Problem 58 - A Complete Resource Book in Chemistry for JEE Main

Question

Aniline first reacts with acetyl chloride producing compound ' \(A\) '. 'A'reacts with nitric acid/sulphuric acid mixture and produces compound 'B', which hydrolyses to compound 'C'. What is the identity of ' \(\mathrm{C}^{\prime}\) ? (a) acetanilide (b) p-nitroacetanilide (c) p-nitroaniline (d) aniline

Step-by-Step Solution

Answer

The identity of compound 'C' is p-nitroaniline.

Step 1: Understanding the Reaction of Aniline with Acetyl Chloride

When aniline (426H52N4) reacts with acetyl chloride (3H3ClO) in a nucleophilic acyl substitution reaction, it forms an amide known as acetanilide (426H52NCOCH3), which we will call compound 'A'. The amino group of aniline (7NH2) reacts with the acetyl group of acetyl chloride, replacing the chlorine atom.

Step 2: Nitration of Acetanilide to Form Compound 'B'

Next, compound 'A' (acetanilide) undergoes nitration when it reacts with a mixture of nitric acid (1NO3) and sulfuric acid (3HSO4). This reaction introduces a nitro group into the aromatic ring, usually at the para position relative to the amide group, due to the directive effect of the acetamido group. This results in the formation of p-nitroacetanilide, which we call compound 'B'.

Step 3: Hydrolysis of Compound 'B' to Obtain Compound 'C'

The compound 'B' (p-nitroacetanilide) is then hydrolysed, typically using an acid or base-catalyzed method, to remove the acetyl group. This process converts p-nitroacetanilide into p-nitroaniline, compound 'C'. During hydrolysis, the amide bond is cleaved, releasing the acetic acid and leaving the p-nitroaniline (C42H7NO2) as the product.

Key Concepts

Amines

Amines are a fascinating class of organic compounds derived from ammonia by replacing one or more of its hydrogen atoms with an alkyl or aryl group. They're popular in the world of organic chemistry because they serve as building blocks for many molecules, including pharmaceuticals and dyes.
There are several types of amines:

• Primary amines: One hydrogen atom is replaced, such as in \( \text{NH}_2\) group of aniline.
• Secondary amines: Two hydrogen atoms replaced.
• Tertiary amines: All three hydrogens are replaced.

In the context of the present chemical reaction, aniline acts as a primary amine when it reacts with acetyl chloride. It's important to understand that the reactivity of amines stems from the lone pair of electrons on the nitrogen, which makes them excellent nucleophiles. This is why aniline readily reacts with acetyl chloride in the acylation process.

Acylation

Acylation is a fundamental reaction in organic chemistry and involves introducing an acyl group into a compound. An acyl group is denoted as \( \text{RCO}- \), where \( \text{R} \) is typically an organic substituent.
In the exercise, aniline reacts with acetyl chloride in a classic acylation reaction. When this occurs, the lone pair on the nitrogen atom of the aniline attacks the carbon atom of the acetyl chloride. This results in the substitution of the chloride ion with the amino group, forming an amide bond and producing acetanilide, the compound 'A'.

• This process illustrates the nucleophilic acyl substitution mechanism.
• Acylation often makes a compound less reactive, which is an important consideration for further reactions.

The formed amide group in acetanilide is a key structural feature that affects subsequent reactions, particularly the orientation during nitration.

Nitration

Nitration is a chemical process usually employed to add a nitro group \( \text{-NO}_2 \) to an organic substrate. It often involves a mixture of nitric and sulfuric acids as the nitrating agent.
In the case of acetanilide, this nitration step leads to the formation of compound 'B', which is p-nitroacetanilide. The reason p-nitroacetanilide is primarily formed \( (\text{para-positioned}) \) is due to the electron-withdrawing nature of the acetamido group, which directs incoming substituents to the para position on the ring. Key specifics of nitration include:

• Nitration is an electrophilic substitution reaction.
• The acetamido group's influence helps avoid ortho-substitution due to steric hindrance.

Understanding nitration in the context of directing effects is crucial for predicting the structure of the product and planning further transformations.

Hydrolysis

Hydrolysis is the chemical breakdown of a compound in the presence of water, often catalyzed by an acid or base. It converts compounds by cleaving chemical bonds, typically removing protecting groups or sidechains.
Following the nitration of acetanilide to p-nitroacetanilide, the exercise involves a hydrolysis step to form compound 'C'. This step removes the acetyl group from the p-nitroacetanilide, leaving behind p-nitroaniline as the final product.

• During hydrolysis, the amide bond in p-nitroacetanilide is broken.
• The process releases acetic acid as a by-product.

Knowing the hydrolysis process is fundamental to understanding how complex synthetic routes can be efficiently planned and executed, especially in organic synthesis where protecting groups like acetyl groups are routinely used.