17.1.2—Describe
- Syllabus
- 9701–2028–2029
- Objective
- 17.1.2
- Level
- AS
Carbonyl compounds are reduced to alcohols or add HCN to form hydroxynitriles
NaBH₄ or LiAlH₄ reduces an aldehyde to a primary alcohol and a ketone to a secondary alcohol. HCN adds across the C=O bond, with KCN acting as a catalyst, to form a hydroxynitrile.
Reduction supplies hydrogen/electron density to the carbonyl; cyanohydrin formation adds CN and OH to the former carbonyl carbon. The nitrile group adds one carbon to the skeleton.
Ethanal + NaBH₄ → ethanol; ethanal + HCN → CH₃CH(OH)CN. Propanone gives a tertiary alcohol on reduction and a substituted hydroxynitrile on addition.
Do not confuse reduction with oxidation, and do not forget the carbon-count increase in cyanohydrin formation.
ConceptA-Level CAIE Chemistry AS