Organic Chemistry

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18. Reactions of Aromatics:EAS and Beyond

EAS:Halogenation Mechanism

EAS Bromination and Chlorination both require complexing with a Lewis Acid Catalyst before the reaction can begin.

General Overview:



EAS Halogenation

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let's dive into the exact mechanism of E s hallucination. So E s Bram in ation and chlorination are both gonna have to complex with a lewis acid catalyst before any reaction can take place. Remember that in our general reaction, you need that Lewis acid catalysts in order to go anywhere, and we're gonna be using that to start off this reaction. So, in our very first step for our mechanism, before the benzene could get involved, it all we need to complex are diatonic halogen with the Lewis acid catalyst. And that happens through the bro. Mean sharing some of its electrons with the lewis acid that you know, it's missing electrons. It's a great electron pair, except er Now what this is going to do is it's going to form a complex. That's very Electra Filic. Let's see what it's gonna look like. It's gonna be a bro ming attached to a bro mean attached to an iron, which is attached to three bro means I'm just gonna put br three. Is that fine? You guys know what that stands for? All three Brahms. Okay, awesome. Now, are there any formal charges included in this molecule? Yeah, Well, now the iron was neutral before it has an extra bond that's going to get a negative charge, bro. Mean, as we know, likes to have seven valence electrons. Right now, it only has six. Because as two lone pairs to bonds, who impairs, that's going to get a positive charge. It's missing a Valence electron. Okay, so this is our active Electra file. This is our active Electra file. This is the one that reacts with bending. Okay, So what's gonna happen in this mechanism is that my bending is the nuclear file. Now it's gonna attack the Electra file. What's it might seem a little bit weird is that you would think that it would go straight for the positive charge, because usually negatives attack positives. But actually, let's just bring down the benzene here. What's gonna happen is that the bending is not gonna attack the positive. It's gonna attack the bro. Me next to the positive. Why? Because if it can attack that one and remove it, then this condone eight. This bro. Mencken donates electrons, loons, missing some, which is the actual, positively charged bro. Man. Okay, so now going down. But this is going to cause is an interruption of air. Metis City, This is gonna be our intermediate. Okay, so now we're gonna have is one double bond here, Wendell one here. We had an each before, but now we've also got a bro me in. Okay, here. We also have an H, but it's missing its fourth bond. So that's where that's gonna be. Where are Carbo? Catalan goes and this specific carbo Catalan is called. What? This is our arena, my on or sigma complex. Okay, so this is gonna be our sigma complex, okay? And as we know, that Sigma complex has resonance structures. Let's just draw those really quick. We know that this soul bond can resonate to three different positions. It's gonna move over h br. And what we're gonna end up with is three resonant structures. Okay, that air stabilizing that intermediate. Okay, as you guys recall, this is the slow step of the reaction. Making the intermediate. All right, So now we want to do the elimination step. That was addition. Let's do the elimination steppin end this reaction. So what do you guys think is gonna be the nuclear file that reacts with my a rainy, um, cat. I am good. It's gonna be the f E b R four that's negatively charged. So we've still got this f e b r three that has an extra extra br on it, and the whole thing is negatively charged. So how is that going to react? Well, we can use the electrons from the bond from the extra bond to eliminate the hydrogen. Okay, Now, to me personally, the mechanism that makes the most sense and you're going to see this a lot in this chapter is what I think makes sense is that the bro mean grabs its electron, says, Hey, I'm taking him back, and then it gives its electrons to the H. Actually, don't draw this, okay? So use your race or really quick, and then it gives its electrons to the age. So to me, if I were writing your textbook, that's the way I would have drawn it, because it makes the most sense that it takes its electrons and then it gives them to the H. Okay, But the way that textbooks usually write this mechanism is all in one shot. They'll write that the electrons just go straight for the H, Okay, but it's the same thing. This this notation of showing that the electrons go from that bond to the age literally just means that the Brahman is taking its electrons and giving them out to the H and now making a bond with the H. Now we know that hydrogen can't make to bonds, So if we make that bond, we would then break this bond and reform the aromatic compound. Okay, so that was just a note to say, guys, that if you ever see me, you know, in the next few mechanisms drawing straight from a bond, that means that you could just think of it as that to arrow mechanism instead. So we're gonna get now is we're gonna get Since I reacted with this resident structural job like this, bro Mean here. Plus, we're going to get what else we're gonna get. FBR three notice that this is why it's called a Lewis acid catalyst. Because we regenerated it at the end. Okay. And we're gonna get HBR excellent guys. And if you're wondering if the residents structure matters No, it doesn't. Guys, you could have drawn that resident structure. However, because It's constantly change. It's constantly. It's in a resident structure. You could draw however you want. You could just draw it as a circle if you want, but that is our product and that's it. So let's go ahead and move on to the next mechanism.