Electrophilic Aromatic Substitution

Alright, guys. So now I just want to switch gears a little bit and talk about the most important mechanism that benzene undergoes. And that mechanism is called Electra Filic. Aromatic substitution. So in prior videos, we've already discussed that benzene really doesn't like to react with anything. And the reason is because it doesn't want to break its air. Metis ity. We've already learned how to test for air authenticity, and we know that if any of those four tests of air Metis City are broken, the molecule become less stable. So let's look back at a reaction that we learned a long time ago and see how that would apply to benzene. Like, for example, halogen nation Halogen nation was a reaction that took a di atomic halogen halogen nation and added it across a double bond and you would wind up getting a die. Hey, light is a product. Now that we have three double bonds, you may think that halogen issue would happen three times and just completely halogen it the ring. But now we understand why this reaction is not favored, right? Because the product is gonna be non aromatic. Why would this molecule the non aromatic. What rule is it breaking? Well written here? This is not fully conjugated, right? Since it's not fully conjugated, a not fully conjugated product is. Guess what? It's less stable because now this molecule doesn't have Air Mississippi to help it out. So what that basically means is that typical addition. Reactions across double bonds are shot. They're not gonna work on benzene. So how can we get bending to react with anything? Well, let's theorize. Here, let's be scientists. If we could somehow get benzene to react in a substitution reaction instead of an addition reaction like for example, let's get some re agent and get it to switch out with one of the hydrogen. This is not a mechanism, by the way. I'm just saying, If you could just get them to switch, then your final product would remain aromatic. So this is the thought process that we go through when we say, Hey, if we're looking for reaction is gonna work on benzene, it needs to preserve air. Metis idiot. The end. Well, how do we do that? It turns out, guys that very, very strong electro files that would be like an E plus right. Electrify all. It's an electron lover. Okay? Something with the positive charge can temporarily disrupt the air. Metis city to create a substitution product as long as by the end of the reaction, it goes back to being an aromatic compound. Okay, this process is what we call Electra Filic, Aromatic substitution or what? I'm going to continue. Just call E A s for short, okay? And this is by far the most important mechanism of benzene and one that we're gonna spend several hours discussing in the next few videos. All right, so let's go ahead and take a look at the general mechanism of s.

the first thing that should strike us when you look at this mechanism is that it's a two step mechanism and it could be summarized by these words addition and elimination thes air the two stages of an E. A s mechanism. Now, remember that in addition, reaction is one that breaks pi Bonn's okay, Typically, you break a pi bon and you add two Sigma bonds. Remember that an elimination is the opposite. Remember that an elimination reaction makes pi bonds, and that makes sense. Because remember, we said that at the end we want a rented city to be restored. So what we're saying is that we're gonna break that double bond for a little bit, but at the end, we're gonna make it again. We're going to get the aromatic products. Okay. For that reason, this mechanism is also known as the Electra Filic. Just write this down addition elimination mechanism, okay? And that just explicitly says the two steps of s. There's on a new addition elimination addition, first elimination, and the whole thing happens through an Electra filigree agent. Okay, so let's go ahead and take a look at this first step and see if you guys can theorize. Where do you think this arrow would first come from? Now, this molecule e a just means that it's some electro file. So some kind of very strong, partial, positive or positive attached to some conjugate that will use later. Okay, So since we have a positive there, you know what the arrows are always going to start from the most negatively charged thing. And for E. A s, that's always the benzene. The benzene has tons of electrons, so even more so than a typical double bond, this thing's got loads of electrons that it can use toe attack the electro files. Let's go ahead and draw that. We're going to attack our electro file. And if we make that bond, we have to break a bond. So we're gonna break up onto the A forming the conjugate base, which is gonna be the conjugate for this for this reaction. Okay, Now we're gonna form. Is this intermediate? And this intermediate is called the Sigma Complex, or it's also called on uranium I on This is one of the most important one of the most important intermediates of organic chemistry too. So I definitely want you guys to really understand it. Okay. What happens is that two of the double bonds stay exactly the same one to like that. But now we've got the lecture file, adding toe one side and a cat eye on on the other because we're missing a bond, right? We just broke the pie bond. Nothing else came to replace it. So we've got literally a missing bond at that site. Okay? Now, one of the things that stabilizes this Sigma complex is the fact that there's tons of resonance possible. Okay, so you might be asked to draw the resonance, Um, the resonance structures of this complex. So let's go ahead and do that now. We're just gonna do that underneath here. So let's bring the top structure down and let's go ahead and just draw the first one exactly the way that I drew it up at the top and then will resonate it. Okay, so that's our first resident structure of the Sigma complex. The next structure would have the double bond moving to take its spot. Remember that cat ions can always move with one arrow, so we would swing this door like a door hinge and we would make the next resonance structure. So the next resonant structure is in the same. You know, all the atoms are the same, but now we have our Catalan Over here. Now we're gonna move this pi bon one more time and we're going to get the last resonance structure of my uranium ion or sigma complex. Okay? And as you guys can imagine, this cat ion is much less stable than an aromatic compound. But it is stabilized by the fact that it can resonate three times. So it's distributed across all five of those carbons. And as you guys might guess, the resonance hybrid would be drawn. Um, would be drawn simply by drawing a dotted line around all five of those and adding a positive okay. And that would, you know, that would be the way that we would represent our Sigma complex hybrid? Okay, awesome. So this signal complex, even though it's stabilized by resonance, it's still the highest. You know, it's still much higher energy on my higher energy on my energy diagram than the reacting to the products. So this is gonna be my slow step because it's difficult to make this intermediate. Okay, so now what we do now we need an elimination step. Remember, that elimination makes double bonds on the way that's gonna work is that's where my conjugate base comes in. My conjugate base is gonna come in and do an elimination reaction, basically like an E one. So this is essentially gonna be an e one mechanism if you recall back to organic chemistry one. Okay, so you've got a Carvel cat ion, and we're gonna go ahead and we're gonna take out do a beta elimination. If this is my Alfa carbon thin, this is my beta carbon. So we're gonna do a beta elimination on a hydrogen and reform the double bond. Okay, Now, this step is the fast step because it's easy to eliminate because we're making an aromatic compound. The slow step or the rate determining step of this reaction is how quickly we can make that sigma complex. Okay, so as you guys see, what are we gonna have it? The end we're gonna have now my substituted airing, and we're gonna have h a Okay, so some kind of acid. Alright. But what we're really concerned about and what your professors really concerned with is the actual benzene ring the actual aromatic product. Okay, so, guys, that pretty much does it for this mechanism. Now, we're going to spend some time talking about the specific Electra follows that we're gonna use because this mechanism just used E. Okay, Now, this mechanism is gonna apply for all the different electric falls we learned in this chapter. But it's gonna be your job to know exactly which Electra files we can use. So let's go ahead and learn about the Electra files of Elektra Filic. Aromatic substitution.