Concept: Concept: EAS Review4m
All right guys, so now I just want to switch gears a little bit and talk about the most important mechanism that benzene undergoes. That mechanism is called electrophilic aromatic substitution.
In prior videos, we've already discussed that benzene really doesn't like to react with anything. The reason is because it doesn't want to break its aromaticity. We've already learned how to test for aromaticity. And we know that if any of those four tests of aromaticity are broken, the molecule will become less stable.
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, halogenation. Halogenation was a reaction that took a diatomic halogen, halogenation, and added it across a double bond. You would wind up getting a dihalide as a product. Now that we have three double bonds, you may think that halogenation would happen three times and just completely halogenate the ring. But now we understand why this reaction is not favored because the product is going to be non-aromatic.
Why would this molecule be non-aromatic? What rule is it breaking? Well, written here, this is not fully conjugated. 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 aromaticity to help it out. What that basically means is that typical addition reactions across double bonds are shot. They're not going to work on benzene.
So how can we get benzene 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 reagent and get it to switch out with one of the hydrogens. 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. This is the thought process that we go through when we say if we're looking for a reaction that's going to work on benzene, it needs to preserve aromaticity at the end.
Well, how do we do that? It turns out guys that very, very strong electrophile so that would be like an E+, electrophile. It's an electron lover. Something with a positive charge. Can temporarily disrupt the aromaticity to create a substitution product as long as by the end of the reaction it goes back to being an aromatic compound. This process is what we call electrophilic aromatic substitution or what I'm going to continue to just call EAS for short.
This is by far the most important mechanism of benzene and one that we're going to spend several hours discussing in the next few videos. Let's go ahead and take a look at the general mechanism of EAS.
Concept: Concept: EAS General Mechanism7m
The first thing that should strike us when you look at this mechanism is that a two step mechanism and it can be summarized by these words addition and elimination these are the two stages of an E A S mechanism now remember that in addition reaction is one that breaks pi bonds typically you break a pi bond and you add 2 sigma bonds and remember that in elimination is the opposite remember that in elimination reaction makes pi bonds and that makes sense because remember we said that at the end we want aromaticity to be restored so what we are saying is that we're going to break that double bond for a little bit but at the end we're going to make it again we're going to get that aromatic product for that reason this mechanism is also known as the electrophilic just write this down addition elimination mechanism and that just explicitly says the two steps of E A S there's an additional elimination addition first elimination and the whole thing happens through an electrophilic region filigreed it. 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 some electrophiles some kind of very strong partial positive or positive attached to some conjugate that we use later. So since we have a positive there we know 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 double bond these thing's got loads of electrons that it can use to attack the electrophile so lets go ahead and draw that we're going to attack our electrophile and if we make that bond we have to break a bond so we are going to break a bond to the A forming the conjugate base which is going to the conjugate for this for this reaction. Now we're going to form is this intermediate and this intermediate is called the sigma complex or it's also called a uranium ion this is one of the most important or one of the most important intermediates of organic chemistry too.
So I definitely want you guys to really understand it, what happens is that two of the double bond stay exactly the same 1, 2 like that. But now we've got the electrophile adding to one side and a cation on the other because we're missing a bond right we just broke the pi bond nothing else came to replace it so we've got literally a missing bond at that site. Now one of the things that stabilizes this sigma complex is the fact that there's tons of resonance possible, so you might be asked to draw the resonance structures of this complex so lets go ahead and do that now we're just going to 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 at the top then we will resonance it. So that's our first resonance structure of the sigma complex the next structure would have the double bond moving to take its spot remember that cations 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 resonance structure is in the same you know all the atoms are the same but now we have our cation over here now we're going to move this pi bond one more time and we're going to get the last resonance structure of my uranium ion or sigma complex and as you guys can imagine this cation 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 simply by drawing a dotted line around all five of those and adding a positive and that would you know that would be the way that we would represent our sigma complex hybrid.
So this signal complex even though its stabilized by resonance it's still the highest you know it's still much higher energy on my energy diagram than the reactance or the products so this is going to be my slow step because its difficult to make this intermediate. So now what do we do? Now we need an elimination step remember that elimination makes double bonds and the way that's going to work is that's where my conjugate base comes in my conjugate base is going to come in and do an elimination reaction basically like in E 1, so this is essential going to be an E 1 mechanism if you recall back to organic chemistry one. So you've got a carbocation and we're going to go ahead and we're going to take out do a beta elimination if this is my alpha carbon then this is my beta carbon. So we're going to do a beta elimination on a hydrogen and reform of the double bond. 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 you can make that sigma complex. So as you can see what we're going to have at the end? We're going to have now my substituted aren and we're going to have H A, so some kind of acid right. But what we're really concerned about and what your professor is really concerned with is the actual benzene ring the actual aromatic product. So guys that pretty much does it for this mechanism now we're going to spend some time talking about the specific electrophiles that we're going to use because this mechanism just used be E now this mechanism is going to apply for all the different electrophiles we learned in this chapter but it's going to be your job to know exactly which electrophiles we can use. So let's go ahead and learn about the electrophiles of electrophilic aromatic substitution.
Provide terms to describe each of the two mechanistic steps of an electrophilic aromatic substitution
Which of the following is most likely to be the first step in the general mechanism for electrophilic substitution reactions?