Decarboxylation Mechanism - Online Tutor, Practice Problems & Exam Prep

Decarboxylation is a crucial reaction in organic chemistry, where a beta carbonyl carboxylic acid loses CO₂ upon heating. This process involves a concerted mechanism, where the oxygen atom grabs a hydrogen, forming a pi bond and cleaving the carboxylic acid. The resulting enol undergoes tautomerization to yield a ketone. Understanding this transformation is essential for grasping enol and enolate chemistry, which plays a significant role in various organic reactions.

General Mechanism

Hey guys. In this video, we're going to talk about an extremely important way to get rid of carboxylic acids, and that's called decarboxylation. Decarboxylation is going to be a very important reaction for the rest of organic chemistry. It's really important that you understand how to do it now. The way that it works is that a beta carbonyl carboxylic acid can decarboxylate in the presence of heat. What this does is it produces CO2 gas as a byproduct. It evolves out of the material. What a beta carbonyl carboxylic acid is, it's just that you have some kind of carbonyl beta to your carboxylic acid. When you heat that up, what's going to wind up happening is that you literally cleave the entire carboxylic acid off and it becomes CO2 gas, leaving only the carbonyl portion behind. Obviously, we need to get to the mechanism for this because it doesn't really make sense just drawn like that. Let's do the mechanism. For this mechanism, it's going to make a lot more sense if you do one thing: if you rotate the bond. We're going to rotate this bond up when I draw it because it's going to make more sense with the mechanism. I'm actually going to draw my OH facing like this and my carbonyl down. But this is still the same exact molecule. I just rotated it. And you should too. When I draw this mechanism, it's actually a series of 4 arrows, and there's no right place to start or no right place to end. It's all concerted. But I like to start from the oxygen grabbing the H because that's kind of like an easy place to remember for me. I would start with the O grabbing this H. It's going to start off like that. If I make a bond, I have to break a bond because H can't have 2. This single bond is going to give its electrons and become a pi bond between that carbon and that O. Now that carbon has 4 bonds. Instead of breaking the bond to the carbonyl though, I'm actually just going to break the single bond to the carbon completely. Notice what this is going to do is it's going to release my C with 2 Os on it because now there's no single bond connecting it to the alkyl group. Finally, once I make that bond, I have to break this bond. There you go. Those are the 4 arrows. This is a heat-mediated reaction. I wind up getting something that looks like this: RO2 with a double bond, and I'm going to get CO2. Now it makes sense why I get the CO2 gas. And it should even make sense why the carboxylic acid's gone. What might be a little confusing for you is that I told you that you get a carbonyl from this. Right now, I have an alcohol. That doesn't seem to make sense. But this is something that we're going to talk about a lot more if you haven't talked about it yet. But there's something special about vinyl alcohols. Does anyone remember? What's special about vinyl alcohols? They undergo this completely separate special process called tautomerization. If you want more information on tautomerization, I have an entire video for that, and there's an entire section of your textbook dedicated to tautomerization and enol chemistry. This is called an enol. Enol or enolate chemistry is a huge deal. You're going to get there if you haven't yet. But the whole point being that when you tautomerize, what you do is you switch the locations of a double bond and a hydrogen. What this actually comes after tautomerization is this. This is the keto form. You wind up getting the ketone. Just take my word for it that it tautomerizes, and then you're fine. Awesome. What I want you guys to do is for the next example, go ahead and predict what the product would look like. I know it's a big complicated molecule. Predict it, and then I'll explain what's going on.

Predict the correct structure

Before I say anything, we're just going to draw the product and then we will discuss. That's the annoying part, just drawing this whole thing. Just bear with me. Follow along. We're getting close. I've got this chain and that's it. Notice that when I decarboxylate, I'm going to get rid of my carboxylic acid. I'm going to need heat to do that. Does anyone know what this molecule is? This molecule is called tetrahydroquinan acid or THCA for short. This molecule is called Tetrahydrocannabidiol. I met a girl with that tattoo one time. It was awesome because it was organic chemistry. What is this? THC is the active component of marijuana. This is the good stuff, right? It turns out that what we just did was a decarboxylation reaction that actually happens in real life. Obviously not with you because it's illegal. At least in my state, it's still illegal. But where you are, we'll see what state law says. Tetrahydrocannabinolic acid decarboxylates at about 120 degrees to produce the active form of THC. THC A is an inactive form of the drug. In order for the drug to actually work, to actually do what it's supposed to do, it's going to need to decarboxylate it first which is why they actually smoke it, right? You smoke it to decarboxylate it to activate it. Anyway, I just thought that was an interesting application. It is probably the most famous application of decarboxylation in the real world but it happens all the time in all kinds of organic reactions, so good for you to know. I know you're not going to forget it now. Let's move on to the next video.

Provide the mechanism for the following reaction.

Hey guys. Let's look at the following practice question dealing with providing a mechanism. So here we have to provide a mechanism for the following reaction. What we need to be here is observe it and realize what exactly changed because that'll tell me what kind of mechanism I need to do. So we're going to say here that this carbon here has an H we don't see. Now, it still has that hydrogen there but what we need to realize here is we lost this carbon plus these 2 oxygens. So, we lost CO2. Now, this H over here, we didn't lose it. It's still there. What kind of reaction helps us lose carbon dioxide? We're going to say the answer is decarboxylation. Now, you may ask, how do I know when decarboxylation is going to occur? Well, the first thing, your compound needs at least 2 carbonyls. Well, it needs at least 2 carbonyls. So, you have to have 2 carbonyls. Here's the thing, at least one of them has to be a carboxylic acid. So at least one of them must be a carboxylic acid. So, if we take a look, here are 2 carbonyls. One carbonyl, 2 carbonyls. And one of them is a carboxylic acid. If both are carboxylic acids, that's fine too. But we don't need both of them to be carboxylic acids. We just need 1. Next, the 2 carbonyl carbons must be separated by one carbon. So, those 2 carbonyl carbons must be separated by one carbon. So here's a carbonyl carbon, here's a carbonyl carbon. They are separated by this one carbon. So you have to be separated by just one carbon. And then finally, you just need some heat. And then in that way, we're going to lose CO2 and that'll be decarboxylation. So you need to remember these requirements. So, here we have to show the mechanism. Here, what we're going to do here is we're going to switch around this molecule a little bit and twist it. Now that the carboxylic acid carbonyl is pointing the opposite direction and the OH is closer to the other oxygen. So what we have to do here is we have to show how exactly did we make that product. So there's an H here. Now, we're going to say here is this. What's going to happen here is this, we can say that this double bond here will shoot out and grab this hydrogen here. When it does that, this bond breaks and oxygen uses it immediately to make a double bond to that carbon. But then this carbon here will be making 5 bonds, one too many. So what happens now? This bond here breaks and falls here. So what we wind up getting is we're going to get an OH. Here's a double bond. We lose this piece here as CO2 because remember, carbon dioxide is a carbon double bonded to 2 oxygens. Remember, here we have a double-bonded carbon with an OH which is an enol. Remember, an enol is an alkene and an alcohol at the same time. They are not stable. What do they undergo? They undergo tautomerization. And since enols are less stable, they get a smaller arrow. And it points to the more stable ketone form or carbonyl form. So basically, the way to think of tautomerization is to think of this H here moving over here, but at the same time this double bond coming up here. And it's in that way that we make our final product there. So remember the requirements for decarboxylation and then remember the general mechanism of how all of this rearranges itself to lose CO2 in order to make the enol which then tautomerizes quickly to give us our more stable keto form. So remember all the steps involved in this particular question.

Which carbonyl group will undergo decarboxylation most readily? 

• A. A
• B. B
• C. C
• D. D

Here’s what students ask on this topic:

What is decarboxylation in organic chemistry?

Decarboxylation is a reaction in organic chemistry where a carboxyl group (COOH) is removed from a molecule, releasing carbon dioxide (CO₂). This reaction is particularly important for beta carbonyl carboxylic acids, which decarboxylate upon heating. The process involves a concerted mechanism where the oxygen atom grabs a hydrogen, forming a pi bond and cleaving the carboxylic acid. The resulting enol then undergoes tautomerization to yield a ketone. Understanding decarboxylation is crucial for grasping enol and enolate chemistry, which are significant in various organic reactions.

What is the mechanism of decarboxylation?

The mechanism of decarboxylation involves a series of concerted steps. First, the oxygen atom of the carboxyl group grabs a hydrogen atom, forming a pi bond between the carbon and oxygen. This causes the single bond between the carbonyl carbon and the carboxyl carbon to break, releasing CO₂ gas. The remaining molecule is an enol, which then undergoes tautomerization to form a more stable ketone. This entire process is heat-mediated and is essential for understanding enol and enolate chemistry.

Why is tautomerization important in decarboxylation?

Tautomerization is crucial in decarboxylation because it stabilizes the product formed after the loss of CO₂. The initial product is an enol, which is less stable. Through tautomerization, the enol converts to a more stable keto form. This involves the migration of a hydrogen atom and the shift of a double bond. Tautomerization ensures that the final product is a ketone, which is more stable and significant in various organic reactions.

What are beta carbonyl carboxylic acids?

Beta carbonyl carboxylic acids are compounds where a carbonyl group (C=O) is located at the beta position relative to a carboxylic acid group (COOH). In other words, the carbonyl group is two carbon atoms away from the carboxylic acid group. These compounds are significant in decarboxylation reactions because they can lose CO₂ upon heating, leading to the formation of a ketone after tautomerization. Understanding the structure and reactivity of beta carbonyl carboxylic acids is essential for mastering decarboxylation mechanisms.

How does heat facilitate decarboxylation?

Heat facilitates decarboxylation by providing the necessary energy to break the bonds in the carboxylic acid group. When a beta carbonyl carboxylic acid is heated, the increased thermal energy allows the oxygen atom to grab a hydrogen atom, forming a pi bond and cleaving the carboxylic acid group. This results in the release of CO₂ gas and the formation of an enol, which then tautomerizes to a ketone. Heat is thus essential for driving the decarboxylation reaction forward.