I have been asked the following question in my problem sheet:
The only differemce between reaction A and reaction B is the electron withdrawing group that stabilises the anionic transition state changes from $CN$ in reaction A to $NO_2$ in reaction B. I can see no glaringly obvious reason why there should be a different rate. The problem is really (at least in my mind): what is the better electron withdrawing group out of $CN$ and $NO_2$. Is this a reasonable interpretation? If so, what reasoning can I bring to the answer to suggest that one is a better EWG than the other?