Intermolecular forces in organic compounds
| Series | Strongest intermolecular force |
|---|
| Alkanes, alkenes, alkynes | London (dispersion) forces only |
| Haloalkanes, aldehydes, ketones, esters | Dipole–dipole forces (plus London forces) |
| Alcohols | Hydrogen bonding (one site per molecule) |
| Carboxylic acids | Hydrogen bonding at two sites — molecules form dimers |
📖 Definitions
Boiling point: the temperature at which the vapour pressure of a liquid equals the atmospheric (external) pressure.
Vapour pressure: the pressure exerted by a vapour at equilibrium with its liquid in a closed system.
Melting point: the temperature at which the solid and liquid phases of a substance are in equilibrium.
Stronger intermolecular forces → more energy needed to overcome them → higher boiling and melting points and lower vapour pressure.
The three trends
- Chain length (same series): longer chain → larger surface area / molecular size → stronger London forces → higher boiling point. Methanol 65 °C, ethanol 78 °C, propan-1-ol 97 °C, butan-1-ol 117 °C.
- Branching (chain isomers): more branches → more compact, smaller contact surface area → weaker London forces → lower boiling point. Pentane 36 °C, 2-methylbutane 28 °C, 2,2-dimethylpropane 10 °C.
- Functional group (similar molar mass): alkane < aldehyde/ketone < alcohol < carboxylic acid. Butane −1 °C, propanal 49 °C, propan-1-ol 97 °C, ethanoic acid 118 °C (all about 58–60 g·mol⁻¹).
✏️ Model answer: explaining a difference in boiling point
1Compare the structure: “Propan-1-ol has a hydroxyl group; propanal has a carbonyl group.”
2Compare the intermolecular forces: “Propan-1-ol has hydrogen bonds between molecules; propanal has dipole–dipole forces, which are weaker.”
3Compare the energy: “More energy is needed to overcome the intermolecular forces in propan-1-ol, so its boiling point is higher.”
Note: carboxylic acids boil higher than alcohols of similar molar mass because they form hydrogen bonds at two sites (dimers).
Investigation skills
Most questions on this topic present a table of data from an investigation. Learn this language:
- Independent variable: what the investigator changes (chain length, type of functional group, number of branches).
- Dependent variable: what is measured (boiling point, vapour pressure, time to evaporate).
- Controlled variables: kept the same for a fair test — for example molar mass when comparing functional groups, or the functional group when comparing chain length.
- Fair test: only the independent variable changes; explain why compounds of similar molar mass are chosen (so the strength of London forces is about the same).
- Investigative question: “What is the relationship between chain length and boiling point of alcohols?”
- Conclusion: link the variables in one sentence: “As the chain length increases, the boiling point increases.”
Vapour pressure and boiling point are opposite: the compound with the highest boiling point has the lowest vapour pressure at a given temperature. A volatile compound has weak intermolecular forces and a high vapour pressure.