18. Reactions of Aromatics:EAS and Beyond
The birch reduction is a dissolving metal reduction, except reacting with benzenes instead of alkynes. The product of an unsubstituted benzene is a simple isolated cyclohexadiene.
Birch Reduction Mechanism
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Hey, guys. So in this video, we're gonna talk about a specific type of reduction reaction that can happen with benzene, and that's called the birch Reduction. So let's just take a look at the general reaction for a second. What a birth reduction does is it combines elemental sodium within a mean and alcohol to turn a benzene into what we call an isolated dying. So specifically, if this were to happen with an unsub stewed, bending like we have here are products would be an isolated cyclo hexane dine two double bonds that air far apart from each other in a position on a cyclo hexane. Okay, now, if you take a closer look at these re agents, they might look familiar because these air very similar to the re agents that we use on a dissolving metal reduction. This is a reaction from Oracle, one that we learned a long time ago that worked with al kinds and it was a radical mediated mechanism. Well, it turns out that this mechanism is really the same exact mechanism, except it's gonna work with benzene instead of within Al Qaeda. So let's get right into it. The mechanism for this reduction is gonna proceed through elemental sodium, which means it's going to possess just one electron. Okay, When that one electron donates toe any of the carbons, we're gonna have to break a bond. Okay, but this is gonna be a mechanism where we have a combination of half headed arrows and normal arrows, just like the dissolving metal reduction. How? There were some arrows that moved one radical and some arrows that moved alone. Pair. So when we make that bond, we have to break this bond in order to make room for the radical and in order to keep these charges as far away from each other as possible with these intermediates as far away from each other as possible, the this old bond is going toe ionized into a lone pair onto the very bottom. So basically the furthest position possible from the radical we're going to get an anti on. So let's go ahead and draw the product of this first step. What we're now going to get is a single radical of the top double bonds on both sides and now a lone parent. The bottom which is gonna be ah, carbon on it. Okay, so this intermediate is called a radical, and I am, which makes sense, because that's what it is. It's a radical, and it's an anti on now, guys, this is where ethanol comes in. Our ethanol is gonna serve as appropriating agent. Okay, now, just you know, ethanol isn't the Onley alcohol you can use. Some text will use Turk. Beautiful. It doesn't matter, guys. It's a source of hydrogen. That's the biggest deal, so e t o h my an ion is gonna grab the h and give a negative charge of the So now what I'm gonna get is a molecule. Looks like this. I've got my toodle wants. I still have my radical, but now I have to h is at the bottom because I had one originally. And now I just added a second one, which is the one that came from the ethanol. Okay, at this point, I react with another equivalent off my elemental sodium that elemental sodium is going to donate electron to that same location. And now I'm going to get a lone pair in Ion. Okay, so this is just a carbon ion intermediate, okay? And guys, this reaction just repeats itself. That's one thing about maybe dissolving metal reduction if you recall, it was the same thing twice. So here we would react again with another equivalent of ethanol, and we would wind up getting are isolated, dying because now we've got hs two h is on the bottom. I've got two inches on the top. And what's the ugliest H? Ever. Sorry. And it got my isolated dying, Which is this molecule here? Okay, for this reason, the fact that it reacts twice sometimes you might see professors, actually, right. Ethanol times two or alcohol times too. It doesn't matter, guys. It's just gonna have enough equivalent to make the reaction go to completion. All right, so that's really it. That's the mechanism for Burt production. And now what we're gonna do is we're gonna talk about specific regional chemistry you have to consider with a birth reduction
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since this reaction always passes through an anti an intermediate, we can actually use activating groups and deactivating groups to direct the site of the isolated dying. How does that work? Well, let's just take a look at the anti on or the carbon. I an intermediate. So this would be the point where we have the two day old ones. We have the two ages, and we have a lone pair Negative at the top. OK, let me ask you a question. If I add an electron withdrawing group to that anti, what do you think it does for stability? Do you think it makes that an ion more stable or less stable? Okay, so hold that thought. Now, what happens if I add an electron donating group to that? And I am. So what if I add something is gonna give mawr electrons to the negative? What does that do for the stability? So the answer was that the first one is gonna make it more stable because it pulls electrons away, right? An electron donating group is actually gonna make it less stable, right? Because it's gonna push mawr electrons into the anti on, so it turns out that these different groups are gonna direct where the double bonds go. Okay, So as you guys can see, withdrawing groups are going to what I say isolate themselves from the dying. And I specifically chose that word for a reason. Because withdrawing and isolate kind of mean the same thing. If you're withdrawing from the crowd, that means you're isolating yourself. So withdrawing Group is going to be isolated from the double bond that's gonna be away from the door bonds. And why is that? It's not just because we memorized it. It's because you know that it's going to stabilize the negative charge, right? So it's gonna want to be where the negative charge waas, whereas donating groups are going to attach themselves directly to the dying in this situation. Why? Because I have electrons going into the ring and I don't want it to be here, Okay, because it was there. It would make my anti on less stable, so I'm trying to put it in a place where it's not going to affect the stability where it's gonna be fine. Okay, So electron donating groups attached to the ring and withdrawing groups isolated from the ring. If you don't remember the mechanism, you could at least remember the way that I'm telling you. Right? Which is that withdrawing isolates? In case you can think of just you're isolating yourself from the crowd. Your withdrawn or donating attach is which is basically the opposite. Okay, awesome guys. So really, that's it for this topic. Let's move onto the next one.
Substituents affect the course of the mechanism, yielding regiospecific products.
Predict the major product from the Birch Reduction
Predict the major product from the Birch Reduction
Additional resources for Birch Reduction
PRACTICE PROBLEMS AND ACTIVITIES (3)
- Predict the major products of the following reactions. (c) o-xylene + H2 (1000 psi, 100 °C, Rh catalyst)
- Predict the major products of the following reactions. (b) benzamide (PhCONH2) + Na (liquid NH3, CH3CH2OH)
- Propose mechanisms for the Birch reductions of anisole. Show why the observed orientation of reduction is favo...