Short Answer

Dipole–dipole interactions are attractive forces that occur between molecules that have permanent dipoles. In such molecules, one end is slightly positive (δ+) and the other is slightly negative (δ–). The positive end of one molecule is attracted to the negative end of another, creating an intermolecular attraction.

These interactions are stronger than London dispersion forces but weaker than hydrogen bonds. Dipole–dipole interactions influence boiling points, melting points, solubility, and the physical behaviour of polar molecules.

Detailed Explanation :

Dipole–Dipole Interactions

Dipole–dipole interactions are a type of Van der Waals force that occur between molecules possessing permanent dipoles. A permanent dipole forms when atoms in a molecule have different electronegativities, causing unequal sharing of electrons. This results in a partial positive charge (δ+) on one atom and a partial negative charge (δ–) on another. The attraction between these opposite charges on different molecules forms dipole–dipole interactions.

These interactions play an important role in physical and chemical properties such as boiling point, melting point, solubility, and molecular structure. They are stronger than London dispersion forces because they arise from permanent charge separations rather than temporary ones.

1. How Dipole–Dipole Interactions Form

Dipole–dipole interactions arise from electrostatic attraction between partially positive and partially negative ends of polar molecules.

For dipole–dipole interaction to occur:

1. The molecule must be polar.
2. It must have a permanent dipole moment.

Example:

• In HCl, hydrogen is δ+ and chlorine is δ–.
• The δ+ end of one HCl molecule is attracted to the δ– end of another.

This attraction holds the molecules closer together and influences their physical properties.

2. Conditions Required for Dipole–Dipole Interactions

Dipole–dipole forces occur when:

• There is an electronegativity difference between atoms.
• The molecule has an asymmetrical shape that prevents dipole cancellation.
• The molecule has a net dipole moment.

Molecules like HCl, SO₂, CH₃Cl, and NH₃ all show dipole–dipole interactions because they possess permanent dipoles.

3. How Strong Are Dipole–Dipole Interactions?

Dipole–dipole interactions have moderate strength:

• Stronger than: London dispersion forces
• Weaker than: Hydrogen bonding and ionic attraction

Their strength depends on:

• Magnitude of dipole moment
• Distance between molecules
• Orientation of dipoles

When molecules are aligned correctly (δ+ facing δ–), the force is strongest.

4. Examples of Molecules Showing Dipole–Dipole Interactions

(a) Hydrogen Chloride (HCl)

• Chlorine is more electronegative
• Creates a permanent dipole
• Molecules attract through dipole–dipole forces

(b) Sulfur Dioxide (SO₂)

• Bent shape → dipoles do not cancel
• Shows strong dipole–dipole interactions

(c) Chloroform (CHCl₃)

• One hydrogen and three chlorine atoms create asymmetry
• Dipole–dipole interactions occur

These examples show how molecular shape and electronegativity influence dipole creation.

5. Dipole–Dipole Interactions vs. Other Intermolecular Forces

Dipole–dipole interactions differ from other Van der Waals forces:

(a) Dipole–Dipole vs. Dispersion Forces

• Dipole–dipole requires permanent dipoles
• Dispersion forces occur in all molecules but arise from temporary dipoles
• Dipole–dipole is stronger

(b) Dipole–Dipole vs. Hydrogen Bonding

• Hydrogen bonding is a special, stronger type of dipole–dipole interaction
• Occurs only when H is bonded to N, O, or F
• Dipole–dipole interactions occur between any polar molecules

Thus, all hydrogen bonds are dipole–dipole interactions, but not all dipole–dipole interactions are hydrogen bonds.

6. Effects of Dipole–Dipole Interactions on Physical Properties

Dipole–dipole interactions significantly influence how substances behave.

(a) Boiling and Melting Points

Polar molecules with dipole–dipole interactions have higher boiling and melting points compared to non-polar molecules of similar mass.

Example:

• HCl boils at a higher temperature than F₂ due to dipole–dipole attraction.

(b) Solubility

Polar molecules dissolve well in polar solvents due to similar intermolecular forces.

Example:

• CH₃Cl dissolves in water better than non-polar gases.

(c) Viscosity and Surface Tension

Stronger intermolecular attraction increases viscosity and surface tension.

(d) Interactions in Liquids

Dipole–dipole interactions help explain why polar liquids exhibit strong attractions and form stable structures.

7. Orientation Dependence

Dipole–dipole interactions depend heavily on the orientation of molecules:

• When the δ+ end faces the δ– end, attraction is strong.
• When two δ+ ends face each other, repulsion occurs.

Because molecules are constantly moving, the attractions and repulsions average out, but overall attraction remains.

8. Role in Chemical and Biological Systems

Dipole–dipole interactions are important in:

• Protein folding
• Binding of drugs to receptors
• Formation of molecular complexes
• Solvent–solute interactions

They help determine the shapes and functions of many biological molecules.

Conclusion

Dipole–dipole interactions are attractive forces between polar molecules with permanent dipoles. They arise from the electrostatic attraction between δ+ and δ– ends of molecules. These forces are stronger than dispersion forces but weaker than hydrogen bonds. Dipole–dipole interactions influence boiling points, solubility, viscosity, and many physical and biological properties. Understanding these interactions helps explain the behaviour of polar substances in different conditions.