What happens when acetone reacts with hydrogen cyanide?


> 
> Which of the following is the major organic product when acetone is allowed to react with hydrogen cyanide in the presence of a catalytic amount of sodium cyanide?
> 
> 
> ![](https://i.stack.imgur.com/Odv4r.png)
> 
> 
> 


The correct answer is (B). 


However, I got this answer without using the catalytic sodium cyanide. In my proposed mechanism, the dissociated proton from $\ce{HCN}$ activates the carbonyl, which facilitates the nucleophilic attack of $\ce{CN-}$.


What role does the $\ce{NaCN}$ play?




The acetone carbonyl is attacked by the CN$^-$ that comes from the NaCN. This tetrahedral intermediate has a oxygen anion, which then grabs a proton from the HCN, regenerating the CN$^-$.


 6

The product that is formed is [cyanohydrin](https://en.m.wikipedia.org/wiki/Cyanohydrin).



> 
> Cyanohydrins can be formed by the cyanohydrin reaction, which involves
>  treating a ketone or an aldehyde with hydrogen cyanide ($\ce{HCN}$) in the
>  presence of excess amounts of sodium cyanide ($\ce{NaCN}$) as a catalyst:
> 
> 
> $$\ce{RR’C=O + HCN → RR’C(OH)CN}$$
> 
> 
> 


Since, the reactant is acetone both R and R' are $\ce{CH3 -}$ group. 



> 
> In this reaction, the nucleophilic $\ce{CN^−}$ ion attacks
>  the electrophilic carbonyl carbon in the ketone, followed by
>  protonation by $\ce{HCN}$, thereby regenerating the cyanide anion.
> 
> 
> 




---


Now, what is the function of catalyst sodium cyanide?



> 
> For successful cyanohydrin formation it is important to have free
>  cyanide ions available to react with the ketone or aldehyde. This can
>  be achieved by using a salt (e.g. $\ce{KCN}$ or $\ce{NaCN}$) or a silylated
>  (e.g. $\ce{Me3SiCN}$) form of cyanide under acidic conditions or by using $\ce{HCN}$
>  with some base added to produce the needed $\ce{CN^−}$ nucleophile.([Here](https://chem.libretexts.org/LibreTexts/Athabasca_University/Chemistry_360%3A_Organic_Chemistry_II/Chapter_19%3A_Aldehydes_and_Ketones%3A_Nucleophilic_Addition_Reactions/19.06_Nucleophilic_Addition_of_HCN%3A_Cyanohydrin_Formation))
> 
> 
> 


 2