Here’s a walkthrough of how a typical nucleophilic aromatic substitution works.
Just like the bottom line of electrophilic aromatic substitution was stabilizing positive charge on carbon, the bottom line of nucleophilic aromatic substitution is all about stabilizing negative charge on carbon in the intermediate anion.
So what factors can stabilize negative charge?
Well, carbanions are electron-rich. And we can make them more stable by taking electrons away from them. How is this done? Through adding electron withdrawing substituents, such as NO2, COOR, CN, or others. In other words – the list of meta directors you met when you learned about electrophilic aromatic substitution.
Let’s compare two anions: one which comes from a reaction where NO2 is a substituent (electron withdrawing) and the other where NH2 is a substitutent.
If you compare the NO2 case, you’ll see there’s an extra resonance form available which lets you “offload” the charge to oxygen. And since oxygen is more electronegative than carbon, this means it will do a much better job of stabilizing negative charge. [The same reasoning can be applied to other groups like CN, COR, COOR, SO3H, and so on]
None of that is possible for the case with NH2. Therefore, it’s going to be less stable, and slower to form.
Bottom line (again): understanding nucleophilic aromatic substitution is all about understanding the factors that stabilize negative charge.
Tomorrow: let’s wrap up the week’s events.
Thanks for reading! James