Aldehydes and Ketones
Chemistry (Year 12) - Organic Chemistry
Aldehydes have a carbonyl group, C = O, located on the terminal (end) carbon. Aldehydes, therefore, have the general formula of R - CHO.
Aldehydes are named with the suffix '-al'. As the functional group is always located on the terminal carbon, the functional group does not need to be numbered.
Like aldehydes, ketones also have a C = O bond. This bond is on a non-terminal carbon, giving the general formula of R' - CO - R'.
Ketones are named with the suffix '-one', and numbering the ketone is required when naming the molecule.
Boiling Point of Aldehydes and Ketones
Aldehydes and ketones have very similar boiling points to each other as they exhibit the same intermolecular forces.
The polar C = O functional group allows aldehydes and ketones to both exhibit dispersion forces and dipole-dipole forces between molecules. When dissolved in water, they can also form hydrogen bonds between the lone pair electrons on the O atom, in the C = O group, and the H atoms in water.
For boiling point, dispersion forces and dipole-dipole forces make the attraction between aldehyde/ketone molecules moderately strong, giving them a higher boiling point than alkanes/alkenes of similar molecular mass. Molecules that are capable of forming hydrogen bonds (e.g. alcohols) have a higher boiling point than aldehydes and ketones of similar molecular mass.
Solubility of Aldehydes and Ketones
For solubility, aldehydes and ketones can form hydrogen bonds with water between the lone pair electrons on the O atom in the C = O group and the H atoms in water. However, aldehydes and ketones are less soluble than alcohols.
The solubility of aldehydes and ketones in water decreases with increasing carbon chain length. This is due to dispersion forces becoming more significant in longer carbon chain molecules, and hence becoming non-polar.