Aldehydes and Ketones

  • Aldehydes and ketones are carbonyl compounds - they contain the carbonyl functional group (C=O). In aldehydes, the carbon atom in the carbonyl group is bonded to at least one hydrogen atom. In ketones, the carbonyl carbon atom is bonded to two other carbon atoms.

  • Naming of these compounds is based on the longest carbon chain that contains the carbonyl group. For aldehydes, the suffix ‘-al’ is used, while for ketones the suffix ‘-one’ is used.

  • Both aldehydes and ketones can undergo a number of reactions due to the polar nature of the carbonyl group. These include nucleophilic addition, reduction and oxidation reactions.

  • The simplest aldehyde is methanal (formaldehyde) and the simplest ketone is propanone (acetone).

  • Aldehydes can be oxidised to carboxylic acids using an oxidising agent such as potassium dichromate(VI) solution.

  • Ketones are resistant to oxidation. Only very strong oxidising agents, under harsh conditions, can oxidise a ketone to a carboxylic acid.

  • Both aldehydes and ketones can be reduced to alcohols. Aldehydes are reduced to primary alcohols, while ketones are reduced to secondary alcohols.

  • Aldehydes and ketones react with hydrogen cyanide (HCN) to form hydroxynitriles. This is an example of nucleophilic addition.

  • Both aldehydes and ketones can undergo a reaction known as the Tollens’ test. Aldehydes reduce Tollens’ reagent (silver nitrate in ammonia) to silver mirror, whereas ketones do not.

  • The Brady’s test (2,4-DNP test) can be used to differentiate between carbonyl compounds and other compounds. Both aldehydes and ketones react with Brady’s reagent to form yellow-orange precipitates.

  • Spectroscopy, including infrared and nuclear magnetic resonance (NMR), can be used to identify the functional groups in aldehydes and ketones.

  • The carbonyl stretch appears as a strong, sharp peak in the infrared spectrum at around 1700 cm⁻¹.

  • The chemical shifts in the NMR spectrum can provide information about the hydrogen environments in the molecule; for example, a shift between 9 and 10 ppm can indicate the presence of an aldehyde hydrogen.

Please review these points thoroughly with practical examples to understand the topic around Aldehydes and Ketones.