Short Answer

Hydrogen bonding is a special type of strong intermolecular attraction that occurs when hydrogen is bonded to highly electronegative atoms like nitrogen (N), oxygen (O), or fluorine (F). These atoms pull electrons strongly, making hydrogen partially positive and able to attract nearby electronegative atoms.

Hydrogen bonding is stronger than other Van der Waals forces but weaker than covalent bonds. It plays an important role in determining boiling points, solubility, molecular structure, and the behavior of substances like water, alcohols, DNA, and proteins.

Detailed Explanation :

Hydrogen Bonding

Hydrogen bonding is an important intermolecular force that occurs when a hydrogen atom is directly bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine. These atoms strongly attract electrons, leaving the hydrogen atom with a significant partial positive charge (δ+). This hydrogen can then attract the lone pair of electrons on another electronegative atom in a nearby molecule, creating a hydrogen bond.

Hydrogen bonding is stronger than dipole–dipole interactions and dispersion forces because the charge separation is greater. However, it is still weaker than covalent or ionic bonds. Hydrogen bonding is responsible for many unique physical and chemical properties of substances, especially water.

1. How Hydrogen Bonding Occurs

Hydrogen bonding forms through the following conditions:

1. Hydrogen must be bonded to N, O, or F.
2. The hydrogen atom becomes highly δ+ due to strong electronegativity difference.
3. A nearby molecule must have a lone pair on N, O, or F.
4. Hydrogen forms an attraction between its δ+ end and the lone pair of the electronegative atom.

Example:
In water (H₂O):

• O is highly electronegative → pulls electron density
• H becomes δ+
• Each water molecule can form hydrogen bonds with four others

This strong network creates many of water’s unusual properties.

2. Types of Hydrogen Bonding

Hydrogen bonding occurs in two forms:

(a) Intermolecular Hydrogen Bonding

Occurs between molecules.

Examples:

• Water molecules bonding with each other
• Hydrogen bonding in alcohols (ethanol)
• HF molecules bonding in chains

This type influences boiling points, melting points, viscosity, and solubility.

(b) Intramolecular Hydrogen Bonding

Occurs within the same molecule.

Example:

• o-Nitrophenol forms an internal hydrogen bond, reducing solubility and boiling point compared to other isomers.

Intramolecular hydrogen bonding reduces the ability of molecules to interact with others.

3. Strength of Hydrogen Bonding

Hydrogen bonds are relatively strong compared to other intermolecular forces:

Strength order:

Even though hydrogen bonds are much weaker than covalent bonds, many hydrogen bonds together can create very strong interactions, as seen in DNA.

4. Examples of Hydrogen Bonding

(a) Water (H₂O)

Each water molecule forms up to four hydrogen bonds.
This leads to:

• High boiling point
• High surface tension
• Ice being less dense than liquid water

These unusual properties are due to hydrogen bonding.

(b) Hydrogen Fluoride (HF)

Forms strong hydrogen bonds because fluorine is highly electronegative.

(c) Ammonia (NH₃)

Forms hydrogen bonds, though fewer than water, because N has fewer lone pairs.

(d) Alcohols and Carboxylic Acids

Hydrogen bonding explains their:

• High boiling points
• Solubility in water

(e) DNA and Proteins

Hydrogen bonds between base pairs stabilize the double helix structure of DNA.
Protein folding also relies on hydrogen bonding between amino acid groups.

5. Impact of Hydrogen Bonding on Physical Properties

Hydrogen bonding affects many properties:

(a) Boiling and Melting Points

Substances with hydrogen bonding have unusually high boiling points.

Example:
Water boils at 100°C but H₂S (which lacks hydrogen bonding) boils at –60°C.

(b) Solubility

Hydrogen bonding increases solubility in polar solvents like water.

Example:
Alcohols dissolve easily in water due to hydrogen bonding.

(c) Density of Ice

Hydrogen bonding arranges water molecules into an open structure, making ice less dense than liquid water.

(d) Surface Tension and Viscosity

Water has high surface tension and viscosity, mainly because of hydrogen bonding.

6. Role of Hydrogen Bonding in Biology

Hydrogen bonding is essential in biological systems:

(a) DNA Stability

A–T pairs form two hydrogen bonds; G–C pairs form three.
These hydrogen bonds hold the DNA double helix together.

(b) Protein Structure

Hydrogen bonds stabilize α-helices and β-sheets.

(c) Enzyme–substrate interaction

Hydrogen bonding helps enzymes recognize and bind to substrates.

Without hydrogen bonding, biological molecules would not maintain their shapes or functions.

Conclusion

Hydrogen bonding is a special type of intermolecular attraction that occurs when hydrogen is bonded to highly electronegative atoms such as N, O, or F. This creates a strong partial positive charge on hydrogen, allowing it to attract a lone pair on a nearby electronegative atom. Hydrogen bonding plays a crucial role in determining physical properties like boiling points, solubility, and molecular structure. It is also essential in biological systems such as DNA and proteins.