BRAINCAKENucleophilic Addition Mechanism
The reaction of aldehydes and ketones with HCN in the presence of KCN is a nucleophilic addition. The C=O bond is polar: oxygen is more electronegative than carbon, so the carbon is δ+ and the oxygen is δ−. The electron-deficient carbon is attacked by a nucleophile, an electron-pair donor.
Step 1: the cyanide ion, :CN−, uses the lone pair on its carbon atom to attack the δ+ carbonyl carbon. At the same time the π bond of C=O breaks and its pair of electrons moves onto the oxygen. This forms a negatively charged intermediate with an O− group.
Step 2: the O− uses a lone pair to accept a proton, H+, from an HCN molecule (or from H+ in solution), forming the OH group. Taking H+ from HCN regenerates a CN− ion.
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More in Carbonyl Compounds
- Physical Properties of Carbonyls
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- Tollens' Reagent
- Fehling's and Benedict's Solutions
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All 11 lessons in Carbonyl Compounds · All Edexcel A-level Chemistry topics