Assertion (A): The activating effect of the –NHCOCH3 group in acetanilide is less than that of the –NH2 group in aniline.
Reason (R): To prepare a monosubstituted derivative of aniline, the –NH2 group is first protected by acetylation with acetic anhydride, and after the substitution the substituted amide is hydrolysed to the substituted amine.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
Aniline is acetylated with acetic anhydride to protect the –NH2 group before a substitution in the ring:
\( \displaystyle \ce{C6H5NH2 + (CH3CO)2O -> C6H5NHCOCH3 + CH3COOH} \)
Assertion (A): The activating effect of the –NHCOCH3 group in acetanilide is less than that of the –NH2 group in aniline. — True. This is why acetanilide can be monosubstituted, for example nitrated to give mainly the p-nitro derivative.
Reason (R): To prepare a monosubstituted derivative of aniline, the –NH2 group is first protected by acetylation with acetic anhydride, and after the substitution the substituted amide is hydrolysed to the substituted amine. — True. This is how the very high reactivity of aniline is controlled: protect the group, carry out the substitution, then hydrolyse the amide back to the amine.
Why R does not explain A?
R gives the steps of the protection method; it does not say why the –NHCOCH3 group activates the ring less.
In fact A is the reason the method works.
The correct reason for A is that the lone pair on nitrogen of acetanilide interacts with the oxygen atom by resonance, so it is less available for donation to the benzene ring.
NCERT (2026-27 edition):
9.6 Chemical Reactions – “If we have to prepare monosubstituted aniline derivative, how can the activating effect of –NH2 group be controlled ? 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.”