How do intermolecular forces explain the boiling points and solubilities of different organic families?
Relate the physical properties of organic compounds to their intermolecular forces and functional groups
A focused answer to the WACE Year 12 Chemistry dot point on the physical properties of organic compounds, explaining boiling points and water solubility in terms of dispersion forces, dipole-dipole forces and hydrogen bonding across the organic families, with a worked example and common exam mistakes.
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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.
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.