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?
  
  


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$^-$.