Problem 147 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following methods is neither meant for the synthesis nor for separation of amines? (a) Hinsberg method (b) Hoffmann method (c) Wurtz reaction (d) Curtius reaction

Step-by-Step Solution

Answer

The Wurtz reaction (c) is not meant for the synthesis or separation of amines.

Step 1: Understanding the methods

First, let's briefly identify the purpose of each method mentioned: 1. **Hinsberg Method**: Used for the separation of primary, secondary, and tertiary amines. 2. **Hoffmann Method**: Refers to Hoffmann elimination or degradation used to synthesize amines. 3. **Wurtz Reaction**: Involves coupling of alkyl halides to form higher alkanes, not typically involving amines. 4. **Curtius Reaction**: Involves converting an acid chloride to an amine via an isocyanate intermediate.

Step 2: Identifying the outlier

From the descriptions: - Hinsberg, Hoffmann, and Curtius reactions all involve the synthesis or separation of amines. - The Wurtz reaction is purely for forming carbon-carbon bonds, typically alkanes, and doesn't generally involve amines.

Step 3: Determine the correct answer

Since the Wurtz reaction does not involve either the synthesis or separation of amines, it is the method that is neither meant for synthesis nor separation of amines.

Key Concepts

Hinsberg Method

The Hinsberg method is a classic technique used to distinguish between primary, secondary, and tertiary amines. This method employs the use of Hinsberg's reagent, which is benzene sulfonyl chloride. Primary and secondary amines react with this reagent under basic conditions.
During the process:

• Primary amines form sulfonamides, which are soluble when they are in an alkaline medium but can be precipitated upon acidification.
• Secondary amines generate insoluble sulfonamides in the alkaline medium.
• Tertiary amines do not react in this method, making this a simple yet effective separation technique.

These differences allow for easy separation based solely on the chemical reactivity with Hinsberg's reagent. It's particularly valuable in identifying the type of amine present in a mixture.

Hoffmann Method

The Hoffmann method, often referred to as the Hoffmann elimination, is a synthetic technique used to form alkenes from amines. This involves the conversion of an amine into a quaternary ammonium salt, which then undergoes elimination to yield an alkene.
Some key steps include:

• First, an alkylamine is treated with excess methyl iodide to form a quaternary ammonium iodide salt.
• This salt is then transformed into a less stable hydroxide by means of silver oxide in water.
• Upon heating, the hydroxide ion facilitates the elimination of an alkene from the ammonium salt.

While this method does not directly produce amines, it relates to amines' transformations and is crucial in the synthesis of various alkenes.

Wurtz Reaction

The Wurtz reaction is a famous organic chemistry reaction that involves the coupling of alkyl halides in the presence of sodium metal. This method is primarily used for forming carbon-carbon bonds, thereby producing symmetrical alkanes.
Key aspects include:

• Two identical alkyl halides react with sodium metal, resulting in the formation of a new carbon-carbon bond.
• It generates higher, often symmetrical alkanes.
• This reaction does not involve amines, which distinguishes it from others that do in the context of manipulation or synthesis.

While incredibly useful for constructing larger carbon skeletons, its lack of involvement with amines makes it less relevant in discussions centered around such compounds.

Curtius Reaction

The Curtius reaction is a transformation that involves the conversion of an acyl azide into an isocyanate upon heating, which can then form an amine. This reaction proceeds through the Curtius rearrangement.
Here's how it happens:

• An acid chloride reacts with sodium azide to form an acyl azide.
• Upon thermal decomposition, the acyl azide undergoes rearrangement to form isocyanate.
• The isocyanate can then react with water to produce a primary amine.

This reaction is particularly valuable in organic synthesis for deriving amines from carboxylic acid derivatives, specifically when other methods might be less feasible.