Problem 46 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \stackrel{\left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2} \mathrm{O}}{\longrightarrow} \mathrm{X} \stackrel{\mathrm{Br} / \mathrm{CCl}_{4}}{\longrightarrow}\) \(\mathrm{Y} \frac{\mathrm{HOH}}{\mathrm{H}^{*}}{\longrightarrow} \mathrm{Z}\) (a) bromoacetophenone (b) o-bromoacetophenone (c) p-bromoaniline (d) o-bromoacetanilide

Step-by-Step Solution

Answer

The final product Z is o-bromoaniline.

Step 1: Identify the starting material

The starting material is aniline, which has the chemical formula \( ext{C}_6 ext{H}_5 ext{NH}_2\). This compound is an aromatic amine.

Step 2: Reaction with acetic anhydride

Aniline reacts with acetic anhydride \((\mathrm{CH}_3\mathrm{CO})_2\mathrm{O}\) to form acetanilide. This is an acylation reaction where the \(\text{NH}_2\) group of aniline is converted to an amide group \(-\text{NHCOCH}_3\), forming \(\text{C}_6\text{H}_5\text{NHCOCH}_3\).

Step 3: Bromination in presence of CC\(_4\)

Acetanilide undergoes bromination in the presence of \(\text{Br}_2/\text{CCl}_4\). This reaction takes place mainly at the ortho and para positions relative to the amide group. The para position is more accessible, but since both ortho positions have the same identity, only one ortho position will be substituted, forming o-bromoacetanilide as a major product.

Step 4: Hydrolysis under acidic condition

The brominated acetanilide undergoes hydrolysis when treated with water in the presence of an acid \((\text{H}^*)\). This reaction breaks down the amide group \(-\text{NHCOCH}_3\) back to the amino group \(-\text{NH}_2\), regenerating the aniline derivative. Because the substitution occurred at the ortho position, the final product is o-bromoaniline.

Key Concepts

Acylation Reaction

In an acylation reaction, an electrophile is introduced to a compound to create a more complex molecule. In this exercise, acylation involves composing a reaction between aniline (\( \mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2 \)) and acetic anhydride (\( (\mathrm{CH}_3\mathrm{CO})_2\mathrm{O} \)). Aniline, an aromatic amine, reacts through its nitrogen atom. This partnership yields acetanilide, transforming the amine group into an amide group |\( \text{-NHCOCH}_3 \). This chemical transformation shields the nitrogen atom, making it less reactive in subsequent reactions.

• The amino group becomes an amide group.
• Acetanilide is less reactive than aniline because the nitrogen lone pair gets delocalized.

This initial step is important because it directs further bromination and determines the reactivity and position of substitutions on the aromatic ring.

Bromination Reaction

Bromination is a common type of electrophilic aromatic substitution reaction where a bromine atom is introduced into an aromatic ring. For acetanilide, bromination happens primarily in the ortho and para positions. The amide substituent, \( \text{-NHCOCH}_3 \), acts as an ortho/para-directing group due to the resonance effect.
During bromination in carbon tetrachloride (CCle3), bromine reacts with acetanilide to form o-bromoacetanilide as the primary product.

• The para position is more accessible but minimized due to its sterics.
• An ortho position is favored for substitution without competing sterics balancing the reaction outcome.

In a laboratory setting, it is crucial to control temperature and concentration to manipulate product formation selectively.

Hydrolysis of Amides

The hydrolysis of amides is a chemical process where amides are converted back to their corresponding amines or carboxylic acids. In this exercise, the brominated acetanilide undergoes hydrolysis under acidic conditions, effectively breaking down the amide \( \text{-NHCOCH}_3 \) group back into the amino group \( \text{-NH}_2 \). This reaction signifies a return to the building blocks for further chemical reactions.

• The reaction uses water and a catalyst, usually strong acid.
• Yields o-bromoaniline from o-bromoacetanilide by removing the acetyl group.

The hydrolysis of amides serves two purposes: regenerating the amine functional group and confirming the bromination location as ortho to the amide group, impacting the final structure and reactivity of the molecule.