Presence of electron releasing groups on the benzene ring of aromatic amines enhances the electron density on nitrogen, thus increase the basicity. On the contrary, presence of electron withdrawing groups reduces the basicity of amines.
Presence of −NO2 group indicates −I and −R effect, thus enhancing the delocalisation of lone pair on nitrogen with π-electrons of the benzene ring system and reducing the basicity of p-nitroaniline compared to aniline. The electron density on nitrogen is increased due to +R effect of methoxy group, further enhancing its basicity compared to aniline.
The basicity of aromatic amines depends on the following two factors:
Inductive effect dependent on the substituent’s position and occurring in the order ortho- > meta- > para-.
Resonance effect observed in maximum intensity at ortho- and para-positions.
The presence of electron-withdrawing groups on aromatic ring decreases the basicity of substituted anilines. The −R effect of nitro group is maximum at ortho- and para-positions; thus, the basicity of m-nitroaniline is more than that of ortho- and para-isomers as it withdraws electrons via −I effect (inductive effect) only. Since the -I effect is stronger in o-nitroaniline than in p-nitroaniline, the o-nitroaniline experiences a strong −R and −I effect and is least basic.
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