissolve in KOH.
• Tertiary amines do not react with benzene sulfonyl chloride, resulting in no product.

These distinct reactions enable chemists to use benzene sulfonyl chloride effectively to determine the type of amine present. Its role is essential and forms the cornerstone of the Hinsberg test's differentiating power.