If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:7:09

Video transcript

in this video we're going to look at the general mechanism for the birch reduction so we start with benzene until we add an alkali metal like sodium and liquid ammonia and also an alcohol and the end result is to reduce the benzene ring to form one for cyclo hexa diene 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 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 right so 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 some movement of electrons in our benzene ring so when I think about when I think about these electrons in here alright 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 started the movement of those electrons are gonna come back and move some more but I just want to do this really slowly here so we can follow along alright so we had we had our hydrogen's attached to our ring so let me go ahead and sketch those in really fast here alright so these PI electrons are going to stay put for our mechanism and let me show the electron in magenta right 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 these electrons right 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 all right like that and the other one moved off onto this carbon like that so the one in red and the one 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 you have that one unpaired electron in green and it's an anion since you have a negative one formal charge over here on this top 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 one formal charge the anion here the carvanha is going to function as a base and so this this negative these electrons here are going to pick up a proton from the alcohol so these electrons would 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 had we had now have two hydrogen's right 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 is 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 these two electrons here so the electron in red and electron in magenta all right 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 alright we had two hydrogen's bonded to the top carbon we had these hydrogen's 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 right and the carb anion 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 alright we had these hydrogen's on our top carbon these carbons all get hydrogen's 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 right pick up a proton so we protonate our ring and we finish so this is our one for cyclo hexa diene a product here now a simple way of thinking about this mechanism for the birch reduction is to think about is to break it into into these four steps and to make those steps very simple so in the first step right sodium is donating an electron so you could think electron for step one second step we know that the anion is picking up a proton right from our alcohol so you could think proton for the second step for the third step once again sodium was going to donate an electron so you think electron and then finally once again the anion is picking up a proton so you think proton so you think electron proton electron 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