(c) Why is –NH₂ group of aniline acetylated before carrying out nitration?
(c) Why is –NH₂ group of aniline acetylated before carrying out nitration?
The –NH₂ group of aniline is acetylated (protected as acetanilide) before nitration to moderate the ring's very high reactivity and to avoid unwanted products.
Reason
- The –NH₂ group is a strong activator, so an unprotected ring is over-reactive towards the electrophile.
- In the strongly acidic nitrating mixture (conc. HNO₃ + H₂SO₄), aniline is protonated to the anilinium ion (C₆H₅NH₃⁺), which is deactivating and meta-directing — giving a large amount of m-nitroaniline.
- Nitric acid also oxidises the easily oxidisable –NH₂ group, forming tarry products.
How acetylation helps
- Aniline + (CH₃CO)₂O → acetanilide (C₆H₅NHCOCH₃).
- The –NHCOCH₃ group is only moderately activating and mainly o-/p-directing, so nitration occurs predominantly at the para position.
- Acid (or base) hydrolysis then regenerates the amino group, giving p-nitroaniline as the major product.
Marking Scheme
- 11 mark: stating that acetylation converts –NH₂ to –NHCOCH₃ (acetanilide) to reduce the strong activating/oxidisable and meta-directing (anilinium) behaviour, so nitration gives mainly the para product; hydrolysis restores –NH₂.
- 2Full mark accepted for any correct reason: control of high reactivity, prevention of oxidation by HNO₃, or avoidance of the deactivating meta-directing anilinium ion.
Hint
Think about what happens to –NH₂ in strong acid (it becomes –NH₃⁺) and how reactive/oxidisable a bare –NH₂ ring is.
Quick Oral Answer
We acetylate aniline to acetanilide so the amino group is protected — this lowers the ring's reactivity, stops oxidation by nitric acid and removes the meta-directing anilinium ion, letting nitration give mainly the para product, which is recovered by hydrolysis.
Analysis & Explanation
Concept
Electrophilic aromatic substitution on aniline is complicated by two problems: excessive reactivity and salt formation in acid.
- The lone pair on nitrogen is delocalised into the ring, making aniline so reactive that oxidation and polysubstitution compete with clean mononitration.
- Because nitration needs a strongly acidic medium, most of the aniline exists as the anilinium ion; the positively charged –NH₃⁺ is deactivating and meta-directing, so direct nitration gives a significant meta fraction (about 47% meta in practice).
Exam trap
Students often say acetylation only 'increases para product'. The complete answer must mention all three ideas: controlling high reactivity, preventing oxidation by HNO₃, and avoiding the meta-directing anilinium ion.
Real-world
This 'protect–react–deprotect' strategy is a classic of synthetic chemistry; p-nitroaniline made this way is a key intermediate for dyes and pharmaceuticals.
Common Mistakes
- 1Saying acetylation is done 'to increase basicity' — it actually reduces reactivity by protecting the lone pair.
- 2Forgetting that in acidic medium aniline forms the meta-directing anilinium ion, and only citing the oxidation reason.
- 3Not mentioning the final hydrolysis step that regenerates the –NH₂ group to give p-nitroaniline.
Interesting Facts
Direct nitration of aniline gives roughly 51% ortho, 47% meta and only 2% para product, which is why chemists protect the amine first.
Acetic anhydride is the usual acetylating agent because it acylates the strongly nucleophilic aniline nitrogen far faster than it reacts with the ring.
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Frequently Asked Questions
Which reagent is used to acetylate aniline?
Acetic anhydride, (CH₃CO)₂O (acetyl chloride also works), converts aniline to acetanilide, C₆H₅NHCOCH₃, by acylating the nitrogen lone pair.
Why does direct nitration of aniline give a meta product?
In the strongly acidic nitrating mixture, aniline is protonated to the anilinium ion (–NH₃⁺), which is deactivating and meta-directing, so a large fraction of m-nitroaniline is formed.