Hydrogen bonding is a special type of intermolecular force that occurs when a hydrogen atom is covalently bonded to a highly electronegative atom (like N, O, or F) and is also attracted to another electronegative atom in a nearby molecule. This attraction is stronger than typical dipole-dipole interactions but weaker than a covalent bond.

Let's analyze each phenomenon to see if hydrogen bonding is the central role:

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

This is NOT primarily due to hydrogen bonding. In aqueous solution, tertiary amines are stronger bases than primary amines because of the +I (inductive) effect of alkyl groups, which stabilizes the conjugate acid. Hydrogen bonding with water solvates the conjugate acid of primary amines more effectively, making them less basic in water. So, hydrogen bonding actually reduces the basicity of primary amines compared to tertiary amines in water; it is not the reason for their higher basicity.

2. Dimerisation of acetic acid in benzene

This is directly due to hydrogen bonding. In non-polar solvents like benzene, acetic acid molecules form dimers through two strong intermolecular hydrogen bonds between the carbonyl oxygen of one molecule and the hydroxyl hydrogen of another. The structure can be represented as a cyclic dimer.

$2\mathrm{CH}{\mathrm{CO}}_{2}H\rightleftharpoons (\mathrm{CH}{\mathrm{CO}}_{2}H{)}_2$

3. Formic acid is more acidic than acetic acid

This is NOT primarily due to hydrogen bonding. The higher acidity of formic acid (pK_a ≈ 3.75) compared to acetic acid (pK_a ≈ 4.76) is due to the +I (electron-donating) effect of the methyl group (-CH₃) in acetic acid. This group destabilizes the acetate anion by increasing the electron density on the carboxylate group, making it harder to lose a proton. Hydrogen bonding is similar in both their conjugate bases and is not the differentiating factor.

4. Ice floats in water

This is a classic example of hydrogen bonding. In liquid water, hydrogen bonds are constantly forming and breaking. In ice, water molecules form a rigid, hexagonal crystalline lattice held together by hydrogen bonds. This structure is more open and has a lower density than liquid water. Therefore, ice is less dense than water and floats.

Conclusion:

Hydrogen bonding plays the central role in Dimerisation of acetic acid in benzene and Ice floats in water.

Related Topics & Formulae

Intermolecular Forces: The strength of intermolecular forces influences physical properties like boiling point, solubility, and density. The order of strength is: Ion-Ion > Hydrogen Bonding > Dipole-Dipole > London Dispersion.

Hydrogen Bonding Criteria: For a hydrogen bond X-H...Y to form, both X and Y must be highly electronegative (usually N, O, F) and Y must have a lone pair of electrons.

Effect on Properties: Substances with hydrogen bonding have anomalously high melting and boiling points (e.g., H₂O vs. H₂S).