Nucleophilic substitution with chloroethane: hydrogen cyanide vs. potassium cyanide Chloroethane can be used to make sodium propanoate.
$$\ce{chloroethane -&gt; Q -&gt; sodium propanoate}$$
The Intermediate, $\ce{Q}$, is hydrolysed with boiling aqueous sodium hydroxide, to give sodium propanoate.
Which reagent would produce the Intermediate, $\ce{Q}$, from chloroethane?
A concentrated ammonia solution
B dilute sulfuric acid
C hydrogen cyanide
D potassium cyanide

What is the difference between using hydrogen cyanide or potassium cyanide? Why would the answer be D?
Because hydrogen cyanide is not a very strong acid.

Potassium cyanide exists as potassium and cyanide ions, so here the cyanide ions can act as strong nucleophiles. Hydrogen cyanide is only a weak acid and so it will not dissociate to produce a significant amount of the cyanide ion nucleophiles. So it is a better idea to use potassium cyanide.