Dipole–Dipole Interactions - Chemistry

1. Fundamental Concepts

Definition: Dipole–dipole interactions occur between molecules with permanent dipoles, where the positive end of one dipole is attracted to the negative end of another.
Polarity: Molecules with a significant difference in electronegativity between atoms will have a permanent dipole moment.
Strength: Dipole–dipole interactions are generally stronger than London dispersion forces but weaker than hydrogen bonds and ionic bonds.

2. Key Concepts

Dipole Moment: $\\mu = Q \cdot d$

Where $$ \\mu $$ is the dipole moment, $$ Q $$ is the charge, and $$ d $$ is the distance between the charges.

Electronegativity Difference: $\\Delta EN > 0.5$

A significant difference in electronegativity (greater than 0.5) typically results in a polar molecule with a permanent dipole.

Application: Dipole–dipole interactions explain the higher boiling points and melting points of polar substances compared to nonpolar substances of similar molar mass.

3. Examples

Example 1 (Basic)

Problem: Identify the type of intermolecular force present in HCl.

Step-by-Step Solution:

Determine the electronegativity values: Chlorine (Cl) has an electronegativity of 3.16, and Hydrogen (H) has an electronegativity of 2.1.
Calculate the electronegativity difference: $$ \\Delta EN = 3.16 - 2.1 = 1.06 $$.
Since $$ \\Delta EN > 0.5 $$, HCl is a polar molecule with a permanent dipole.
Therefore, the primary intermolecular force in HCl is dipole–dipole interaction.

Validation: HCl has a higher boiling point (−85.05°C) compared to nonpolar H₂ (−252.87°C), confirming the presence of dipole–dipole interactions. ✓

Example 2 (Intermediate)

Problem: Compare the boiling points of CH₄ and CH₃Cl.

Step-by-Step Solution:

CH₄ (methane) is a nonpolar molecule because the electronegativity difference between carbon and hydrogen is negligible.
CH₃Cl (methyl chloride) is a polar molecule due to the significant electronegativity difference between chlorine and carbon/hydrogen.
CH₃Cl has dipole–dipole interactions, which are stronger than the London dispersion forces in CH₄.
Therefore, CH₃Cl has a higher boiling point (−24.2°C) compared to CH₄ (−161.5°C).

Validation: The higher boiling point of CH₃Cl confirms the presence of stronger dipole–dipole interactions. ✓

4. Problem-Solving Techniques

Electronegativity Chart: Use a periodic table with electronegativity values to determine the polarity of a molecule.
Molecular Geometry: Consider the molecular geometry to predict the direction of the dipole moment.
Boiling Point Comparison: Compare the boiling points of similar compounds to infer the strength of intermolecular forces.