The amino group is ortho, para directing and a powerful activating group, so aryl amines undergo the typical electrophilic substitution reactions readily. In fact, the rate of these reactions is so great that in certain cases poly-substitution, rather than mono-substitution, occurs even under mild conditions. If the reaction has to be stopped at the mono-substitution stage, the activating effect of the –NH2 group has to be controlled.
To control the activating effect of the –NH2 group, aniline is first acetylated to acetanilide. Why is the –NHCOCH3 group a weaker activating group than the –NH2 group?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → The lone pair of electrons on nitrogen interacts with the oxygen atom of the acetyl group by resonance, so it is less available for donation to the benzene ring.
The –NH2 group activates the ring because the lone pair on nitrogen is donated into the ring by resonance.
In acetanilide the same lone pair is also in conjugation with the C=O group of the acetyl group, so it is shared between the ring and the oxygen. Less of it reaches the ring, and the activation is milder.
\( \displaystyle \ce{C6H5NH2 + (CH3CO)2O -> C6H5NHCOCH3 + CH3COOH} \)
After the substitution, hydrolysis of the amide gives back the –NH2 group.
Why the other options are incorrect
Option (2) The nitrogen of acetanilide is protonated in the reaction mixture, so the group becomes positively charged and meta directing. — Incorrect.
Protonation to a meta directing ion is what happens to aniline in a strongly acidic medium (the anilinium ion).
It is not the reason why the acetyl group lowers the activation; acetanilide gives the para product.
Option (3) The bulky acetyl group blocks the ortho and para positions of the benzene ring from the attacking electrophile. — Incorrect. The acetyl group does not block the ring positions; substitution still takes place at the para position. The change is in the availability of the lone pair, not in crowding.
Option (4) The lone pair of electrons on nitrogen is used up in forming the bond with the acetyl group, so nitrogen has no lone pair left. — Incorrect.
Nitrogen in acetanilide still has its lone pair; the bond to the acetyl group uses one of its unpaired electrons, as the N–H bond did.
The lone pair is only delocalised towards oxygen, so –NHCOCH3 is still an activating group, only a weaker one.
NCERT (2026-27 edition):
9.6 Chemical Reactions – “…This can be done by protecting the -NH2 group by acetylation with acetic anhydride, then carrying out the desired substitution followed by hydrolysis of the substituted amide to the substituted amine.”
“The lone pair of electrons on nitrogen of acetanilide interacts with oxygen atom due to resonance … Hence, the lone pair of electrons on nitrogen is less available for donation to benzene ring by resonance. Therefore, activating effect of –NHCOCH3 group is less than that of amino group.”