When it comes to the mechanism of Wolff Kishner reduction, there are two things that I want you to keep in mind. One, we're starting off with hydrazone. Don't worry about drawing that mechanism again. Oops, no H. NHH. 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. There are two objectives that we're trying to achieve. One thing we're trying to do is we're trying to add Hs to the imine carbon. 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 N2 gas. If you guys don't know what N2 gas looks like, N2=N2 lone pair, lone pair. In fact, nitrogen gas makes up about 78% of the atmosphere—78% of every breath you take tonight is N2 gas. Isn't that romantic? Okay, so we just drew that. We're trying to somehow make a triple bond between those nitrogens. Maybe those objectives will help you remember this bottom carbon.
The way we do that is through a base-catalyzed proton transfer. 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. 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. What I wind up doing is I wind up getting something like this. N=NH. Now I have an extra H down here that I didn't have before. 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. 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 my base-catalyzed proton transfer. Perfect. Now what can we do? We can do it again. I can react with another equivalent of base and do the reaction again. 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 nitrogen. Now that that nitrogen has three bonds, the one on the bottom, it doesn't need any more bonds. 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≡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. This anion, however, is very unstable.
This anion, remember it had one H already. Let's draw in that H. It was here. That anion is just going to grab another hydrogen to regenerate that base. What I'm going to get at the end is I'm going to get an alkane that now I added two hydrogens to, so I get an alkane product plus I get my N2 gas and I get my base left over at the end. I don't know if your professor is going to require you to memorize this. Usually with Wolff Kishner, what I teach my students is to 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 Kishner," then you should learn it. If not, then just let this help you understand the reaction. Next video.