Wolff-Kischner Reduction

Concept: Concept: General Reaction

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.