Problem 11 - A Complete Resource Book in Chemistry for JEE Main

Question

Rearrangement of an oxime to an amide in presence of strong acid is called (a) Beckmann rearrangement (b) Fries rearrangement (c) Sandmeyer reaction (d) Curtius rearrangement

Step-by-Step Solution

Answer

(a) Beckmann rearrangement

Step 1: Identify the Reaction Type

First, we need to identify the specific type of chemical rearrangement described in the problem. The exercise mentions the rearrangement of an oxime to an amide. We need to recall which rearrangement process involves this type of transformation.

Step 2: Recall the Definition of Beckmann Rearrangement

The Beckmann rearrangement is a reaction that specifically involves the conversion of an oxime into an amide using acidic conditions. Therefore, if the question asks about the transformation of an oxime to an amide in the presence of strong acid, the Beckmann rearrangement is the appropriate one.

Step 3: Compare with Other Options

Let's briefly compare the given options: - Fries rearrangement is not related to oximes; it involves the rearrangement of phenyl esters to hydroxyketones. - Sandmeyer reaction doesn't involve oxime conversion; it refers to the conversion of aryl diazonium salts into aryl halides. - Curtius rearrangement involves isocyanates, not oximes.

Step 4: Select the Correct Answer

Based on the definitions and comparisons, the rearrangement of an oxime to an amide in the presence of strong acid aligns with the Beckmann rearrangement. Hence, option (a) is the correct one.

Key Concepts

Oxime to Amide Conversion

The transformation of an oxime to an amide is a fascinating aspect of organic chemistry and plays a crucial role in the synthesis of many compounds. Simply put, an oxime has a nitrogen-oxygen double bond structure ( ext{R}_2C=N-OH), whereas an amide is known for its carbon-nitrogen double bond structure with a carbonyl group ( ext{RCONR}_2).
In this context, the conversion entails breaking the nitrogen-oxygen bond in oximes and rearranging the structure to form an amide, characterized by the new nitrogen-carbon bond.

During this conversion, the role of strong acid is pivotal. The acidic environment helps to protonate the oxime, making it more susceptible to structural changes. This protonation facilitates the rearrangement, leading to the formation of the more stable amide compound.
Understanding this process allows chemists to synthesize a wide range of amides, which are important in the formulation of pharmaceuticals and synthetic materials.

Rearrangement Reactions

Rearrangement reactions are a broad category of organic reactions where the structure of a molecule changes through the relocation of atoms or bonds, without adding or removing any atoms within the molecule.
These reactions are essential for creating complex molecules from simpler ones by rearranging their internal structures.

The Beckmann rearrangement is a prime example of such a reaction, where an oxime is transformed into an amide. This rearrangement is specific in that it occurs under acidic conditions, triggering a shift in bonds and atoms within the molecule.
Some general characteristics of rearrangement reactions include:

• Involvement of structural changes within the molecule.
• Creation of more stable compounds.
• Often induced by external catalysts, such as heat, acids, or bases.

Rearrangement reactions are crucial in synthetic chemistry, helping in the creation of a variety of chemical products.

Strong Acid Catalysis

Strong acid catalysis is a vital concept in chemistry that involves using a strong acid to accelerate or induce chemical reactions.
This approach is particularly important in catalyzing rearrangement reactions like the Beckmann rearrangement.

In the Beckmann rearrangement, the use of strong acid can help:

• Protonate the nitrogen atom in the oxime, increasing its susceptibility to bond breaking.
• Facilitate the migration of the alkyl or aryl group attached to the nitrogen.
• Stabilize the intermediates formed during the rearrangement, allowing for the smooth completion of the conversion process.

An example of a strong acid used in these conditions is sulfuric acid (H₂SO₄) or hydrochloric acid (HCl).
Both these acids help in breaking existing bonds and forming new ones, by donating protons to the molecules involved, thereby accelerating the rearrangement process. Understanding the role of acid catalysis enables chemists to predict reaction pathways and outcomes more effectively.