SolutionsPhysical Properties
1. Order by basicity
Order from most to least basic: aniline, 4-methylaniline, 4-nitroaniline and 4-chloroaniline (pKb: 9.4; 8.9; 13.0; 10.0).
4-Methylaniline (8.9) > aniline (9.4) > 4-chloroaniline (10.0) > 4-nitroaniline (13.0).
The methyl group donates electron density (inductive effect and hyperconjugation); chlorine withdraws it inductively; the para nitro group also withdraws it by resonance, delocalizing the nitrogen lone pair onto the oxygens.
2. Meta versus para
Why is 3-nitroaniline (pKb 11.5) more basic than 4-nitroaniline (pKb 13.0)?
From the meta position the nitro group only withdraws electrons inductively. In para, in addition, the lone pair of the amino group is delocalized by resonance onto the nitro group (a resonance form with N+= on the amino group and negative charge on the oxygens), which makes it much less available to take up a proton.
3. The methoxy group
A para methoxy group increases the basicity of aniline (pKb 8.7), but a meta one decreases it (9.8). Explain this difference.
In para, the oxygen lone pair is delocalized into the ring and increases the electron density on the carbon bonded to nitrogen; the nitrogen lone pair is less delocalized and the amine is more basic. In meta the resonance does not reach that carbon and the electron-withdrawing inductive effect of oxygen, more electronegative than carbon, dominates.