Hydrogen Bonding

Full Notes

Introduction to Hydrogen Bonding

Hydrogen bonding is a special type of dipole–dipole interaction that arises when hydrogen is covalently bonded to a highly electronegative atom such as fluorine (F), oxygen (O), or nitrogen (N). Due to the small size and high electronegativity of these atoms, the bonded hydrogen carries a significant partial positive charge (δ⁺), leaving its proton relatively exposed.

This exposed δ⁺ hydrogen is strongly attracted to a lone pair of electrons on another nearby electronegative atom, forming a hydrogen bond. Though weaker than covalent bonds, hydrogen bonds are stronger than van der Waals forces, and they play a critical role in determining the structure and properties of many molecular systems.

4.9.1 Cause of Formation of Hydrogen Bond

Hydrogen bonding arises under two specific conditions:

• A highly electronegative atom (F, O, or N) covalently bonded to hydrogen, creating a strongly polar bond.
• A lone pair of electrons on a neighbouring electronegative atom, available to interact with the exposed hydrogen nucleus.

In molecules such as N–H, O–H, or F–H, the hydrogen atom’s nucleus becomes partially uncovered due to the strong electron pull of the bonded atom. This allows a lone pair on another N, O, or F atom to form a strong electrostatic attraction – the hydrogen bond.

4.9.2 Types of Hydrogen Bonds

Hydrogen bonding can occur both between molecules and within a single molecule, giving rise to two main types: intermolecular hydrogen bonding and intramolecular hydrogen bonding.

Intermolecular Hydrogen Bonding

This occurs between separate molecules, often leading to higher boiling and melting points than otherwise expected and greater solubility in polar solvents.

Examples:

• Water (H₂O): Each molecule can form up to four hydrogen bonds, giving water its high boiling point, high surface tension, and unique density anomaly.
• Hydrogen fluoride (HF): Forms extended zig-zag chains through hydrogen bonding, resulting in an unusually high boiling point for a diatomic molecule.
• Alcohols (e.g., ethanol): The –OH group forms hydrogen bonds with water, enhancing miscibility and solubility.

Intramolecular Hydrogen Bonding

Occurs within the same molecule, typically when the hydrogen atom is situated between two electronegative atoms that are part of the same molecule. This type of bonding can lead to the formation of a closed ring structure.

Examples:

• Ortho-nitrophenol: Shows intramolecular hydrogen bonding between the –OH group and the –NO₂ group. As a result, it is less soluble in water and has a lower boiling point than its para-isomer.

Note: Intramolecular bonding may lead to a non-polar structure overall, despite the presence of polar functional groups.

Summary

• Hydrogen bonding occurs when H is bonded to F, O, or N and interacts with a lone pair on an electronegative atom.
• Intermolecular hydrogen bonding raises boiling and melting points and increases solubility in polar solvents.
• Intramolecular hydrogen bonding can reduce solubility and lower boiling point compared to isomers.
• Hydrogen bonding influences structure, properties, and many biological functions.