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? \([\mathbf{2 0 0 5}]\) (a) Hinsberg method (b) Hoffmann method (c) Wurtz reaction (d) Curtius reaction

Step-by-Step Solution

Answer

The Wurtz reaction (c) is neither meant for the synthesis nor separation of amines.

Step 1: Understanding the Methods

First, let's understand what each method is generally used for: - The Hinsberg method is used to separate amines. - The Hoffmann method refers to Hofmann rearrangement, used to synthesize primary amines. - The Wurtz reaction is used to couple two alkyl halide molecules forming hydrocarbons, not amines. - The Curtius reaction is involved in converting acyl azides into isocyanates or primary amines.

Step 2: Evaluation of Methods

Now, evaluate the role of each method one-by-one in terms of amines: - Hinsberg method: involves separation of amines, so it's related to amines. - Hoffmann method: synthesizes amines. - Wurtz reaction: does not involve amines; it is for making alkanes. - Curtius reaction: synthesizes primary amines from acyl azides.

Step 3: Identify the Outlier

Since the Wurtz reaction does not involve amines at all but rather involves the synthesis of alkanes using alkyl halides, it is neither meant for the synthesis nor separation of amines. The other methods (Hinsberg, Hoffmann, Curtius) are all related to working with amines.

Key Concepts

Hinsberg method

The Hinsberg method is a classic approach to separating amines based on their chemical properties. This technique relies on the unique reactions that different types of amines undergo when treated with benzene sulfonyl chloride.
Here’s how it works:

• Primary amines react to form a sulfonamide that is soluble in alkali but insoluble in water.
• Secondary amines form a sulfonamide that is insoluble in alkalis, appearing as a precipitate that can be filtered.
• Tertiary amines do not react with benzene sulfonyl chloride, remaining insoluble and distinct from the other amines.

This differentiation based on solubility allows for the systematic separation of these amines. Importantly, this method is crucial in both laboratory and industrial settings for the selective separation of amine mixtures, thus simplifying their analysis and further processing.

Hoffmann method

The Hoffmann method, also known as the Hofmann rearrangement, plays a vital role in the synthesis of primary amines. This process transforms amide groups into amines, with one less carbon atom than the original compound.
Carried out in the presence of a halogen (usually bromine or chlorine) and a strong base, like sodium hydroxide, the rearrangement steps include:

• The formation of an intermediate called an isocyanate.
• Subsequent hydrolysis of the isocyanate gives the final primary amine product.

The Hoffmann method is particularly valued for its ability to produce amines directly from less reactive starting materials, offering efficiency in synthetic organic chemistry. This makes it a go-to choice for preparing specific amines needed in various chemical manufacturing processes.

Curtius reaction

The Curtius reaction is another fascinating chemical transformation that turns acyl azides into isocyanates. These isocyanates can further transform into primary amines.
The sequence begins with heating the acyl azide, leading to the loss of nitrogen gas and formation of an isocyanate. This isocyanate then typically undergoes hydrolysis to yield the primary amine.
This reaction is valuable in synthetic organic chemistry for its capacity to modify the functional groups of molecules, particularly in the preparation of amines from different acyl starting materials.

• Acyl azide is the starting substrate.
• Isocyanate is the intermediate product.
• The final product is a primary amine, often achieved through hydrolysis.

By facilitating the introduction of an amine group while simultaneously simplifying the reactant structure, the Curtius reaction remains a tool of choice for achieving versatile synthetic objectives.