The Addition of Hydrogen Cyanide Cyanohydrins

The aminoalcohol intermediate is protonated by the catalytic acid. Contribution of an unshared electron pair from the nitrogen atom and departure of a water molecule lead to an iminium cation intermediate. 

A proton is removed from the carbon adjacent to the iminium group. Proton removal occurs from the carbon because there is no proton to remove from the nitrogen of the iminium cation (as there would have been if a primary amine had been used). This step forms the enamine, neutralizes the formal charge, and regenerates the catalytic acid. (If there had been a proton to remove from the nitrogen of the iminium cation, the final product would have been an imine.) 

0 Hydrogen cyanide adds to the carbonyl groups of aldehydes and most ketones to form compounds called cyanohydrins. (Ketones in which the carbonyl group is highly hindered do not undergo this reaction.) 

Cyanohydrins form fastest under conditions where cyanide anions are present to act as the nucleophile. Use of potassium cyanide, or any base that can generate cyanide anions from HCN, increases the reaction rate as compared to the use of HCN alone. The addition of hydrogen cyanide itself to a carbonyl group is slow because the weak acidity of HCN (pKa 9) provides only a small concentration of the nucleophilic cyanide anion. The following is a mechanism for formation of a cyanohydrin. 

Great care must be taken when working with hydrogen cyanide due to its high toxicity and voiatiiity. Reactions invoiving HCN must be conducted in an efficient fume hood. 

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