Wolff-Kischner Reduction

Concept: Concept: General Reaction

3m
Video Transcript

On this page, I wanna discuss a specific reaction that hydrazones can undergo and that’s Wolff-Kishner reduction. The Wolff-Kishner reduction is a reaction that completely removes carbonyls. I’m not sure if you guys are already familiar with these reactions or if you’ve studied them yet but there's two reactions that you learned in Organic Chemistry 2 that also completely remove carbonyls. One is called Clemmensen reduction. Does that sound familiar? The other one is called thioacetals plus Raney nickel.
It turns out that this is just going to be another method. There’s a third method that we can use to completely get rid of a carbonyl and turn it into an alkane. The way we do this is by using an ammonia derivative to make an imine derivative. Let's see how. What we do is we take a carbonyl, you react it with hydrazine. Hydrazine is going add and make an imine derivative specifically the imine derivative that we make is called hydrazine. If you recall, hydrazine is the combination of hydrazine with a ketone, so it’s hydrazine. Once you have your hydrazine, usually you would be done, usually this would be the end of the reaction. It’s reversible. But if you react this hydrazone in a base-catalyzed environment, you’re going to get a completely different product. What you’re actually going to get is the generation of N2 gas, which I’ll show you. You’re going to get N2 gas to evolve and you're going to get an alkane.
The reagents we usually use for this are some strong base. NaOH works just great. In your textbook, you might see KOH or 2-tertbutoxide. It doesn't matter. It’s just some strong base. Usually there's an alcohol present to help the reaction along. This alcohol is not in the mechanism. It’s just there to provide correct conditions for the mechanism to take place. Ethylene glycol as shown here is a really common one and you need heat. You need some kind of heat to get the reaction going. Now what I’m going to do is in the next, I’m going to show you the whole mechanism for Wolff-Kishner reduction.

Concept: Concept: Mechanism

5m
Video Transcript

Alright guys, so when it comes to the mechanism of Wolff-Kischner reduction, there's two things that I want you to keep in mind. One, we're starting off with hydrazones so don't worry about drawing that mechanism again, oops no H, N H H, perfect. Don't worry about drawing the mechanism for hydrazone because we've already done that, that would be an imine reaction so don't worry about that. What we're going to do is we're going to react this with base and there's two objectives that we're trying to do. One thing we're trying to do is we're trying to add H's to the imine carbon. So I'm just saying that this is an imine derivative so that would be this guy right here. Another thing we're trying to do is we're trying to evolve N 2 gas. Now if you guys don't know what N 2 gas looks like, N 2 gas is N triple bond N, lone pair, lone pair in fact nitrogen gas like seventy eight percent of the atmosphere is N 2 gas so seventy eight percent of every breath you take tonight is an N 2 gas, isn't that romantic?

So we just drew that so we're trying to somehow make a triple bond between those nitrogens maybe that will, those objectives, will help you to remember this mechanism because it's a little weird. So the first thing we're going to do is we're going to transfer a proton to the very bottom carbon. The way we do that is through base-catalyzed proton transfer so my base is going to grab an H, that's going to cause a double bond to form here, make a bond break a bond. If I make that bond I have to break a bond so then this double bond is going to break but what it's going to do is it's going to grab an H off of the conjugate of my base. So what I wind up doing is I wind up getting something like this, N now double bond N, H and now I have an extra H down here that I didn't have before. So notice that I just got closer to my goal in two different ways. One, I was able to add an H to the bottom carbon, awesome. Two, I was able to get closer to putting a triple bond between my nitrogens. I'm trying to get a triple bond. By the way this was called,this was my base-catalyzed proton transfer. Perfect, so now what can we do? We can do it again. So I can react with another equivalent of base and do the reaction again. So I'm going to take away this H, if I make a bond I break a bond, I'm going to break a bond and make one to the N and now that that nitrogen has three bonds the one on the bottom it doesn't need any more bonds so it's literally just going to break this single bond and turn it into an anion at the bottom. What this is going to do is it's going to give me a molecule that looks like this. Now I have a lone pair here, so I have a negative charge, I have an anion plus I have N triple bond N plus I have water which doesn't really matter but notice that now this nitrogen gas is gone. It can just leave, it's not tied back to anything it's just going to take off. It's going to go into the atmosphere. Now this anion however is very unstable so this anion remember it had one H already. Let's draw in that H, it was here. Well that anion is just going to grab another hydrogen to regenerate that base. So what I'm going to get at the end is I'm going to get an alkane that now I added to hydrogens to so I get an alkane product plus I get my N 2 gas and I get my base left over at the end.

Cool, right? So guys I don't know if your professor's going to require you to memorise this. Usually with Wolff-Kischner what I teach my students is recognize it, know the reagents, it's not that often that your professor actually wants you to draw the whole mechanism but I'm going to leave that up to you and your discretion. If you have a very mechanistic professor that said you better know Wolff-Kischner, then you should learn it. If not, then just let this help you understand the reaction. I would focus much more on the reaction and the reagents more than the actual arrows. Awesome guys, let's move on to the next video.