Birch Reduction: Videos & Practice Problems

The Birch reduction is a reduction reaction that transforms benzene into an isolated diene using elemental sodium, an amine, and alcohol. The mechanism involves the formation of a radical anion intermediate, which is stabilized by electron-withdrawing groups that direct the position of double bonds away from themselves, while electron-donating groups attach directly to the diene. This regioselectivity is crucial for understanding the stability of intermediates and the final product's structure. The reaction typically requires two equivalents of alcohol to complete the process, yielding a stable isolated diene.

Birch Reduction Mechanism

Hey guys. So in this video, we're going to talk about a specific type of reduction reaction that can happen with Benzene and that's called the Birch reduction. Let's just take a look at the general reaction for a second. What a Birch reduction does is it combines elemental sodium with an amine and alcohol to turn a benzene into what we call an isolated diene. Specifically, if this were to happen with an unsubstituted benzene like we have here, our product would be an isolated cyclohexadiene. Two double bonds that are far apart from each other in a 1,4 position on a cyclohexane. Now, if you take a closer look at these reagents, they might look familiar because these are very similar to the reagents that we use on a dissolving metal reduction. This is a reaction from Organic Chemistry 1 that we learned a long time ago that worked with alkynes and it was a radical-mediated mechanism. It turns out that this mechanism is really the same exact mechanism except it's going to work with benzene instead of with an alkyne. Let's get right into it. The mechanism for this reduction is going to proceed through elemental sodium, which means it's going to possess just one electron. When that one electron donates to any of the carbons, we're going to have to break a bond. But this is going to be a mechanism where we have a combination of half-headed arrows and normal arrows just like the dissolving metal reduction. How there were some arrows that moved one radical and some arrows that moved a lone pair. When we make that bond, we have to break this bond in order to make room for the radical. And in order to keep these charges as far away from each other as possible or these intermediates as far away from each other as possible, this double bond is going to ionize into a lone pair onto the very bottom. So basically, the furthest position possible from the radical, we're going to get an anion. So let's go ahead and draw the product of this first step. What we're now going to get is a single radical at the top, double bonds on both sides, and now a lone pair at the bottom which is going to be a carbanion. This intermediate is called a radical anion, which makes sense because that's what it is. It's a radical and it's an anion. This is where our ethanol comes in. Our ethanol is going to serve as a protonating agent. Just so you know, ethanol isn't the only alcohol you can use. Some texts will use tert-butanol. It doesn't matter guys. It's a source of hydrogen. That's the biggest deal. EtOH, my anion is going to grab the H and give a negative charge to the O. Now what I'm going to get is a molecule that looks like this. I've got my two double bonds. I still have my radical. Ethanol. At this point, I react with another equivalent of my elemental sodium. That elemental sodium is going to donate electrons to that same location. Now I'm going to get a lone pair anion. This is just a carbanion intermediate. This reaction just repeats itself. That's one thing about maybe dissolving metal reduction if you recall. It was the same thing twice. Here, we would react again with another equivalent of ethanol and we would wind up getting our isolated diene because now I've got H's, two H's on the bottom. I've got two H's on the top. It's the ugliest H ever. And I've got my isolated diene which is this molecule here. For this reason, the fact that it reacts twice, sometimes you might see professors actually write ethanol times two or alcohol times two. It doesn't matter. It's just going to have enough equivalents to make the reaction go to completion. That's really it. That's the mechanism for Birch reduction. And now what we're going to do is we're going to talk about specific regiochemistry that you have to consider with a Birch reduction.

Regiospecific products

Since this reaction always passes through an anion intermediate, we can actually use activating groups and deactivating groups to direct the site of the isolated diene. How does that work? Let's just take a look at the anion or the carbanion intermediate. This would be the point where we have the 2 double bonds. We have the 2 H's and we have a lone pair negative at the top.

Let me ask you a question. If I add an electron withdrawing group to that anion? What do you think it does for stability? Do you think it makes that anion more stable or less stable? Hold that thought. What happens if I add an electron donating group to that anion? What if I add something that's going to give more electrons to the negative? What does that do for the stability? The answer is that the first one is going to make it more stable because it pulls electrons away. An electron donating group is actually going to make it less stable because it's going to push more electrons into the anion.

It turns out that these different groups are going to direct where the double bonds go. As you guys can see, withdrawing groups are going to what I say isolate themselves from the diene. I specifically chose that word for a reason because withdrawing and isolate kind of mean the same thing. If you're withdrawing from the crowd, that means you're isolating yourself. A withdrawing group is going to be isolated from the double bond. It's going to be away from the double bond. And why is that? It's not just because we memorized it. It's because you know that it's going to stabilize the negative charge. It's going to want to be where the negative charge was.

Whereas donating groups are going to attach themselves directly to the diene like in this situation. Why? Because I have electrons going into the ring and I don't want it to be here. Because if it was there, it would make my anion less stable. I'm trying to put it in a place where it's not going to affect the stability, where it's going to be fine. Electron donating groups attach to the ring and withdrawing groups isolate from the ring. If you don't remember the mechanism, you can at least remember the way that I'm telling you which is that withdrawing isolates. You can think of just isolating yourself from the crowd, you're withdrawing or donating attaches which is basically the opposite.

Awesome, guys. So really that's it for this topic. Let's move on to the next one.

Predict the major product from the Birch Reduction

• A.
• B.
• C.
• D.

Answer: (B)

Predict the major product from the Birch Reduction

• A.
• B.
• C.
• D.

Answer: (B)

Here’s what students ask on this topic:

What is the Birch reduction and how does it work?

The Birch reduction is a chemical reaction that reduces benzene to an isolated diene using elemental sodium, an amine, and alcohol. The mechanism involves the formation of a radical anion intermediate. Initially, an electron from sodium is donated to the benzene ring, creating a radical anion. This intermediate is then protonated by the alcohol, forming a cyclohexadienyl radical. Another electron from sodium is added, creating a carbanion, which is again protonated by the alcohol, resulting in an isolated diene. The reaction typically requires two equivalents of alcohol to complete the process.

What reagents are used in the Birch reduction?

The Birch reduction uses elemental sodium (Na), an amine (such as liquid ammonia, NH₃), and an alcohol (commonly ethanol, EtOH, or tert-butanol, t-BuOH). The sodium provides electrons for the reduction process, the amine serves as a solvent, and the alcohol acts as a proton source to stabilize the intermediates formed during the reaction.

How do electron-withdrawing and electron-donating groups affect the Birch reduction?

In the Birch reduction, electron-withdrawing groups (EWGs) and electron-donating groups (EDGs) influence the regioselectivity of the reaction. EWGs stabilize the radical anion intermediate by pulling electron density away, directing the double bonds away from themselves. Conversely, EDGs destabilize the radical anion by donating electron density, causing the double bonds to form adjacent to the donating group. This regioselectivity is crucial for predicting the structure of the final isolated diene product.

What is the mechanism of the Birch reduction?

The mechanism of the Birch reduction involves several steps: (1) An electron from elemental sodium is donated to the benzene ring, forming a radical anion. (2) This radical anion is protonated by the alcohol, creating a cyclohexadienyl radical. (3) Another electron from sodium is added, forming a carbanion. (4) This carbanion is protonated again by the alcohol, resulting in an isolated diene. The process typically requires two equivalents of alcohol to complete the reaction.

Why is the Birch reduction important in organic chemistry?

The Birch reduction is important in organic chemistry because it provides a method to selectively reduce benzene rings to isolated dienes, which are valuable intermediates in the synthesis of various organic compounds. This reaction allows chemists to manipulate the structure of aromatic compounds, enabling the synthesis of complex molecules with specific functional groups and regiochemistry. The ability to control the position of double bonds and the influence of substituents makes the Birch reduction a versatile tool in synthetic organic chemistry.