Question

The first step in conversion of aniline to 4-bromoaniline involves

• A. Acetylation of amino group
• B. Formation of diazonium salt in the presence of \(\text{NaNO}_2\) and \(\text{HCl}\).
• C. Reaction with Bromine water at room temperature.
• D. Reaction with Bromine in the presence of \(\text{NaOH}\).

Hint:

To obtain mono-substituted derivatives of aniline (such as 4-bromoaniline or 4-nitroaniline), ALWAYS protect the \(-\text{NH}_2\) group first by acetylation to form acetanilide!

Answer 1

The Correct Option is A

Concept:
The amino group (\(-\text{NH}_2\)) in aniline is a very strong activating and ortho/para-directing group in electrophilic aromatic substitution due to resonance donation of its unshared electron pair into the benzene ring.
To achieve controlled monobromination and avoid polybromination, the activating power of the amino group must first be reduced by protecting it via acetylation.

Step 1: The Problem of Direct Bromination:
When aniline is treated directly with bromine water at room temperature, the amino group activates the ring so strongly that electrophilic bromination occurs instantaneously at all available ortho- and para-positions.
This produces 2,4,6-tribromoaniline as a white precipitate, making selective synthesis of 4-bromoaniline impossible by direct halogenation.

Step 2: Protection via Acetylation:
To moderate the activating effect, the first step is the acetylation of the amino group by treating aniline with acetic anhydride in pyridine:
\[ \text{C}_6\text{H}_5\text{NH}_2 + (\text{CH}_3\text{CO})_2\text{O} \xrightarrow{\text{pyridine}} \text{C}_6\text{H}_5\text{NHCOCH}_3 + \text{CH}_3\text{COOH} \] In the resulting acetanilide, the nitrogen lone pair is delocalized onto the carbonyl oxygen of the acetyl group:
\[ \text{Ar}-\overset{\cdot\cdot}{\text{N}}\text{H}-\overset{\text{O}}{\overset{\parallel}{\text{C}}}-\text{CH}_3 \longleftrightarrow \text{Ar}-\overset{+}{\text{N}}\text{H}=\overset{\text{O}^-}{\overset{\parallel}{\text{C}}}-\text{CH}_3 \] This delocalization reduces the availability of the nitrogen lone pair to the aromatic ring, lowering its activating power.

Step 3: Subsequent Monobromination and Deprotection:
The protected acetanilide then undergoes electrophilic bromination with \(\text{Br}_2\) in acetic acid to give 4-bromoacetanilide as the major product due to steric hindrance at the ortho-position.
Subsequent acid or alkaline hydrolysis removes the acetyl group to deliver 4-bromoaniline selectively.
Final Answer:
The first step is the acetylation of the amino group, corresponding to option (A).