Nucleophilic addition of HCN to ethanal

Aldehydes and ketones have a polar C=O bond with a δ+ carbon, which nucleophiles attack. Cyanide ions (from KCN with acid, or a little base with HCN) add to ethanal, and protonation gives a hydroxynitrile, a useful way of adding a carbon atom to a chain.

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Nucleophilic addition

The cyanide ion attacks the δ+ carbonyl carbon and the C=O π electrons move onto oxygen. The resulting alkoxide takes a proton from HCN (or acid) to give 2-hydroxypropanenitrile, regenerating cyanide.

Overall reaction

1. 1Cyanide attacks the carbonyl carbonnucleophilic addition

   cyanide ion

   +

   ethanal

   alkoxide intermediate

   Electron movement

   1. The lone pair on the carbon of cyanide attacks the δ+ carbonyl carbon.
   2. The C=O π bond breaks and its electrons move onto the oxygen.

   Oxygen is more electronegative than carbon, so the carbonyl carbon is δ+ and the C=O π bond can accept the incoming electrons.

2. 2Protonation of the alkoxideproton transfer · reversible

   alkoxide intermediate

   +

   hydrogen cyanide

   2-hydroxypropanenitrile

   +

   cyanide ion

   Electron movement

   1. A lone pair on the negative oxygen takes the proton from HCN.
   2. The H–C bond breaks, regenerating the cyanide ion.

   The negative oxygen is a strong base. Protonating it gives the neutral product and returns the cyanide ion, which can attack another ethanal molecule.