Why does the nucleophilic addition of a cyanide ion to an aldehyde form a racemic mixture?
Because the plane of the C=O bond in an aldehyde is flat, and so the nucleophilic attack of CN- onto the C can occur above or below the plane of the molecule, producing two different stereoisomers of a 2-hydroxyalkane molecule with 50/50 split (as both structures are equally thermodynamically favourable). This produces a racemic mixture
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