[tropicanapure]

I have a practise exam question for uni about substitution on a fused ring with KCN, i think its napthaline, with a Methoxy group and a nitro group para to it on one side of the ring, the question is, the reaction of kcn with this ring can be accelerated or retarded by a single class of additives, what are they and how do they exert their influence. I was thinking halides, but i dont know if they can slow down the reaction as well? can anyone point me in the right direction, thanks