reaction of 1,2 dibromoethane with KCN.......
Answers
In the first step — aside from the fact that nobody wants to work with gaseous HCN due to its toxicity — using HCN wouldn’t work. The reaction in question is a bimolecular nucleophilic substitution. For that, you need to have a nucleophilic lone pair that is capable of attacking and it has to be on carbon (otherwise we would arrive at a diisonitrile). However, in hydrogen cyanide there is no free lone pair on carbon; it would need to get deprotonated before it can attack. Rather than using the toxic, gaseous HCN and deprotonating it, it is much quicker to just immediately start from an ionic compound such as KCN wherein the cyanide is already present as −C≡N anions ready for nucleophilic attack.
Br−CH2−CH2−Br+2CN−⟶NC−CH2−CH2−CN+2Br−(1)
In your second step, it really doesn’t matter which acid you use. In fact, many problem sets wouldn’t even specify the acid, they would just write what I did in equation (2). Thus, you should get the same points for using HCl as you would for using H2SO4. You should use a strong acid, though; acetic acid will not do the trick.