EAS:Any Carbocation Mechanism: Videos & Practice Problems

Electrophilic aromatic substitution (EAS) mechanisms involving carbocations are crucial in organic chemistry. Two common methods include hydrofluoric acid (HF) and boron trifluoride (BF3) catalyzed reactions. In HF reactions, a double bond generates a carbocation, which is then attacked by benzene, yielding an alkylated product. BF3 promotes carbocation formation from alcohols, leading to similar outcomes. Notably, BF3 is consumed in the reaction, classifying it as a promoter rather than a catalyst. Understanding these mechanisms enhances comprehension of Friedel-Crafts alkylation and the role of carbocations in EAS processes.

Any Carbocation

Now we're going to talk about another category of EDS mechanisms that are all related in one very important way, which is that they use carbocations as their active electrophiles. I'm just going to call this category the any carbocation category of EAS mechanisms. As you guys know, pretty much any reaction that you learn from organic chemistry 1 or 2 that can generate carbocations could be used in an EAS because the benzene could attack the carbocation immediately when it sees it. However, we're really just going to focus on the two most common ways that we see this because I don't want you guys to spend your time trying to remember 15 different reactions. These are the ones that come up the most often. They're the ones that are catalyzed by HF, hydrofluoric acid, or promoted by boron trifluoride. As I said earlier, obviously, this entire category is defined by carbocations, so you have to watch out for carbocation shifts or rearrangements.

How do these occur? Basically, there are 2 main ways that we find these. A double bond with HF is going to wind up yielding a positive charge because it's an addition reaction. That positive charge can react with the benzene. Another popular way that we see these reactions work is with alcohols and boron trifluoride. It's also a very strong Lewis acid catalyst. As we'll see with the mechanism, it makes sense how it makes a carbocation. Let's first look at the HF mechanism. Just so you know, I'm going to be skipping the drawing of the sigma complex for these because it's literally the same thing as Friedel-Crafts alkylation.

The first step of this mechanism is going to be that my double bond attacks the electrophilic hydrogen. The reason that hydrogen is electrophilic, guys, is because remember there's a very strong dipole making a partial negative and a partial positive, so that double bond is attracted to the hydrogen. That's going to give me an intermediate that looks like this: I'm now going to have an extra hydrogen on one side, so 2 hydrogens because I already had one prior. But I'm going to have a positive charge on the other. This carbocation would not shift in this specific situation. But you do have to be concerned about shifts in general. Definitely make sure that you take care of that before you would ever react with the benzene. What we're used to seeing in a typical reaction in organic chemistry is that the fluorine would attack. This would be called a hydrohalogenation reaction. What do you think is going to happen here? Does that happen? No, guys. What's going to happen is that the benzene is going to compete with the fluoride to be the nucleophile and the benzene is going to win. You're going to get a very high yield of benzene attacking the carbocation instead of fluorine. After all of your resonance structures, you get something that looks like this. What do you think we can use as the conjugate to make the elimination reaction take place? You could use the fluoride ion. The fluoride ion could come in and do the elimination step and what do we get? We get an R group. Here's my benzene. Here's my R group. In this case, it's a cyclobutyl group. Notice I get HF at the end, so that means that this hydrofluoric acid was a true catalyst because it's regenerated.

Now let's look at an alcohol with BF3. What's going to happen here, guys, is that BF3, once again, is a very strong Lewis acid. It's got that empty p orbital. The bond between the carbon and the oxygen can easily break and donate its electrons to the orbital. Obviously, what that means is that the oxygen is picking up its electrons and giving them to the boron. But we're just going to draw it all in one step. What that's going to give us is it's going to give a carbocation plus BF3 with an oxygen negative. Now you know what's going to happen. We've got this positive charge. Now remember, it can't resonate. It could rearrange. Just be mindful that in this case, it won't. But if, for example, I had a methyl group over here, you know that it would wind up doing a 1,2-shift. But let's just say now the benzene reacts with it. I'm going to wind up getting a benzene ring that after drawing my sigma complex, it's going to look like this, with hydrogen and once again cyclobutyl. What do you guys think is going to be the conjugate that reacts with the hydrogen? Actually, guys, this one's a little bit tricky. You guys are right. I know that you guys are saying that negative charge. But it's a little bit tricky because what winds up actually reacting is actually one of the fluorides. It's going to be F-BF2OH. What actually winds up being more reactive is that one of these fluorides is going to do the elimination. Why is that important? It's not really that important. But let me just show you something that's very interesting about this which is that now you get your same cyclobutyl group. But now what are our byproducts? Now I'm actually getting HF. Even though I didn't start off with any HF, I get it. I'm also getting now BF2OH. This is the only part that's worth mentioning, guys. Do you think that BF3 is a true catalyst in this case? Would we call it a catalyst? No, because notice it actually is being consumed. It actually is a reactant that changes at the end of the reaction. We would say that this is a reaction that is promoted by BF3, not catalyzed because it actually is consumed in the reaction. So you have to say that it's promoted. It's an acid-promoted reaction, not an acid-catalyzed reaction. But other than that, this is a Friedel-Crafts alkylation in every other sense of the word. You've got your carbocation and you've got your final alkylated benzene. I hope that made sense, guys. Let's move on to the next video.

Here’s what students ask on this topic:

What is the role of hydrofluoric acid (HF) in EAS mechanisms involving carbocations?

In EAS mechanisms involving carbocations, hydrofluoric acid (HF) acts as a catalyst to generate carbocations from double bonds. The double bond attacks the electrophilic hydrogen (H) of HF, creating a carbocation intermediate. This carbocation is then attacked by benzene, leading to the formation of an alkylated benzene product. The HF is regenerated at the end of the reaction, confirming its role as a true catalyst. This process is crucial for understanding Friedel-Crafts alkylation and the behavior of carbocations in EAS reactions.

How does boron trifluoride (BF3) promote carbocation formation in EAS reactions?

Boron trifluoride (BF3) promotes carbocation formation in EAS reactions by acting as a strong Lewis acid. It accepts electron pairs from alcohols, breaking the bond between carbon and oxygen. This results in the formation of a carbocation and BF3 with an OH^- group. The carbocation can then be attacked by benzene, leading to an alkylated benzene product. Unlike HF, BF3 is consumed in the reaction, making it a promoter rather than a catalyst. This distinction is important for understanding the role of BF3 in Friedel-Crafts alkylation and EAS mechanisms.

What are the main differences between HF-catalyzed and BF3-promoted EAS reactions?

The main differences between HF-catalyzed and BF3-promoted EAS reactions lie in their roles and mechanisms. HF acts as a true catalyst, facilitating the formation of carbocations from double bonds and being regenerated at the end of the reaction. In contrast, BF3 is a promoter, not a catalyst, as it is consumed during the reaction. BF3 promotes carbocation formation from alcohols by accepting electron pairs, leading to the formation of a carbocation and BF3 with an OH^- group. Both methods result in the alkylation of benzene, but their roles and mechanisms differ significantly.

Why is it important to consider carbocation shifts or rearrangements in EAS mechanisms?

Considering carbocation shifts or rearrangements in EAS mechanisms is crucial because carbocations are highly reactive intermediates that can undergo structural changes to form more stable carbocations. These shifts can significantly impact the outcome of the reaction, leading to different products. For example, a 1,2-hydride or alkyl shift can occur, resulting in a more stable carbocation that benzene will attack. Ignoring these potential shifts can lead to incorrect predictions of the reaction products. Understanding these rearrangements is essential for accurately predicting and controlling the outcomes of EAS reactions involving carbocations.

How does the mechanism of HF-catalyzed EAS reactions compare to Friedel-Crafts alkylation?

The mechanism of HF-catalyzed EAS reactions is similar to Friedel-Crafts alkylation in that both involve the formation of a carbocation intermediate that is attacked by benzene. In HF-catalyzed reactions, a double bond reacts with the electrophilic hydrogen of HF, forming a carbocation. This carbocation is then attacked by benzene, leading to an alkylated product. In Friedel-Crafts alkylation, an alkyl halide reacts with a Lewis acid (like AlCl₃) to form a carbocation, which is then attacked by benzene. Both mechanisms involve carbocation intermediates and result in the alkylation of benzene, but the catalysts and specific steps differ.