The C=O bond in aldehydes and ketones is polar, because oxygen is much more electronegative than carbon. This permanent dipole controls their physical properties.Boiling points
Between carbonyl molecules there are London forces and permanent dipole-dipole forces. Carbonyl compounds cannot form hydrogen bonds with each other, because they have no H atom bonded to an oxygen atom. So:
• their boiling points are higher than those of alkanes of similar Mr, which have only London forces;
• their boiling points are lower than those of alcohols of similar Mr, which also form hydrogen bonds.Worked comparison (similar Mr, so similar London forces):
• propane, Mr = 44, boiling point −42 °C (London forces only)
• ethanal, Mr = 44, boiling point 20 °C (plus permanent dipole-dipole forces)
• ethanol, Mr = 46, boiling point 78 °C (plus hydrogen bonds)Within each homologous series, boiling point rises as the chain gets longer, because the London forces get stronger. Methanal is a gas at room temperature, ethanal boils at about room temperature and propanone is a liquid that boils at 56 °C.Solubility in water
Although carbonyl compounds cannot hydrogen bond to each other, the lone pairs on the δ− oxygen atom can form hydrogen bonds with water molecules. So small aldehydes and ketones, such as ethanal and propanone, mix completely with water.As the carbon chain gets longer, the non-polar hydrocarbon part gets larger. It cannot hydrogen bond with water, so solubility decreases with chain length.
Between carbonyl molecules there are London forces and permanent dipole-dipole forces. Carbonyl compounds cannot form hydrogen bonds with each other, because they have no H atom bonded to an oxygen atom. So:
• their boiling points are higher than those of alkanes of similar Mr, which have only London forces;
• their boiling points are lower than those of alcohols of similar Mr, which also form hydrogen bonds.Worked comparison (similar Mr, so similar London forces):
• propane, Mr = 44, boiling point −42 °C (London forces only)
• ethanal, Mr = 44, boiling point 20 °C (plus permanent dipole-dipole forces)
• ethanol, Mr = 46, boiling point 78 °C (plus hydrogen bonds)Within each homologous series, boiling point rises as the chain gets longer, because the London forces get stronger. Methanal is a gas at room temperature, ethanal boils at about room temperature and propanone is a liquid that boils at 56 °C.Solubility in water
Although carbonyl compounds cannot hydrogen bond to each other, the lone pairs on the δ− oxygen atom can form hydrogen bonds with water molecules. So small aldehydes and ketones, such as ethanal and propanone, mix completely with water.As the carbon chain gets longer, the non-polar hydrocarbon part gets larger. It cannot hydrogen bond with water, so solubility decreases with chain length.