JEE Advanced 2016 · Paper 2 · Chemistry Q23 · Dronacharya Exams

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ChemistrySingle correct+3 / -1Question 23 of 54

The correct order of acidity for the following compounds is

Four hydroxybenzoic acids: I, 2,6-dihydroxybenzoic acid; II, 2-hydroxybenzoic acid (salicylic acid); III, 3-hydroxybenzoic acid; IV, 4-hydroxybenzoic acid.

A

I > II > III > IV

B

III > I > II > IV

C

III > IV > II > I

D

I > III > IV > II

Answer and worked solution

Official answer: A

I (2,6-dihydroxybenzoic acid), pKₐ ≈ 1.3: two ortho-OH groups H-bond to and stabilise the carboxylate.
II (salicylic acid), ≈ 3.0: one such H-bond.
III (meta-OH), ≈ 4.1: −I effect only.
IV (para-OH), ≈ 4.6: +R donation destabilises the anion.

Order: I > II > III > IV.

(A)

Topics:Acidity of carboxylic acids Hydrogen bonding

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