Wolff Kishner Reduction: Videos & Practice Problems

The Wolff-Kishner reduction is a method for completely removing carbonyl groups, converting them into alkanes. This reaction begins with hydrazine reacting with a carbonyl to form a hydrazone. In a base-catalyzed environment, typically using NaOH or KOH and heat, nitrogen gas (N₂) is evolved, facilitating the transformation. The mechanism involves base-catalyzed proton transfers that lead to the formation of a nitrogen triple bond and ultimately result in the generation of an alkane product alongside nitrogen gas.

General Reaction

On this page, I want to discuss a specific reaction that hydrazones can undergo, and that's called Wolf Kishner reduction. The Wolf 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 are 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 to add and make an imine derivative. Specifically, the imine derivative that we make is called hydrazone. If you recall, hydrazone is the combination of hydrazine with a ketone, so it's hydrazone. Once you have your hydrazone, usually you'd 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 tert-butoxide. 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. 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 video, I'm going to show you guys the whole mechanism for Wolf Kishner reduction.

Mechanism

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.

Here’s what students ask on this topic:

What is the Wolff-Kishner reduction?

The Wolff-Kishner reduction is a chemical reaction used to completely remove carbonyl groups from aldehydes and ketones, converting them into alkanes. The process involves the formation of a hydrazone intermediate by reacting the carbonyl compound with hydrazine. This hydrazone is then subjected to a base-catalyzed environment, typically using strong bases like NaOH or KOH and heat. The reaction results in the evolution of nitrogen gas (N₂) and the formation of an alkane.

What reagents are used in the Wolff-Kishner reduction?

The Wolff-Kishner reduction typically uses hydrazine (N₂H₄) to form a hydrazone intermediate from the carbonyl compound. The reaction is then carried out in a strongly basic environment, often using bases such as sodium hydroxide (NaOH) or potassium hydroxide (KOH). Heat is also required to drive the reaction to completion. An alcohol, like ethylene glycol, is often present to provide the correct conditions for the reaction, although it is not directly involved in the mechanism.

What is the mechanism of the Wolff-Kishner reduction?

The mechanism of the Wolff-Kishner reduction involves several steps. First, the carbonyl compound reacts with hydrazine to form a hydrazone. In a base-catalyzed environment, the hydrazone undergoes proton transfers facilitated by the base. This leads to the formation of a nitrogen-nitrogen triple bond (N≡N) and the evolution of nitrogen gas (N₂). The final step involves the addition of hydrogen atoms to the carbon, resulting in the formation of an alkane. The overall reaction can be summarized as:

${R-C=O}_H+{N-H-NH}_2\to {R-C=N-NH}_2\to {R-H}_2+{N\equiv N}_2$

What are the differences between Wolff-Kishner reduction and Clemmensen reduction?

Both Wolff-Kishner and Clemmensen reductions are used to convert carbonyl groups into alkanes, but they differ in their reagents and conditions. The Wolff-Kishner reduction uses hydrazine and a strong base (e.g., NaOH or KOH) under high temperatures. In contrast, the Clemmensen reduction employs zinc amalgam (Zn(Hg)) and hydrochloric acid (HCl) under acidic conditions. The choice between these methods depends on the sensitivity of the substrate to either basic or acidic conditions.

Why is nitrogen gas (N₂) evolved in the Wolff-Kishner reduction?

Nitrogen gas (N₂) is evolved in the Wolff-Kishner reduction as a result of the formation of a nitrogen-nitrogen triple bond (N≡N) during the reaction. The hydrazone intermediate undergoes base-catalyzed proton transfers, leading to the formation of this triple bond. Once formed, the nitrogen gas is released from the reaction mixture, driving the reaction to completion and resulting in the formation of the alkane product.