Definition: Hydrogen bonding is the electrostatic attraction between a hydrogen atom attached to a strongly electronegative atom (N, O, F) and a lone pair of another electronegative atom.
1. Why only N, O, F?
These elements are small and highly electronegative. The X–H bond becomes strongly polar; the bare H⁺-like δ+ end can approach lone pairs closely, making the bond unusually strong (10–40 kJ/mol).
2. Types of hydrogen bonds
- Intermolecular: between molecules — water, HF, alcohols (causes high boiling points).
- Intramolecular: within one molecule — o-nitrophenol, salicylic acid (chelation, often cyclic).
3. Consequences
- High boiling point of water (100°C) vs H₂S (−60°C).
- Ice is less dense than water (open hexagonal cage).
- DNA double helix — H-bonds between base pairs.
- Protein folding — H-bonds in α-helix and β-sheet.
- o-nitrophenol is less soluble in water than p-nitrophenol (intramolecular H-bonding in o- blocks water interaction).
Note: Exam point: 'water has a higher boiling point than H₂S despite lower molar mass' is the classic question — answer with intermolecular H-bonding.