Why is it aqueous and ethanolic conditions for nucleophillc substitution of cyanide ion with benzyl chloride?

I was doing an exam question where benzyl chloride was undergoing a nucleophilic substitution with cyanide ion to form benzyl cyanide.


Please can someone enlighten me as I have never seen both conditions before just one or the other. I have Googled it but no sites talk about the conditions of nucleophilic substitution with a cyanide ion in a benzene context. Does the benzene alter the conditions?




Since $\mathrm{p}K\_\mathrm{a}$ of $\ce{HCN}$ is 9.21 ([Wikipedia](https://en.wikipedia.org/wiki/Hydrogen_cyanide)), the cyanide ion in water is pretty alkaline and contains significant amounts of hydroxide ions, according to the following equilibrium:
$$\ce{NC- + H2O <=> OH- + HCN}$$
That's why you smell strong $\ce{HCN}$ in aqueous $\ce{KCN}$ or $\ce{NaCN}$ solution. Thus, when you perform the reaction of $\ce{KCN}$ or $\ce{NaCN}$ with the haloalkane (including allyl and benzyl) in water you tend to get the substitution by $\ce{^-OH}$ instead of $\ce{^-CN}$. To get major product as cyanoalkane, you may need to do the reaction in absolute alcohol.


Not relevant, but note worthy that $\ce{^-CN}$ is an ambident nucleophile, meaning a nucleophile which can attack acitve site of a molecule through more than one sites (here either by $\ce{N}$ or $\ce{C}$). Thus, a reaction of alkyl halide ($\ce{R-X}$) with $\ce{KCN}$ gives alkyl nitrile ($\ce{R-CN}$) while a reaction of alkyl halide with $\ce{AgCN}$ gives an alkyl isonitrile ($\ce{R-NC}$).


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The cyanide ion is a very good nucleophile and reacts readily with benzyl chloride. 
The problem is that the cyanide salts are soluble in water, while the benzyl chloride is not. 
Therefore, ethanol is added so that both reagents are slightly soluble in the medium and the reaction proceeds. 


An alternative to this system is to use phase transfer catalysts like tetrabutylammonium salts. 
This cation can then transport the cyanide to the organic phase, for instance toluene, in which the benzyl cyanide is dissolved, and the reaction occurs there. 


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