Formation of Cyanohydrins from ketones and aldehydes
Description: Treatment of aldehydes (or ketones) with cyanide ion in the presence of a proton source (acid) leads to formation of cyanohydrins.
Notes: Acid is required to protonate the anion that is formed after CN(-) adds to the carbonyl. Note that while HCN is a common acid, sometimes H+ or other acid is also used.
In this reaction as drawn K is not important (just used as an example so that the charges are balanced). Cyanide ion [CN(-) ] is required, as it is a good nucleophile.
Notes: Note that the reaction can be drawn with K, Na or (sometimes) with no cation at all.
The specific choice of acid is not crucial.
Mechanism: Cyanide ion adds to the carbonyl carbon in a 1,2- addition (Step 1, arrows A and B) followed by protonation of the oxygen by acid (Step 2, arrows C and D). This regenerates free cyanide ion.
Notes: It’s also reasonable to draw the carbonyl oxygen as being protonated first, and then cyanide ion adding to it.