Trends in Physical Properties of Homologous Series

Specification Reference S3.2.4

Quick Notes

Functional group influences intermolecular forces (e.g. hydrogen bonding vs. dipole–dipole).
Branching tends to lower boiling points due to reduced surface contact.

Within a homologous series (e.g. alkanes, alcohols, acids), each member differs by a –CH₂– unit.

As molecular size increases:
- Boiling points and melting points generally increase
- This is due to stronger intermolecular forces between molecules

Larger molecules have:

More electrons and a larger surface area
This leads to stronger London (dispersion) forces
As a result, more energy is required to overcome intermolecular attractions between molecules, giving higher melting and boiling points.

Structure 2.2 – Linked Course Question

What is the influence of carbon chain length, branching, and functional groups on intermolecular forces?

Longer carbon chains increase surface area and lead to stronger London dispersion forces between molecules, raising boiling points.
Branching reduces how closely molecules can pack, weakening intermolecular forces and lowering boiling points.
The nature of the functional group determines the type of force present:
- Polar groups (e.g. –OH, –COOH) create dipole–dipole or hydrogen bonding, increasing boiling and melting points.
- Non-polar groups rely mainly on London dispersion forces.
Together, these factors affect physical properties like boiling point, solubility, and volatility.

Melting and boiling points increase with chain length in homologous series due to stronger intermolecular forces.
Branching lowers boiling points because of weaker packing interactions.
Functional groups determine the type of intermolecular force present.
Hydrogen bonding and dipole–dipole interactions cause higher melting and boiling points than non-polar forces.