Now that we understand a little bit about the SNAr mechanism, let's talk about something called the Meisenheimer complex. As mentioned earlier, the Dow process was a typical SNAr reaction but it required tons of heat and pressure to proceed forward. This is due to the instability of the anionic intermediate. But it turns out that scientists figured out that there are ways to naturally stabilize that anionic intermediate that are going to make it require less heat and less pressure. The rule that we use for that is WAP. It's going to be withdrawing groups or heteroatoms in the ortho and para positions will stabilize the intermediate. We're basically looking for things. Remember that the anionic intermediate goes through the ortho and para positions relative to the nucleophile. That's exactly what we're trying to do here. What we're trying to do is we're trying to use atoms in those ortho and para positions to stabilize that negative charge. A classical trinitrobenzene. Think about it. Is nitro a good withdrawing group? It's the best. It's one of the best electron withdrawing groups. If you use a trinitrobenzene Meisenheimer complex, the reaction can actually proceed forward at room temperature. What is a Meisenheimer complex? A Meisenheimer complex is just what I'm saying, a WAP. A Meisenheimer complex is like an ultimate WAP where you have a molecule that has either heteroatoms or withdrawing groups in the ortho and para positions. I'm going to just say that those are synonyms of each other. A Meisenheimer complex is just any benzene that is in a WAP formation.
Let's take a look at this. Here I have once again a strong nucleophile, OCH3-, and I have a leaving group. But notice that on my ortho and para positions, I have all withdrawing groups. Normally for the Dow process, I would have required 350 degrees Celsius to proceed forward. But it turns out these withdrawing groups are so stable, are so strong that I'm actually going to be able to proceed forward at room temperature; 35 is a little bit warm for room, but it could be a hot room. Let's look at this mechanism.
Basically, your negative is going to attack the leaving group. But you're going to make an anionic intermediate. You're going to make a negative charge that's now stabilized by my withdrawing group and we can draw resonance structures for this. We would have resonance structures, tons of resonance structures. Let me just show you a few of them. We're not going to draw all of them because there's a lot. CLOCH3. Notice what's going to happen is that nitro looks like this. N, so 1 O, negative. Not only will the negative charge be able to resonate through the ring, it can even resonate with the nitro group. We can get something that looks like this. We can get resonance structures that form within the nitro groups. Giving us something like this. By the way, that was supposed to have a plus. Sorry. So much to draw. See how that resonance structure exists, and we can also draw resonance structures of the negative charge moving to the next nitro. Then that would be another resonance structure.
Altogether, there's going to be like 6 resonance structures. We're not going to draw all of them. That's for sure. But I'm trying to show you guys how a Meisenheimer complex works. Now anywhere that this negative charge goes, it's stabilized by withdrawing groups. Eventually what winds up happening is that the negative charge is going to reform a double bond and it's going to kick out the Cl. What you're going to wind up getting is an SNAr product because you've got a substitution that occurred, but it was for a nucleophilic reason. It wasn't for an electrophilic molecule. It was for a nucleophilic molecule that had occurred. Awesome. That's what a Meisenheimer complex is. It doesn't just have to look like this. It could be any combination of withdrawing groups and heteroatoms. That means if I just put a nitrogen inside the ring here, let me use a different color. If I put a nitrogen inside the ring here, that would qualify as a heteroatom because now I have a non-carbon atom inside the ring and non-carbon atoms are more electronegative, so they're also good at stabilizing negative charges. It's not just withdrawing groups. It's also heteroatoms that will help.
We're going to look at the following 2 reactions. It says use resonance structures to determine which of the following ipso substitutions is more favored. Remember that ipso substitution is just another name for SNAr. Go ahead and look at both of these. Try to draw resonance structures and then figure out which one is going to be the more favored reaction.