18.1 Introduction to Aldehydes and Ketones
What are carbonyl compounds?
1) Carbonyl compounds are compounds that contain the C=O(carbonyl) group. Examples are aldehydes and ketones.
Physical properties of carbonyl compounds
-
- The boiling point of carbonyl compounds is higher than the alkanes with similar Mr.
- The boiling point increases with increasing number of carbon atom. This is because there are more electrons, hence more temporary dipoles can be set up. More energy is required to overcome these forces.
- Besides temporary dipoles, permanent dipole-dipole forces are also present due to carbonyl compounds being polar
- Methanal and ethanal are gases at room temperature, while others are liquids
-
- Carbonyl compounds are soluble in This is because they are able to form hydrogen bond with water molecules.
- The solubility decreases with increasing number of carbon atoms. This is because the long hydrocarbon chain disrupts the hydrogen bonding.
- Carbonyl compounds are soluble in This is because they are able to form hydrogen bond with water molecules.
Preparation of aldehydes and ketones
- Aldehydes and ketones can be made by oxidising primary and secondary alcohol respectively. The oxidising agent used is either acidified potassium dichromate(VI), K2Cr2O7 or acidified potassium manganate(VII), KMnO4.
- To make an aldehyde:
- The primary alcohol used must be in excess and heated under reflux with acidified K2Cr2O7/KMnO4. The aldehyde formed must be distilled off as soon as possible.
- Under these conditions, a primary alcohol is oxidised to an aldehyde. Take ethanol as an example, ethanal is produced.
CH3CH2OH + [O] → CH3CHO + H2O
- To make a ketone:
- The secondary alcohol is heated under reflux with acidified K2Cr2O7/KMnO4.
- Under these conditions, a secondary alcohol is oxidised to a Take propan-2-ol an an example:
CH3CH(OH)CH3 + [O] → CH3COCH3 + H2O