Question:
Why is \(-NH_2\) group of aniline acetylated before carrying out nitration?
Why is \(-NH_2\) group of aniline acetylated before carrying out nitration?
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Acetylation is a standard "protection" strategy in organic chemistry whenever a highly reactive functional group interferes with a specific reaction.
Updated On: Mar 3, 2026
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Solution and Explanation
Step 1: Understanding the Concept:
Direct nitration of aniline is problematic due to the high reactivity of the ring and the formation of anilinium ions in acidic conditions.
Step 2: Detailed Explanation:
1. Reactivity and Oxidation: The \(-NH_2\) group is strongly activating and nitric acid is a strong oxidizing agent. Direct nitration often results in complex oxidation products.
2. Orientation Issue: Nitration is done in a strongly acidic medium (\(conc. H_2SO_4 + conc. HNO_3\)). In this environment, the amino group is protonated to the anilinium ion (\(-NH_3^+\)), which is a powerful electron-withdrawing and meta-directing group. This leads to about 47% formation of meta-nitroaniline.
3. The Solution (Protection): By acetylating aniline to acetanilide (\(C_6H_5NHCOCH_3\)), the electron pair on nitrogen is delocalized through resonance with the acetyl carbonyl. This reduces the overall activation of the ring and protects it from oxidation.
4. Directing Effect: The bulky acetamido group directs the nitronium ion primarily to the para position. The acetyl group can be easily removed by hydrolysis afterwards to recover the amine.
Step 3: Final Answer:
Acetylation reduces ring reactivity, prevents protonation to the meta-directing anilinium ion, and directs nitration predominantly to the para position.
Direct nitration of aniline is problematic due to the high reactivity of the ring and the formation of anilinium ions in acidic conditions.
Step 2: Detailed Explanation:
1. Reactivity and Oxidation: The \(-NH_2\) group is strongly activating and nitric acid is a strong oxidizing agent. Direct nitration often results in complex oxidation products.
2. Orientation Issue: Nitration is done in a strongly acidic medium (\(conc. H_2SO_4 + conc. HNO_3\)). In this environment, the amino group is protonated to the anilinium ion (\(-NH_3^+\)), which is a powerful electron-withdrawing and meta-directing group. This leads to about 47% formation of meta-nitroaniline.
3. The Solution (Protection): By acetylating aniline to acetanilide (\(C_6H_5NHCOCH_3\)), the electron pair on nitrogen is delocalized through resonance with the acetyl carbonyl. This reduces the overall activation of the ring and protects it from oxidation.
4. Directing Effect: The bulky acetamido group directs the nitronium ion primarily to the para position. The acetyl group can be easily removed by hydrolysis afterwards to recover the amine.
Step 3: Final Answer:
Acetylation reduces ring reactivity, prevents protonation to the meta-directing anilinium ion, and directs nitration predominantly to the para position.
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