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Consider four aniline derivatives: I: Aniline (C6H5NH2) II: N,N-dimethylaniline (C6H5N(CH3)2) III: 2,4,6-trinitroaniline IV: N,N-dimethyl-2,4,6-trinitroaniline Which of the following statement(s) is/are correct?

1. The order of basicity is II > I > III > IV.
2. The difference in pKb between I and II is greater in magnitude than the difference between III and IV.
3. The resonance effect (lone pair delocalization into the ring) is more pronounced in III than in IV.
4. The steric effect of the N-methyl groups makes compound IV more basic than III.

Correct answer: The difference in pKb between I and II is greater in magnitude than the difference between III and IV.

Solution

The three ortho and para nitro groups in III and IV strongly withdraw electron density and sterically hinder the nitrogen lone pair from resonating into the ring. The basicity order is II > I > IV > III because in IV the N-methyl groups provide slight steric relief from the ortho nitro groups (reducing coplanarity and thereby allowing a tiny bit more electron density on N), making IV slightly more basic than III.

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