JEE Advanced 2014 · Paper 1 · Chemistry Q25

ChemistryMultiple correct+3Question 25 of 60

Hydrogen bonding plays a central role in the following phenomena:

Ice floats in water.

Higher Lewis basicity of primary amines than tertiary amines in aqueous solutions.

Formic acid is more acidic than acetic acid.

Dimerisation of acetic acid in benzene.

Answer and worked solution

Official answer: A, B, D

(A) Ice has an open, hydrogen-bonded tetrahedral lattice, less dense than liquid water. Hydrogen bonding is central.
(B) In water, the protonated primary amine $\ce{RNH3+}$ is stabilised by hydrogen bonding to water through three N-H bonds, while $\ce{R3NH+}$ has only one. This solvation makes primary amines more basic in water. Hydrogen bonding is central.
(C) Formic acid is stronger because the methyl group of acetic acid (+I effect) destabilises the carboxylate. This is an inductive effect, not hydrogen bonding.
(D) Acetic acid dimerises in benzene through two O-H···O hydrogen bonds. Hydrogen bonding is central.

(A), (B), (D)

Topics:Hydrogen bonding Acidity and basicity

JEE Advanced 2014 · Paper 1, Chemistry question 25