Question

Hydrogen bonding plays a central role in the following phenomena :

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

Answer 1

Answer: (A), (B), (D)

The correct options are
A Ice floats in water.
B Higher Lewis basicity of primary amines than tertiary amines in aqueous solutions.
D Dimerisation of acetic acid in benzene.

(A) Ice has cage-like structure in which each water molecule is surrounded by four other water molecules tetrahedrally through hydrogen bonding. Due to this, density of ice is less than water and it floats in water.

(B) $R-N{H}_2+H-OH\rightleftharpoons R-\stackrel{⨁}{N}{H}_3+O{H}^{-}$
$\left(I\right)$

$(R{)}_{3}N+H-OH\rightleftharpoons (R{)}_3-\stackrel{⨁}{N}H+O{H}^{-}$
$\left(II\right)$

The cation $\left(I\right)$ more stabilized through hydrogen bonding than cation $\left(II\right)$. So, $R-N{H}_2$ is better base than $(R{)}_{3}N$ in aqueous solution.

(C) Formic acid is more acidic because it has lesser number of alkyl groups attached and hence, lesser the +I effect of alkyl groups.

(D) Acetic acid exists as a dimer because of Hydrogen bonding as shown in the diagram.

Hence, option A, B and D are correct.