Q.Answer in one word/sentence: Why tertiary amines not give acylation reaction?
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Acylation of Amines – From Intuition to Precision
Imagine you have a primary amine — say, aniline (CX6HX5NHX2). The nitrogen carries a lone pair and is nucleophilic. Now imagine you bring an acid chloride like acetyl chloride (CHX3COCl) near it. The carbonyl carbon of the acid chloride is electron-deficient (electrophilic). The nitrogen's lone pair attacks that carbon, kicking out the chloride ion. What you get is an amide — a molecule where the nitrogen is now attached to an acyl group (−COCHX3) instead of a hydrogen.
That replacement of an N–H hydrogen by an acyl group (RCOX−) is acylation. It is a nucleophilic acyl substitution reaction.
Acylation is not alkylation. Alkylation adds an alkyl group (RX−) and often leads to over-alkylation (polyalkylation). Acylation adds an acyl group (RCOX−) and stops cleanly at the mono-acylated product because the amide formed is much less nucleophilic than the original amine.
The precise statement
Primary and secondary amines react with acid chlorides (RCOCl) or acid anhydrides ((RCO)X2O) to form substituted amides. One N–H hydrogen is replaced by the acyl group. The by-product is HCl (from acid chlorides) or a carboxylic acid (from anhydrides).
For a primary amine (RNHX2):
RNHX2+RX′COClRX′NHCOR+HCl
For a secondary amine (RX2NH):
RX2NH+RX′COClRX′CONRX2+HCl
Tertiary amines have no N–H hydrogen, so they do not undergo acylation.
A common mistake: thinking acylation works on tertiary amines. It does not — there is no N–H to replace. Tertiary amines can act as bases to neutralise the HCl formed, but they do not form amides.
Why does acylation stop at one step?
After the first acylation, the nitrogen in the amide has its lone pair delocalised into the carbonyl π-system. This makes the amide nitrogen far less nucleophilic than the original amine. So it does not attack another acyl chloride molecule. This is a huge practical advantage over alkylation, where you often get a messy mixture.
Reagents commonly used
- Acid chlorides (e.g., acetyl chloride, benzoyl chloride) — very reactive, often used in the lab.
- Acid anhydrides (e.g., acetic anhydride) — milder, commonly used in industry (e.g., acetylation of aniline to paracetamol intermediate).
- Esters can also acylate, but much more slowly (requires heating).
The Schotten–Baumann technique
In practice, acylation is often done in the presence of a weak base (like aqueous NaOH or pyridine) to neutralise the HCl produced. This prevents the HCl from protonating the unreacted amine (which would stop the reaction). This method is called the Schotten–Baumann reaction. …
Acylation of an amine works by replacing a hydrogen on nitrogen with an acyl group to give a stable, neutral amide, so the reaction depends on the amine actually having that hydrogen available. …
Acylation of an amine substitutes an N-H hydrogen with an acyl group to form a stable amide; a tertiary amine has no N-H, so it cannot form a stable acylated (amide) product.
When a primary or secondary amine reacts with an acyl chloride or acid anhydride, the nitrogen's lone pair first attacks the carbonyl carbon, then the nitrogen loses one of its N-H hydrogens (as HCl or as the leaving acid) to give a neutral, stable amide:
RNH2 + CH3COCl -> RNHCOCH3 + HCl (N-substituted amide)
R2NH + CH3COCl -> R2NCOCH3 + HCl (N,N-disubstituted amide)
…
- CBSE 2024Set A11 markMCQQ.To prepare p-Nitroaniline as a major product from aniline, the amino group is protected by :(a) Acetylation(b) Alkylation(c) Saponification(d) Sulphonation
›Reveal solutionSolution
The amino group is protected by acetylation before nitration to obtain p-nitroaniline as the major product — option (a).
Direct nitration of aniline uses a strongly acidic (nitrating) medium, which protonates −NH2 to the meta-directing anilinium ion and also oxidises aniline, giving substantial meta product and tar. To avoid this, the amino group is first acetylated (with acetic anhydride) to acetanilide, C6H5NHCOCH3. The −NHCOCH3 group is still o,p-directing but less activating, so nit …
- CBSE 2023Set 56/1/11 markMCQQ.Assertion (A) : Acetylation of aniline gives a monosubstituted product. Reason (R) : Activating effect of −NHCOCH3 group is more than that of amino group. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true.
›Reveal solutionSolution
Acetylation of aniline does give a monosubstituted product (at nitrogen), but the reason is wrong: the −NHCOCH3 group is actually less activating than −NH2, not more. Answer: (C)
Understanding Acetylation and Ring Activation
When we talk about acetylation of aniline, we need to be clear about what is being acetylated. Aniline has a nucleophilic amino group that readily reacts with acetic anhydride or acetyl chloride to form acetanilide. This is indeed a monosubstituted product—one acetyl group attaches to the nitrogen.
The question then asks us to evaluate whether the reason given (about relative activating effects) correctly explains this observation.
Step-by-Step Analysis
1. What happens during acetylation of aniline?
Aniline (C6H5NH2) reacts with an acetylating agent like (CH3CO)2O to give:
C6H5NH2+(CH3CO)2O→C6H5NHCOCH3+CH3COOH
The product is acetanilide, where one acetyl group is attached to nitrogen. This is a monosubstituted product, so Assertion (A) is true.
2. Why does acetylation stop at one acetyl group?
Nitrogen in aniline has one lone pair and two hydrogens. After the first acetylation, we get −NHCOCH3, which still has one hydrogen but the nitrogen is now less nucleophilic (the electron-withdrawing carbonyl reduces the availability of the lone pair). A second acetylation is possible under forcing conditions, but under normal conditions we get predominantly the monoacetyl product.
The real reason for monosubstitution is simply that the first acetylation satisfies the typical reaction conditions and the resulting amide nitrogen is much less nucleophilic than the original amine.
3. Now let's examine the Reason (R): Is −NHCOCH3 more activating than −NH2?
This is where we need to understand activating effects toward electrophilic aromatic substitution.
The −NH2 group is one of the most powerful activating groups for the benzene ring. It donates electron density through resonance (the lone pair on nitrogen delocalizes into the ring), making the ring electron-rich and highly reactive toward electrophiles.
When we convert −NH2 to −NHCOCH3, the lone pair on nitrogen is now partially delocalized into the carbonyl group (C=O) through resonance:
−NH−CO−CH3↔−N+H=C−−O−CH3
This means less electron density is available for donation to the benzene ring. The acetyl group is electron-withdrawing by resonance, competing with the ring for nitrogen's lone pair.
ImportantThe activating power follows the order: −NH2>−NHCOCH3>−H …
- CBSE 2022Set HE2181 markQ.Answer in one word/sentence: Why tertiary amines not give acylation reaction?
›Reveal solutionSolution
Acylation of an amine substitutes an N-H hydrogen with an acyl group to form a stable amide; a tertiary amine has no N-H, so it cannot form a stable acylated (amide) product.
When a primary or secondary amine reacts with an acyl chloride or acid anhydride, the nitrogen's lone pair first attacks the carbonyl carbon, then the nitrogen loses one of its N-H hydrogens (as HCl or as the leaving acid) to give a neutral, stable amide:
RNH2 + CH3COCl -> RNHCOCH3 + HCl (N-substituted amide)
R2NH + CH3COCl -> R2NCOCH3 + HCl (N,N-disubstituted amide)
…
- CBSE 2018Set ANNUAL1 markMCQQ.A compound on hydrolysis gives 1°- amine. The compound is-(a) anilide(b) amide(c) cyanide(d) None
›Reveal solutionSolution
Anilide hydrolysis → 1° amine (aniline).
An anilide is the acyl derivative of a primary aromatic amine, e.g. acetanilide C₆H₅–NH–COCH₃. On acid/alkaline hydrolysis:
C₆H₅NHCOCH₃ + H₂O → C₆H₅NH₂ (aniline, a 1° amine) + CH₃COOH.
…
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