EAS:Friedel-Crafts Alkylation Mechanism: Study with Video Lessons, Practice Problems & Examples

Friedel-Crafts alkylation involves the reaction of an alkyl halide with a strong Lewis acid to generate a carbocation, which acts as a strong electrophile. This mechanism can lead to carbocation rearrangements, particularly with primary alkyl halides, which are unstable. The process includes the formation of a sigma complex and the regeneration of the catalyst, typically AlCl₃. Understanding the stability of carbocations and the potential for rearrangements is crucial for predicting the final products in this electrophilic aromatic substitution reaction.

Friedel-Crafts Alkylation

Now we're going to explore the mechanism for Friedel-Crafts alkylation. Friedel-Crafts alkylation is going to be the reaction of an alkyl halide with a strong Lewis acid. Coupled together, they are going to make a strong electrophile. But the active electrophile for this molecule is actually a true carbocation. Remember that carbocations are definitely electron-loving because they have an empty p orbital. But they're also very tricky because what happens when we have carbocation intermediates? We have to watch out for rearrangements. That's going to be kind of the annoying part of this reaction: we will observe carbocation rearrangements in this mechanism because what I'm going to do is I'm going to show you the EAS part. But later on, when you're drawing products for alkylation, it's going to be important that you always think about those rearrangements before drawing your final product. As you guys can see, we have an alkyl halide and a strong Lewis acid. We say here aluminum trihalide but usually, that's going to be a chlorine. Let's look at this following alkyl halide and how it's going to react. The very first thing is going to be that we want to generate that carbocation. We're going to take the electrons from the carbon-chlorine bond and donate them to that empty orbital on the aluminum. This mechanism is similar to when I told you guys that you could just pick up your electrons and then give them away to the aluminum. It's the same thing. I'm just taking them and giving them directly to the aluminum. Now what that's going to give me is it's going to give me a benzene ring with AlCl₄^-. But I'm also going to go to a carbocation, so R⁺. I'm going to get R⁺ and then I can do the rest of my mechanism. So I'm going to get that the double bond attacks the R⁺ and I'm going to get my sigma complex. Let's draw it. I've got my double bond, H-R. Oops, I forgot to draw the positive charge, so let's just do that. Great. Now I've got my resonance structure. So that's got a positive here. We're going to draw one more. There you have it. That is our sigma complex. Now what do you think is going to be left over to eliminate in the second step of my reaction? You got it. The Lewis acid catalyst that's negatively charged. I could draw this as Cl AlCl₃^- and what we're going to do is we're going to take the electrons from that bond and use them to do my beta-elimination. What that's going to give me as a product is now I'm going to have an alkylated benzene with my AlCl₃ catalyst, notice that I regenerated the same catalyst as I had at the beginning. It truly is a catalyst and I'm also going to have HCl, so an acid being generated as a byproduct. Really straightforward mechanism. Now I do want to show you guys one thing though. It turns out that when you have primary alkyl halides, the mechanism does get a little bit more tricky because primary carbocations are unstable. What did we just say about this mechanism? It's carbocation intermediated. How does this reaction happen if primary carbocations are so high energy, they're so difficult to create? In that case, what we're going to do is we're going to make the mechanism very similar, but we're going to have to do the carbocation shift actually in the actual mechanism. Let me just show you really quickly what happens if this was my carbocation or this is my alkyl halide and I'm reacting that with AlCl₃. I've got my empty p orbital at the top. In this case, it would actually be a mistake. Your professor would probably be unhappy if you just grab these electrons and put them into there and that's it, because what you're going to get as a product is you would get this. And that as we know, that's a primary carbocation and that's not very stable. Most professors don't like to see that on your page. How do we fix this? Well, it's pretty easy to fix, guys. All you have to do is if you have this situation, you have to draw the carbocation shift in the same mechanism. Would this carbocation want to shift if it was formed on the primary? Yes, you would shift it to the secondary. Then we would just draw the shift in the same mechanism. I would draw that this is going to give its electrons away but now this bond is going to give its electrons away, so that eventually in the same step, we get our rearranged carbocation plus our AlCl₄^-. And then at this point, this is the active electrophile that my benzene would react with, not the primary. If you have a primary carbocation, then draw the shift within the mechanism. Not so bad. Just a little contingency there because as you're going to see as a general pattern, we're going to avoid primary carbocations at all costs in this course because they're so high energy. It's very difficult to generate them in a lab. That's the end of that mechanism. Let's move on to the next one.

Here’s what students ask on this topic:

What is the mechanism of Friedel-Crafts alkylation?

Friedel-Crafts alkylation involves the reaction of an alkyl halide with a strong Lewis acid, typically AlCl₃. The Lewis acid helps generate a carbocation from the alkyl halide, which acts as a strong electrophile. The benzene ring then attacks this carbocation, forming a sigma complex. This complex undergoes deprotonation to regenerate the aromatic system, producing an alkylated benzene. The catalyst, AlCl₃, is regenerated in the process, making it a true catalyst. The overall reaction can be summarized as:

$C_{6\mathrm{H}6}+R_{\mathrm{Cl}}+{\mathrm{AlCl}}_3\to C_{6\mathrm{H}5R}+{\mathrm{HCl}}_{}+{\mathrm{AlCl}}_3$

Why are carbocation rearrangements important in Friedel-Crafts alkylation?

Carbocation rearrangements are crucial in Friedel-Crafts alkylation because they can significantly affect the final product. When a carbocation intermediate forms, it may rearrange to a more stable carbocation, such as from a primary to a secondary or tertiary carbocation. This rearrangement occurs because primary carbocations are highly unstable. For example, if a primary carbocation forms, it will likely rearrange to a secondary or tertiary carbocation before the benzene ring attacks. This rearrangement must be considered when predicting the final product, as it can lead to unexpected outcomes.

What role does AlCl₃ play in Friedel-Crafts alkylation?

AlCl₃ acts as a strong Lewis acid in Friedel-Crafts alkylation. Its primary role is to generate a carbocation from the alkyl halide. AlCl₃ accepts a pair of electrons from the alkyl halide, breaking the C-Cl bond and forming a carbocation. This carbocation is the active electrophile that reacts with the benzene ring. Additionally, AlCl₃ is regenerated at the end of the reaction, making it a true catalyst. The overall reaction can be summarized as:

$R_{\mathrm{Cl}}+{\mathrm{AlCl}}_3\to R_{}+{\mathrm{AlCl}}_4-$

How do primary alkyl halides behave in Friedel-Crafts alkylation?

Primary alkyl halides are challenging in Friedel-Crafts alkylation because primary carbocations are highly unstable. When a primary alkyl halide reacts with AlCl₃, the initially formed primary carbocation will likely rearrange to a more stable secondary or tertiary carbocation. This rearrangement occurs within the reaction mechanism itself. For example, if a primary carbocation forms, it will shift to a secondary carbocation before the benzene ring attacks. This rearrangement must be considered when predicting the final product, as it can lead to different outcomes than initially expected.

What are the products of Friedel-Crafts alkylation?

The primary product of Friedel-Crafts alkylation is an alkylated benzene. Additionally, the reaction generates HCl as a byproduct and regenerates the Lewis acid catalyst, typically AlCl₃. The overall reaction can be summarized as:

$C_{6\mathrm{H}6}+R_{\mathrm{Cl}}+{\mathrm{AlCl}}_3\to C_{6\mathrm{H}5R}+{\mathrm{HCl}}_{}+{\mathrm{AlCl}}_3$