 We know that aromatic rings are electron-rich and this is why one of the most popular reactions of haloarenes is electrophilic aromatic substitution reactions like nitration, sulfonation, priddlecraft reactions and so on. But what if I told you that we can also make these aromatic rings undergo a nucleophilic substitution reaction? You might find that pretty absurd because how can a nucleophile attack another electron-rich species and bring about a substitution reaction? If you thought so, you are absolutely right. And that is why we need to tweak a couple of conditions to make it happen. As I already mentioned, if I have to make a nucleophile attack this electron-rich ring, then I need to somehow decrease electron density in this ring and make it more electrophilic, right? Only then the nucleophile would want to attack it. And we can decrease electron density on the ring by introducing an electron withdrawing group. When we have a strong electron withdrawing group, it would draw away the electron density from the ring towards itself, make this ring electron deficient and facilitate a nucleophile to attack. Therefore, to make this haloarene undergo a nucleophilic aromatic substitution reaction, it must have a strong electron withdrawing group attached to the ring. In fact, more the number of electron withdrawing groups present in the ring, greater would be the rate of the reaction. The rate of the reaction substantially increases with more number of nitro groups attached to the ring, as you can see here. So, let's now look at the mechanism of this reaction. The first step is the addition of the nucleophile. Now, here we have an orthoisomer and as the nucleophile adds to the haloarene, the electron density gets delocalized. Now in this case, the pi electrons get delocalized onto the electron withdrawing nitro group. And as you can see here, the electron density in the ring is drawn away by the nitro group and delocalized onto the more electronegative oxygen atom here. This negative charge gets further delocalized within the ring and eventually results in the elimination of the halogen atom. And this process is much favored because it also restores the aromaticity of the ring. So, this is why nucleophilic aromatic substitution reactions are also called addition elimination reactions. The first step is the addition of the nucleophile and the second step involves the elimination of the leaving group, which in our case is the halogen. Now, obviously the rate determining step in this reaction is the first step or the addition of the nucleophile. The step is slow because it results in the loss of aromaticity. As we know, the aromatic ring does not want to lose its stability and it takes a good deal of energy to get this step started, whereas the elimination is a much faster step because here the aromaticity is restored. So, the rate determining step in this mechanism is basically the first step. Now, the only small caveat here is that the electron withdrawing group, in our case the nitro group here, should always be present at the orto or para positions to the leaving group. Now pause the video and think why orto and para positions are preferred and not the meta position, okay? So, basically in the case of the orto isomer, the electron withdrawing group stabilizes the negatively charged intermediate via resonance. And the same thing happens in the case of the para isomer. Here again, the pi electrons get delocalized onto the more electronegative oxygen atom of the nitro group and thus withdraws electron density from the ring. And this facilitates a nucleophilic attack, whereas in the case of the meta isomer, no such delocalization is possible. Here, the negative charge remains stuck on the less electronegative carbon atom and cannot move on to the more electronegative oxygen atom. And that means electron density pretty much stays within the ring and does not decrease because of the presence of an electron withdrawing group. So, basically having a nitro group at the meta position is not of much use as it does not decrease electron density in the ring or make it electrophilic enough for a nucleophile to attack. And this is why the rate of the reaction is much faster in the case of orto and para isomers, but very slow in the case of the meta isomer.