JEE Advanced 2020 · Paper 2 · Chemistry Q11

ChemistryMultiple correct+4 / -2Question 29 of 54

Consider the following four compounds I, II, III, and IV.

Choose the correct statement(s).

The order of basicity is II > I > III > IV.

The magnitude of p

K_b

difference between I and II is more than that between III and IV.

Resonance effect is more in III than in IV.

Steric effect makes compound IV more basic than III.

Answer and worked solution

Official answer: C, D

Effects of the substituents.

In III the $\ce{NH2}$ lies in the ring plane, so its lone pair is delocalised strongly into the three nitro groups; III is very weakly basic.
In IV the two N-methyl groups clash with the ortho nitro groups. The $\ce{NMe2}$ twists out of the ring plane, so this resonance is largely lost (steric inhibition of resonance), and IV is much more basic than III.

The statements.

(A) The order is II > I > IV > III, not ending III > IV. False.
(B) I and II differ by less than one pK unit, while III and IV differ by several. False.
(C) Resonance is greater in III than in IV. True.
(D) The steric effect makes IV more basic than III. True.

(C), (D)

JEE Advanced 2020 · Paper 2, Chemistry question 11