(i) Acetylation involves the addition of an acetyl group to a compound. An acetyl group is made up of a carbonyl group (carbon double bonded to oxygen) with a methyl group (CH3) on the end. The part of the acetyl group that is attached to the compound is represented as R.
Acetylation of aniline which is an aromatic amine (NH2 group on benzene) yields acetanilide (NHCOCH3 group on benzene). The lone pair of electrons on the nitrogen of acetanilide interacts with oxygen atom due to resonance
So, the lone pair of electrons on nitrogen is less available for donation to the benzene ring by resonance. Therefore, activating the effect of –NHCOCH3 group is less than that of the amino group. Hence acetylation of aniline reduces its activation effect.
(ii) Basicity of an amine depends upon the ease of formation of the cation by accepting a proton from the acid. If the cation is more stable compared to the amine, more basic is the amine.
Now, in the case of CH3NH2 , due to the electron releasing nature of alkyl group (CH3) it pushes electrons towards nitrogen and thus makes the unshared electron pair more available for sharing with the proton of the acid. Hence, protonation is easy leading to high basicity.
Now, in the case of aniline, NH2 group is directly linked with benzene. It causes the unshared electron pair on nitrogen atom to be in conjugation with the benzene ring and thus making it less available for protonation. Therefore, basicity decreases.
Hence methylamine is more basic than aniline.
(iii) Direct nitration of aniline yields the oxidation products in addition to the nitro derivatives. Moreover, in the strongly acidic medium, aniline is protonated to form
the anilinium ion which is meta directing. That is why , -NH2 being ortho/para directing group gives a significant amount of meta derivative, along with ortho/para derivatives.
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