EAS: Friedel-Crafts Alkylation Mechanism

Now we're going to explore the mechanism for Friedel-Crafts Alkylation. So Friedel-Crafts Alkylation is going to be the reaction of a alkyl halide with a strong Lewis Acid, coupled together they are going to make a strong electrophile but that active electrophile for this molecule is actually a true carbocation. Remember that carbocations are you know 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. So that's going to be kind of the annoying part of this reaction that we can what we will observe carbocation rearrangements when it's favorable for the reaction. So that is going to complicate or that could complicate our products quite a bit. Now we're not going to focus on rearrangement 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. So 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. So 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 carbocation, right? So we're going to take the electrons from the carbon chlorine bond and give them, donate them to that empty orbital on the aluminum. Now 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 I'm going to get ALCL4 minus but I'm also going to get a carbocation so R plus. So I'm going to get R plus and then I can do the rest of my mechanism so I'm going to get that that the double bond attacks the R plus and I'm going to get my sigma complex. Let's draw it. So I've got my double bond, double bond, H, R, oops I forgot to draw the positive charge so let's just do that. Great so now I've got my resonant structure that's got a positive here, you're going to draw one more and 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. So I could draw this as CLALCL3 negative and what we're going to do 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 what? Well with my ALCL3 catalyst, notice that I regenerated the same catalyst as I had in the beginning so it truly is a catalyst and I'm also going to have HCL so an acid being generated as a byproduct. So really straightforward mechanism now I do want to show you guys one thing though. It turns out that when you have a primary alkyl halides, primary alkyl halides, the mechanism does get a little more tricky because primary carbocations are unstable and what did we just say about this? Let me just put a sad face.

Primary carbocations are unstable. What did we just say about this mechanism? It's carbocation intermediated. So how does this reaction happen if primary carbocations are so high energy they're so difficult to create? Well 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. So let me just show you really quick what happens if this was my carbocation or this was my alkyl halide and I'm reacting that with ALCL3 and I've got my empty P orbital at the top. Well in this case it would actually be a mistake your professor would be unhappy if you just grab these electrons and put them into there and that's it because what you're going to get is a product, don't draw this, what you would get as a product is you get this and that as we know that's a primary carbocation and that's not very stable so most professors don't like to see that on your page so how do we fix this? Well it's pretty easy to fix guys all you have to do is if you have the situation you have to draw the carbocation shift in the same mechanism so would this carbocation want to shift if it was formed non-primary? Yes you would shift to the secondary so then we would just draw the shift in the same mechanism so I would draw that, hey, 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 negative and then at this point this is the active electrophile that my benzene would react with not the primary. So if you have a primary carbocation then draw shift within 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 they really don't, they're really difficult to generate them in a lab. Awesome guys that's the end of that mechanism. Let's move on to the next one.