Physical properties of alcohols and phenols
The -OH group forms hydrogen bonds, so alcohols and phenols boil higher than hydrocarbons and ethers of similar mass, and lower alcohols dissolve in water.
Hydrogen bonding and boiling points
In an alcohol, the O-H bond is polar: H carries δ⁺, and O carries δ⁻ and two lone pairs. The H of one molecule is attracted to the O of the next. This intermolecular hydrogen bond links the molecules together. Extra energy is needed to break these bonds, so alcohols and phenols have high boiling points.
Similar molar mass, very different boiling points
| Compound | Molar mass (g mol⁻¹) | Boiling point | Main forces between molecules |
|---|---|---|---|
| Propane, CH₃CH₂CH₃ | 44 | 231 K | Weak van der Waals forces only |
| Methoxymethane, CH₃OCH₃ | 46 | 248 K | Dipole-dipole forces; no O-H, so no hydrogen bonds between molecules |
| Ethanol, CH₃CH₂OH | 46 | 351 K | Intermolecular hydrogen bonds |
Trends in boiling point
- Boiling point rises with the number of carbon atoms, because van der Waals forces grow.
- In isomeric alcohols, branching lowers the boiling point: the molecule is more compact, so its surface area and van der Waals forces are smaller.
- Alcohols and phenols boil higher than hydrocarbons, ethers, haloalkanes and haloarenes of comparable molar mass.
Solubility in water
Alcohols and phenols dissolve in water because their -OH group forms hydrogen bonds with water molecules. Methanol, ethanol and the propanols mix with water in all proportions. As the alkyl or aryl part grows, solubility falls, because this part is hydrophobic (water-repelling). Pentan-1-ol and larger alcohols are only slightly soluble.
Two jobs of the same hydrogen bond
Hand sanitisers sold in India are mostly ethanol or propan-2-ol mixed with water. They mix completely because the -OH group forms hydrogen bonds with water.
2 marks: “Why do alcohols have higher boiling points than ethers and hydrocarbons of comparable molecular mass?” Alcohol molecules are linked by intermolecular hydrogen bonds through their -OH groups; ethers and hydrocarbons have no O-H, so their forces are weaker. Example: ethanol 351 K, methoxymethane 248 K, propane 231 K.
2 marks: “Why does the solubility of alcohols in water decrease as their molecular mass increases?” The -OH group hydrogen bonds with water, but the alkyl part is hydrophobic. As the alkyl part grows, it outweighs the effect of the -OH group.
1 marks: “Arrange in increasing order of boiling point: ethanol, propane, methoxymethane.” Propane < methoxymethane < ethanol.
OH holds hands: with its own kind it raises the boiling point; with water it dissolves.