Aldehydes and ketones

Section 1: Introduction to Aldehydes and Ketones

  • Named under the class of carbonyl compounds, both aldehydes and ketones contain a carbonyl group (C=O).
  • Aldehydes have a carbonyl group at the end of the carbon chain while ketones have it within the carbon chain.
  • Generally, both compounds are polar due to the carbonyl group, resulting in relatively high melting and boiling points.
  • Aldehydes and ketones can be produced through oxidation of alcohols.

Section 2: Physical Properties of Aldehydes and Ketones

  • Both groups of compounds are characterised by the presence of a dipole across the carbonyl group, enhancing intermolecular forces.
  • Since they do not form hydrogen bonds, they have lower boiling points than alcohols and carboxylic acids.
  • Both aldehydes and ketones are soluble in organic solvents and in water if the carbon chain is short.

Section 3: Chemical Reactions of Aldehydes and Ketones

  • Aldehydes can be oxidised to form carboxylic acids unlike ketones.
  • Both aldehydes and ketones are susceptible to nucleophilic addition reactions, often involving a nucleophile attacking the carbon atom of the carbonyl group.
  • Aldehydes and ketones form hydrates, hemiketals, hemiacetals, and undergo Aldol condensation under certain conditions.
  • Frequently used in Cannizzaro’s reaction, aldehydes without alpha-hydrogens are treated with a base to form a primary alcohol and a carboxylic acid.

Section 4: Identification of Aldehydes and Ketones

  • The Tollens’ reagent test can identify aldehydes as they oxidise to form silver mirrors while ketones do not react.
  • The Fehling’s solution test and the Schiff’s test can also distinguish between the two.
  • The Iodoform test helps to identify methyl ketones specifically.

Section 5: Uses of Aldehydes and Ketones

  • Aldehydes and ketones find extensive use in the perfume and flavour industry.
  • These compounds are also fundamental in polymer industry and in the synthesis of pharmaceuticals.
  • They also serve as important solvents in a variety of chemical reactions.