Electrophilic Aromatic Substitution: Study with Video Lessons, Practice Problems & Examples

Benzene undergoes electrophilic aromatic substitution (EAS) to maintain its aromaticity, avoiding typical addition reactions that disrupt stability. EAS consists of two steps: addition, where a strong electrophile attacks the benzene, forming a sigma complex (or arenium ion), and elimination, where a hydrogen is removed to restore aromaticity. The sigma complex, stabilized by resonance, is the rate-limiting step. Understanding EAS is crucial for predicting reactions involving benzene and its derivatives, emphasizing the importance of aromatic stability in organic chemistry.

EAS Review

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 The reason is because it doesn't want to break its aromaticity. 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. So let's look back at a reaction that we learned a long time ago and see how that would apply to benzene. For example, halogenation. Halogenation was a reaction that took a diatomic halogen and added it across a double bond and you would wind up getting a dihalide as a product. Now that we have 3 double bonds, you may think that halogenation would happen 3 times and just completely halogenate the ring. But now we understand why this reaction is not favored right? Because the product is going to be non aromatic. Why would this molecule be non aromatic? What rule is it breaking? 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 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.

How can we get benzene to react with anything? 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, hey, 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 electrophiles, that would be like an E+ Electrophile. It's an electron lover, something with a positive charge, can temporarily disrupt that 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 Let's go ahead and take a look at the general mechanism of EAS.

EAS General Mechanism

The first thing that should strike us when we look at this mechanism is that it's a 2 step mechanism, and it can be summarized by these words: addition and elimination. These are the two stages of an EAS mechanism. Now, remember that an addition reaction is one that breaks pi bonds. Typically, you break a pi bond and you add 2 sigma bonds. Remember that an elimination is the opposite. An elimination reaction makes pi bonds. That makes sense because, remember, we said that at the end, we want aromatic C to be restored. What we're 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 addition elimination mechanism. That just explicitly says the 2 steps of EAS. There's an addition elimination, addition first elimination, and the whole thing happens through an electrophilic reagent. 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 EA just means that it's some electrophile, so some kind of very strong partial positive or positive attached to some conjugate that we'll use later. Since we have a positive there, you know that the arrows are always going to start from the most negatively charged thing. And for EAS, 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 to attack the electrophile. Let's 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're going to break our bond to the A, forming the conjugate base, which is going to be the conjugate for this reaction. Now what we're going to form is this intermediate. And this intermediate is called the sigma complex, or it's also called an arenium ion. This is one of the most important intermediates of organic chemistry too, so I definitely want you guys to really understand it. What happens is that 2 of the double bonds 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. We just broke the pi bond. Nothing else came to replace it. 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. Let's go ahead and do that now. We're just going to do that underneath here. 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, and then we'll resonate it. 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. The next resonance structure is in the same; 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 arenium ion or sigma complex. 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 3 times. So it's distributed across all 5 of those carbons. As you guys might guess, the resonance hybrid would be drawn simply by drawing a dotted line around all 5 of those and adding a positive. That would be the way that we would represent our sigma complex hybrid. Okay. Awesome. So this sigma complex, even though it's stabilized by resonance, it's still the highest, you know, it's still much higher energy on my energy diagram than the reactants or the products. So this is going to be my slow step because it's difficult to make this intermediate. So now what do we do? Now we need an elimination step. Remember that elimination makes double bonds. 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 an E1. This is essentially going to be an E1 mechanism if you recall back to organic chemistry 1. 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. We're going to do a beta elimination on a hydrogen and reform the double bond. 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. As you guys see, what are we going to have at the end? We're going to have now my substituted arene, and we're going to have some kind of acid. But what we're really concerned about and what your professor is really concerned with is the actual benzene ring, the actual aromatic product. 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 E. Now this mechanism is going to apply for all the different electrophiles we learn in this chapter, but it's going to be your job to know exactly which electrophiles we can use. Let's go ahead and learn about the electrophiles of electrophilic aromatic substitution.

Here’s what students ask on this topic:

What is electrophilic aromatic substitution (EAS) and why is it important for benzene?

Electrophilic aromatic substitution (EAS) is a reaction mechanism where an electrophile replaces a hydrogen atom on an aromatic ring, such as benzene. This process is crucial because it allows benzene to maintain its aromaticity, which is a key factor in its stability. Unlike addition reactions that disrupt the aromatic system, EAS preserves the aromatic nature of benzene by undergoing a two-step mechanism: addition of the electrophile to form a sigma complex (or arenium ion) and elimination of a hydrogen to restore aromaticity. Understanding EAS is essential for predicting and controlling reactions involving benzene and its derivatives in organic chemistry.

What are the two main steps in the electrophilic aromatic substitution mechanism?

The electrophilic aromatic substitution (EAS) mechanism consists of two main steps: addition and elimination. In the addition step, a strong electrophile attacks the benzene ring, forming a sigma complex (or arenium ion). This intermediate is stabilized by resonance but is still higher in energy compared to the reactants and products. In the elimination step, a hydrogen atom is removed from the sigma complex, restoring the aromaticity of the benzene ring. This step is typically fast and completes the substitution process, resulting in a substituted aromatic compound.

Why is the sigma complex (arenium ion) important in EAS reactions?

The sigma complex, also known as the arenium ion, is a crucial intermediate in electrophilic aromatic substitution (EAS) reactions. It forms during the addition step when the electrophile attacks the benzene ring. The sigma complex is important because it temporarily disrupts the aromaticity of benzene, allowing the substitution to occur. Despite being less stable than the aromatic starting material, the sigma complex is stabilized by resonance, which distributes the positive charge over several carbon atoms. Understanding the formation and stabilization of the sigma complex is essential for grasping the overall EAS mechanism.

What factors influence the rate of electrophilic aromatic substitution reactions?

The rate of electrophilic aromatic substitution (EAS) reactions is influenced by several factors. The most significant factor is the nature of the electrophile; stronger electrophiles react more readily with benzene. The presence of substituents on the benzene ring also affects the rate. Electron-donating groups (EDGs) increase the electron density on the ring, making it more reactive towards electrophiles, while electron-withdrawing groups (EWGs) decrease the electron density, making the ring less reactive. Additionally, the stability of the sigma complex intermediate and the ease of the elimination step also play roles in determining the overall reaction rate.

How does the presence of substituents on the benzene ring affect EAS reactions?

The presence of substituents on the benzene ring significantly affects electrophilic aromatic substitution (EAS) reactions. Electron-donating groups (EDGs), such as alkyl groups or methoxy groups, increase the electron density on the benzene ring, making it more reactive towards electrophiles. These groups typically direct incoming electrophiles to the ortho and para positions. Conversely, electron-withdrawing groups (EWGs), such as nitro or carbonyl groups, decrease the electron density on the ring, making it less reactive. EWGs usually direct electrophiles to the meta position. Understanding these effects is crucial for predicting the outcomes of EAS reactions.