Question

Hydrogen bonding plays a central role in which of the following phenomena?

• A. Ice floats in water
• B. Dimerisation of acetic acid in benzene
• C. Higher Lewis basicity of primary amines than tertiary amines in aqueous solutions
• D. Formic acid is more acidic than acetic acid

Answer 1

Answer: (A), (B), (C)

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

(a) Density of ice is less than water due to cage-like structure, in which each water molecule is surrounded by four other water molecules tetrahedrally through H- bonding. Hence, ice floats in water.
(b) $\underset{1^{o}amine}{RN{H}_2}+H-OH\rightleftharpoons \underset{I}{RN{H}_3^{+}}+O{H}^{-}\underset{3^{o}amine}{R_3-N}+H-OH\rightleftharpoons \underset{II}{R_{3}N{H}^{+}}+O{H}^{-}$
The cation (I) is more stabilised through hydrogen bonding than cation (II). So, $R-N{H}_2$ is stronger base than $R_{3}N$ in aqueous solution.
(c) $HCOOH$ is stronger acid than $C{H}_{3}COOH$ due to inductive effect and not due to hydrogen bonding.
(d) Acetic acid dimerises in benzene through intermolecular hydrogen bonding.