Key ideas
Boiling points, solubility and acidity all follow from intermolecular forces and from how well a molecule can stabilize charge. Ranking compounds means finding the strongest attractions or the most stable conjugate base.
- Boiling point rises with stronger forces: hydrogen bonding (alcohols, acids, amines) beats dipole–dipole (aldehydes, ketones, ethers), which beats dispersion forces (alkanes).
- For the same type of compound, a longer chain raises the boiling point and branching lowers it, because branching reduces surface contact.
- Small molecules with –OH or –NH₂ dissolve in water. Solubility falls as the hydrocarbon part grows.
- Acidity increases when the conjugate base is more stable: carboxylic acid > phenol > alcohol, and electron-withdrawing groups such as halogens raise acidity.
Key equations
boiling point: H-bonding > dipole–dipole > dispersionacidity: RCOOH > ArOH > H₂O ≈ ROHpKa = −log₁₀ Ka
Worked example
Ethanol (46 g/mol) boils at 78 °C, ethanal (44 g/mol) at 20 °C and propane (44 g/mol) at −42 °C, although their masses are similar, because only ethanol can hydrogen-bond. In acidity, ethanoic acid (pKa 4.76) is stronger than phenol (about 10), which is stronger than ethanol (about 16). Trifluoroethanoic acid is stronger than ethanoic acid because of the CF₃ group.
Common mistakes
- Ranking boiling points by molar mass alone, ignoring hydrogen bonding.
- Expecting a branched isomer to boil higher than its straight-chain isomer.
- Forgetting that a more stable conjugate base means a stronger acid, so a lower pKa.
Practice
Test yourself with practice questions on this topic: Physical properties and acidity