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The mechanism of the Birch reduction. Created by Jay.
Video transcript
In this video, we're going to look at the general mechanism for the Birch reduction. So we start with benzene and to it we add an alkaline metal like sodium and liquid ammonia and also an alcohol, and the end result is to reduce the benzene ring to form 1, 4-cyclohexadiene. Let's look at the mechanism for the Birch reduction. So we know that sodium is in group one of the periodic table and so it has one valence electron, which I will go ahead and color magenta there. And so the start of the mechanism is for sodium to donate its one valence electron to the benzene ring, and so we can show the movement of that one electron with a fish hook arrow or a half-headed arrow here where we show that one electron moving over here to this carbon, so this carbon right here. Now, we're also going to get some movement of electrons in our benzene ring. So when I think about these electrons in here, so these pi electrons in red, we know that there are two electrons there. So let me go ahead and show those two electrons like that. So those two electrons are also going to move. So this electron over here is going to come off on this carbon as well, and then this other electron in red is going to move over to here. So let's go ahead and show the start of the movement of those electrons. We're going to come back and move some more, but I just want to do this really slowly here so we can follow along. So we had our hydrogens attached to our rings so let me go ahead and sketch those in really fast here. So these pi electrons are going to stay put for our mechanism. And let me show the electron in magenta, the one that sodium donated, so it ends up being on this carbon right here. And then one of the electrons in red also moved on to that carbon. So let me show that electron in red right there. So that carbon gets a negative 1 formal charge so we form an anion here. One of those red electrons is going to move over to this position right here, and then we are also going to show these electrons moving around, so the electrons in green here. Let me just go ahead and highlight those. So these electrons, so there's one and there's two. So one of them is going to move to the same position that the red one did in there, and the other one is going to come off onto this carbon. So let me see if I can show that. So one of them moved in here like that, and the other one moved off onto this carbon like that. So the one in red and the one in green are, of course, now a pi bond. You could think about it that way, and so we've now generated what's called a radical anion here. So this is a radical anion. So it's a radical because you have that one unpaired electron in green, and it's an anion since you have a negative 1 formal charge over here on this topic carbon. And then I forgot to put in this hydrogen so let me go ahead and add that one in there like that. Second step of our mechanism, our alcohol comes along so we have our generic alcohol, which is going to function as an acid because the negative 1 formal charge, the anion here, the carbanion, is going to function as a base. And so these electrons here are going to pick up a proton from the alcohol so these electrons will kick off onto the oxygen here. And so let's go ahead and draw the result of that acid base reaction. So we have these pi electrons in here, and we now have two hydrogens on that top carbon. We have one hydrogen on each of our other carbons here. And we still have a radical. So let me go ahead and show this electron is still on that carbon. So now we have a radical instead of a radical anion. And I should point out that for our radical anion and for our radical, the electron density can be delocalized throughout the ring, but here we're just trying to show just moving around some electrons. And so let me go ahead and highlight to these two electrons here. So the electron in red and the electron in magenta are forming a bond with that proton right here on our ring. So next step in our mechanism, we get some more sodium so some more sodium comes along here. Let me go ahead and show that. And, of course, once again, sodium has one valence electron so here's sodium's one valence electron. The sodium can donate that valence electron to our benzene ring, and so it's going to donate it over here to this carbon, the carbon that had the green electron on it already. And so let's go ahead and show the result of that. We would have our ring, we would have our pi electrons. We had two hydrogens bonded to the top carbon. We had these hydrogens around my ring like that. The bottom carbon still has a hydrogen bonded to it. And we started with a green electron on that carbon, and now we're going to add a magenta electron, giving that carbon a negative 1 formal charge so we form an anion again. So now that we have an anion, the last step of the mechanism is another acid base reaction so our alcohol comes along, and the carbanion is going to function as a base and pick up a proton from our alcohol. So the same step that we saw before pretty much. And we go ahead and draw our final product. So we have those pi electrons. We had these hydrogens on our top carbon, these carbons all get hydrogens, and then finally, we have added on a proton to this bottom carbon here. So let me go ahead and highlight those. So these electrons here, those two electrons pick up a proton so we protonate our ring and we finish. So this is our 1, 4 cycylohexadiene product here. Now, a simple way of thinking about this mechanism for the Birch reduction is to break it into these four steps and to make those steps very simple. So in the first step, sodium is donating electrons an electron so you could think electron for step one. Second step, we know that the anion is picking up a proton from our alcohol so you could think proton for the second step. For the third step, once again, sodium is going to donate an electron so you could think electron. And then finally, once again the, anion is picking up a proton so you could think proton. So you could think electron, proton, electron, and proton is a simple way of thinking about the steps for a Birch reduction. In the next video, we're going to look at what happens with the Birch reduction when you get a substituted benzene ring. And I'll show you mechanisms for the two possibilities that you might see on an exam.