Relate the physical properties of organic compounds to their intermolecular forces and functional groups

What this dot point is asking

The physical properties of organic compounds, especially boiling point and water solubility, are governed by the intermolecular forces between molecules. The stronger these forces, the more energy is needed to separate molecules, so the higher the boiling point.

The three intermolecular forces

• Dispersion forces exist between all molecules. They arise from temporary fluctuating dipoles and strengthen as molecular size and surface area increase.
• Dipole-dipole forces occur between polar molecules (those with a permanent dipole, such as ketones and haloalkanes) and are stronger than dispersion forces for molecules of similar size.
• Hydrogen bonding is the strongest, occurring when hydrogen is bonded to nitrogen, oxygen or fluorine. Alcohols, carboxylic acids and amines can hydrogen bond.

Boiling point trends across families

For molecules of comparable carbon number, the boiling point order reflects the strongest force present. Carboxylic acids and alcohols (hydrogen bonding) boil highest; aldehydes and ketones (dipole-dipole) are intermediate; alkanes (dispersion only) boil lowest. Carboxylic acids boil even higher than alcohols of similar size because they can form two hydrogen bonds, dimerising in the pure liquid.

Water solubility

A molecule is water-soluble if it can form favourable interactions (especially hydrogen bonds) with water. Small alcohols, carboxylic acids and amines are very soluble because their polar group hydrogen bonds with water. As the non-polar carbon chain grows, it dominates and solubility falls: methanol and ethanol mix freely with water, but longer-chain alcohols become increasingly insoluble. Alkanes are essentially insoluble because they are non-polar.

Carboxylic acid dimers and the volatility ladder

A detail that earns top marks is the dimerisation of carboxylic acids. Each $\text{-COOH}$ group has both a hydrogen-bond donor (the $\text{O-H}$) and an acceptor (the C=O oxygen), so two molecules pair head-to-head through two hydrogen bonds. The effective particle is therefore roughly double the size and doubly bonded, which is why carboxylic acids boil markedly higher than alcohols of similar molar mass. Reading across a fixed carbon number, the volatility ladder runs from alkanes (most volatile, dispersion only), up through haloalkanes and aldehydes and ketones (dipole-dipole), then alcohols and amines (single hydrogen bonds), to carboxylic acids (double hydrogen bonds, least volatile). Being able to place an unknown family on this ladder, and justify it by the strongest force present, is the recurring physical-properties skill in Unit 4.

Why this matters

These ideas explain why alcohols are good solvents, why carboxylic acids are liquids while similar alkanes are gases, and why long-chain molecules are greasy and water-repellent. They also link to separation and purification techniques (such as distillation) used in synthesis, and underpin the property questions that recur in the examination.