Formation of Cyanohydrins from ketones and aldehydes

by James

Description: Treatment of aldehydes (or ketones) with cyanide ion in the presence of a proton source (acid) leads to formation of cyanohydrins.
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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.

Examples:

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.

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