S3.2.4 Physical trends in homologous series Notes

Melting and Boiling Points, Volatility, and Solubility of Organic Compounds

How Molecular Size Affects Boiling Points

1. Boiling occurs when molecules in a liquid gain enough energy to overcome the forces holding them together.
2. For organic compounds, these forces are primarily intermolecular forces, such as London dispersion forces, dipole-dipole interactions, and hydrogen bonding.
3. As molecular size increases, such as in a homologous series like the alkanes, boiling points also rise.
4. Why? Larger molecules have more electrons, which increases the strength of Van der Waals forces.
5. These forces result from temporary dipoles created by the random movement of electrons.

Effect of Branching on Physical Trends in Homologous Series

Branching significantly influences the physical properties of compounds in a homologous series:

1. Boiling Point:
   1. Branching reduces the surface area available for intermolecular forces, weakening Van der Waals forces and lowering boiling points compared to straight-chain isomers.
2. Melting Point:
   1. Branching disrupts regular crystal packing, often lowering melting points.
   2. However, highly symmetrical branching can enhance packing efficiency, leading to higher melting points.

The Effect of Functional Groups on Boiling Points

Functional groups can significantly impact boiling points by introducing polar interactions or hydrogen bonding, both of which strengthen intermolecular forces.

1. Hydroxyl Groups (-OH):
   1. Alcohols, which contain hydroxyl groups, exhibit hydrogen bonding—a particularly strong type of dipole-dipole interaction.
   2. This raises their boiling points compared to alkanes of similar molecular size.
2. Carbonyl Groups (C=O):
   1. Aldehydes and ketones, with their polar carbonyl groups, experience dipole-dipole interactions.
   2. These are weaker than hydrogen bonds but still stronger than dispersion forces.