Aldehydes and Ketones

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Recall the reactions (reagents and conditions) by which aldehydes and ketones can be produced:

    • (a) The oxidation of primary alcohols using acidified K₂Cr₂O₇ or acidified KMnO₄ and distillation to produce aldehydes
    • (b) The oxidation of secondary alcohols using acidified K₂Cr₂O₇ or acidified KMnO₄ and distillation to produce ketones

  2. Describe:

    • (a) The reduction of aldehydes and ketones using NaBH₄ or LiAlH₄ to produce alcohols
    • (b) The reaction of aldehydes and ketones with HCN, KCN as catalyst, and heat to produce hydroxynitriles as exemplified by ethanal and propanone

  3. Describe the mechanism of the nucleophilic addition reactions of hydrogen cyanide with aldehydes and ketones

  4. Describe the use of 2,4-dinitrophenylhydrazine (2,4-DNPH reagent) to detect the presence of carbonyl compounds

  5. Deduce the nature (aldehyde or ketone) of an unknown carbonyl compound from the results of simple tests (Fehling's and Tollens' reagents; ease of oxidation)

  6. Deduce the presence of a CH₃CO– group in an aldehyde or ketone from its reaction with alkaline I₂(aq) to form a yellow precipitate of tri-iodomethane

1. Introduction to Carbonyl Compounds

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Aldehydes and ketones are both types of carbonyl compounds. This means they both contain a carbonyl group, which is a carbon atom double-bonded to an oxygen atom (C=O).

What makes them different?

  • Aldehydes have at least one hydrogen atom attached directly to the carbonyl carbon. Their names end in -al.

    • Example: Ethanal (CH₃CHO), Propanal (CH₃CH₂CHO)

  • Ketones have two carbon atoms (from alkyl groups) attached to the carbonyl carbon. Their names end in -one.

    • Example: Propanone (CH₃COCH₃), Butanone (CH₃COCH₂CH₃)

Why is the carbonyl group important?

The carbonyl group is polar. This means the electrons are not shared equally between the carbon and oxygen atoms. Oxygen is more electronegative (it pulls electrons towards itself more strongly) than carbon. This creates:

  • A slight positive charge (δ+) on the carbon atom
  • A slight negative charge (δ−) on the oxygen atom

This polarity makes the carbon atom vulnerable to attack by nucleophiles (particles that are attracted to positive charges and can donate electrons).

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