Problem 7 - A Complete Resource Book in Chemistry for JEE Main

Question

Acetamide is treated separately with the following reagents. Which one of these would give methylamine? (a) sodalime (b) \(\mathrm{PCl}_{5}\) (c) \(\mathrm{NaOH}+\mathrm{Br}_{2}\) (d) hot conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\)

Step-by-Step Solution

Answer

(c) NaOH + Br2 will give methylamine through Hofmann Bromamide Degradation.

Step 1: Understanding the Reaction Context

When acetamide is treated with reagents, different chemical reactions occur based on the reagent used. We are asked to identify which reagent converts acetamide into methylamine.

Step 2: Examining Reagent Potential: Sodalime

Sodalime is typically used in decarboxylation reactions where it removes the carboxyl group (-[9;0;]C(=O)OH) from carboxylic acids. It is unlikely to convert acetamide into methylamine.

Step 3: Understanding Acyl Halide Formation: Using PCl5

Treating with PCl5[9;0;] forms acyl chlorides from amides. Thus, using PCl5 will convert acetamide to acetyl chloride, not methylamine.

Step 4: Hofmann Bromamide Degradation

NaOH + Br2 is indicative of Hofmann Bromamide Degradation. In this reaction, primary amides are converted into primary amines, with the loss of one carbon atom in the process. Applying this method will convert acetamide (CH3CONH2) to methylamine (CH3NH2).

Step 5: Identifying Deamidation Potential: Hot Conc. H2SO4

Hot concentrated H2SO4 will hydrolyze amides to form carboxylic acid. Therefore, treating acetamide with H2SO4 will lead to the formation of acetic acid, not methylamine.

Key Concepts

Amide to Amine Conversion

The transformation of amides to amines is an important reaction in organic chemistry. This conversion involves a primary amide, which is a compound with the structure R-CO-NH2, turning into a primary amine, R-NH2. This reaction is unique because it not only removes the carbonyl group but also reduces the carbon number by one.
The reagent that facilitates this conversion is \( \text{NaOH and Br}_2 \), utilized in the Hofmann Bromamide Reaction. When a primary amide such as acetamide is treated with \( \text{NaOH and Br}_2 \), a secondary reaction mechanism initiates, transforming it into methylamine. This process is particularly valuable in synthetic organic chemistry, allowing for the modification and simplification of molecular structures.
This type of conversion is hinged on specific reaction conditions and appears prominently in selective organic transformations where simplicity and efficacy are critical.

Chemical Reaction Mechanisms

Chemical reaction mechanisms reveal the intermediate steps and processes that a chemical reaction undergoes from reactants to products. In the context of the Hofmann Bromamide Reaction, understanding these mechanisms is essential.
Initially, when acetamide reacts with bromine in an alkaline medium (\( \text{NaOH} \)), the bromine acts as a halogenating agent. This reaction modifies the amide nitrogen, forming a bromamide intermediate. The negatively charged hydroxide ion from \( \text{NaOH} \) then attacks the \( \text{C-N} \) bond of the bromamide, which is crucial for the migration step. This step is called the rearrangement step, where the migratory aptitude of the carbon chain causes the cleavage of the carbon-oxygen bond.
Finally, the loss of \( \text{CO}_2 \) results in the formation of the primary amine. The entire mechanism underscores the importance of stepwise transformations in achieving desired chemical products.

Primary Amides

Primary amides are organic compounds characterized by the functional group \( \text{R-CONH}_2 \), where \( \text{R} \) represents an alkyl or aryl group. This group is derived from carboxylic acids by replacing the hydroxyl group with an amino group.
In acetamide, the simplest primary amide, the \( \text{R} \) group is a methyl group, leading to its chemical formula \( \text{CH}_3\text{CONH}_2 \). These compounds have significant biological and industrial importance, serving as building blocks for more complex molecules.
Primary amides can undergo various reactions, including hydrolysis, reduction, and the notable Hofmann Bromamide Reaction. The versatility and reactivity of primary amides make them pivotal players in synthesizing pharmaceuticals, agrochemicals, and dyes. Understanding their structure and reactivity is fundamental to harnessing their potential in chemical synthesis.