Chemistry · General Organic Chemistry

JEE Advanced 2020 — Paper 2 — Question 27

Consider the following four compounds I, II, III and IV. Choose the correct statement(s).

Option A:
The order of basicity is II $>$ I $>$ III $>$ IV.
Option B:
The magnitude of $\mathrm{pK}_{\mathrm{b}}$ difference between I and II is more than that between III and IV.
Option C:
Resonance effect is more in III than in IV.
Correct
Option D:
Steric effect makes compound IV more basic than III.
Correct

Answer: C, D

(III) is the weakest base due to the presence of three $-\mathrm{NO}_{2}$ group, which exert strong -M effect.

In compound (IV) $\sim_{\text {Me }}^{\text {Me }}$ group is thrown out of the plane of aromatic ring due to steric crowding by $-\mathrm{NO}_{2}$

groups at ortho positions as a result of this lone pair of electron on $\mathrm{N}-$ atom does not participate in resonance

with aromatic ring and hence it acts as an aliphatic amine. So, (IV) is the strongest base.

Correct order of basicity is; III < I $<$ II $<$ IV (in basic character).

Difference of $\mathrm{pK}_{\mathrm{a}}$ values is highest between (III) and (IV).

$\mathrm{pK}_{\mathrm{b}} ;(\mathrm{I}) \rightarrow 9.42$ ; (II) $\rightarrow 8.94$ , (IV) $\rightarrow 4.60$

Answer key and solution verified before publishing.

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