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Organic Chemistry

Learn the toughest concepts covered in Organic Chemistry with step-by-step video tutorials and practice problems by world-class tutors.

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18. Reactions of Aromatics:EAS and Beyond

EAS:Friedel-Crafts Alkylation Mechanism

Friedel-Crafts Alkyation requires an alkyl halide to complex with a Lewis Acid Catalyst before the reaction can begin. 


Friedel-Crafts Alkylation

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Now we're going to explore the mechanism for Friedel crafts calculation. So frail craft calculation is gonna be the reaction of a alcohol allied with a strong Lewis acid couple. Together, they are gonna make a strong electro file. But that active Electra file for this molecule is actually a true Carvel cat Ion. Okay, remember that Carvell Catalans are definitely electron loving because they have an empty P orbital. But they're also very tricky because what happens when we have Carvell Canada intermediates? We have toe watch out for rearrangements. So it's gonna be kind of the annoying part of this reaction that we can What? We will observe carbon capture and rearrangements when it's, you know, when it's favorable for the reaction. So that is gonna complicate our That could complicate our products quite a bit. Now, we're not gonna focus on rearrangements in this mechanism because what I'm gonna do is I'm gonna show you the A s part. But later on, when you're drawing products for calculation, it's gonna be important that you always think about those rearrangements before drawing your final product. So as you guys can see, we have a knock your hey lied and a strong Lewis acid. We say here, aluminum try. Hey, lied. But usually that's gonna be a chlorine. Okay, so let's look at this Following alcohol. Hey, light and how it's gonna react. The very first thing is gonna be that we want to generate that carbon county. 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. Okay, Now, this mechanism is similar to what? 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, if that's gonna give me is it's going to give me a benzene ring with I'm gonna get a l c l four minus right. But I'm also gonna go to Carvel, Catalan. So are plus okay, so I'm gonna get our plus, and then I can do the rest of my mechanism. So I'm gonna get that the double bond attacks the r plus, and I'm gonna get my Sigma complex. Let's draw it. So I've got my double bond double bond. H are Oops. Forgot to draw the positive charge. So let's just do that. Great. So now I've got my resident structure, so it's got a positive here, Aaron, draw one more and there you have it. That is our sigma complex. Okay, Now, what do you think is gonna be left over to eliminate in the second step of my reaction? You got it. The Lewis acid catalyst. That's, uh, negatively charged so I could draw this as cl a l c l three negative. Right? And what we're gonna do is we're gonna take the electrons from that bond and use them to do my beta elimination. What that's gonna give me as a product is now I'm gonna have an calculated benzene. Okay? With what? Well, with my ale. Si l three catalyst noticed that I regenerated the same catalyst as I had at the beginning, so it truly is a catalyst. And I'm also gonna have hcl so on acid being generated as a byproduct. Okay, So really straightforward mechanism. Now, I do want to show you guys one thing, though it turns out that when you have primary, um, alcohol slides, primary alcohol. Hey, Allied's, um, the mechanism does get a little bit more tricky because primary Carvel cat ions are unstable. And what did we just say about this? I'm gonna put a sad face. Okay. Primary carbon atoms are unstable. What did we just say about this mechanism? It's carbon kata and Inter mediated. So how does this reaction happen if primary carbon condoms were so high energy, they're so difficult to create. Well, in that case, what we're gonna do is we're gonna make the mechanism very similar, but we're gonna have to do the carbon cutting and shift, actually, in the actual mechanism. So let me just show you really quick what happens if this was my carbon category or this was my This is my, um I'll kill a line, and I'm reacting that with l c. L three and I've got my empty P orbital at the top. Okay, well, in this case, it would actually be a missed A cure Professor would probably be unhappy if you just grab these electrons and put them into their and that's it. Okay, because what you're going to get is a product. Don't draw this. What you would get is a product is you would get this okay. And that, as we know, that's a primary car. Broke a tie in, and that's not very stable. Okay, 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 carbon capture and shift in the same mechanism. So would this carbon capture and want to shift. If if it was formed on the primary, yes, you would shift into 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 rearrange carbon Catalan, plus R l c L four negative. And then at this point, this is the active Electra file that my benzene would react with, Not the primary. Okay, so if you have a primary Carvel cat iron than 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 gonna avoid primary carbo Catalans at all costs in this corpse because they're so high energy, they really don't very difficult to generate them in the lab. Okay, awesome, guys. That's the end of that mechanism. Let's move on to the next one.