Nucleophilic attack on the carbonyl group: reaction between cynide and ethanal

Discussion

1) The cyanide ion is a good nucleophile. Indeed, the negative charge on the carbon atom (! with low electronegativity) pushes the free electron pair away and makes it suitable for a nucleophilic attack. 2) The cyanide ion plays both the role of reagent (it is partly added to the molecule) and catalyst (it is partially reformed at the end of reaction). 3) This is one of the few reactions where the carbon chain is elongated by one unit.