Tautomers of Dicarbonyl Compounds - Online Tutor, Practice Problems & Exam Prep

Tautomerism in dicarbonyl compounds typically favors the ketotautomer due to its greater stability. However, beta dicarbonyls are unique, favoring the enol tautomer because of their structure, which allows for significant resonance stabilization and hydrogen bonding. The acidity of beta dicarbonyls is notable, with pKa values around 9-13, making them comparable to ammonium. This frequent tautomerization can lead to racemization at chiral centers, resulting in a 50/50 mixture of enantiomers. Understanding these properties is crucial for organic synthesis involving beta dicarbonyl compounds.

Tautomers of Dicarbonyls

Hey guys. In this video, I want to discuss the specific tautomers of dicarbonyl compounds. I've already introduced the fact that tautomers happen to carbonyls whether you like it or not. This is just a natural process that occurs at equilibrium. Just because it happens, doesn't mean that it happens in very appreciable amounts. In fact, most carbonyl compounds are going to favor the ketotautomer. Why? Thermodynamics, guys. It's just more stable. The ketotautomer is more stable than a vinyl alcohol. But beta dicarbonyls are special because beta dicarbonyls are shaped in such a way that they actually favor the enol tautomer. How do we explain that? Let's look at the beta dicarbonyl for a second.

First of all, let's define beta dicarbonyl that you have one carbonyl and another one beta to the one that you currently have. Now, we remember that the alpha carbon of a carbonyl has a pKa of around 20. These are pKas that we're talking about here. Does anyone want to take a stab at what the pKa is between a beta dicarbonyl? What's the pKa of this hydrogen here? Notice that now it has 2 carbonyls on both sides. This is depending on 9, 11, 13, around that area. How do you explain that? That's a crazy acidity. That's like as acidic as ammonium. That's as acidic as NH₄⁺ with a positive charge. How do you get something that acidic that's just an alkane? Because of the stability of the enal tautomer. Look at what the enal tautomer is going to look like.

First of all, you're going to have 2 tautomers that are possible. What you're going to get first of all is a bunch of overlapping resonance, atoms that can resonate. This is called conjugation. Conjugation makes molecules stable. Conjugation is going to stabilize these guys. On top of that, we've even got hydrogen bonding because the enol from one side is going to be able to hydrogen bond with the keto from the other. This is the enol side and this is the keto side. It's going to be able to hydrogen bond with itself. What winds up happening is that if you take away a proton from the beta dicarbonyl, the methylene carbon in the middle, you're actually going to get 75 percent enol at equilibrium depending on what the R groups are obviously. If these R groups are different things, the number will change a little bit. I'm just letting you know this is an extremely favored tautomerization.

Beta dicarbonyl compounds are exceptionally acidic due to the high stability of the conjugate base. One thing to keep in mind is that since this is going to happen so frequently, if you happen to have a chiral center at the alpha carbon, so let's say that this had been a chiral center. It's not now because I got 2 H's. But if it was a chiral center, that chiral center will always be racemized. What the heck does that mean? It just means that you can't isolate one enantiomer. You're going to wind up getting a mixture of both, 50-50. Why? Because when you form the enol, you're going to lose your stereochemistry. You're going to turn trigonal planar. You're going to lose the wedge and dash structure. Then when you add, you could either add the proton from the front or from the back. You're going to get a racemization of any alpha carbon. That's pretty much as a rule. You're going to racemize all of your alpha carbons. This being said, beta carbonyls are very important for organic synthesis because they're so good at doing tautomerization.

Let's do this problem here. I want you guys to look at the problem and evaluate which of these beta carbonyls is going to be the most acidic one of all. Think about the one that can form the most tautomers. Do that and then I'll answer.

Here’s what students ask on this topic:

What are tautomers and how do they relate to dicarbonyl compounds?

Tautomers are isomers of a compound that differ only in the position of protons and electrons. In dicarbonyl compounds, tautomerism typically involves the shift between a keto form and an enol form. The keto form is generally more stable due to thermodynamic reasons. However, in beta dicarbonyl compounds, the enol form is favored because of resonance stabilization and hydrogen bonding. This unique structure allows for significant stabilization, making the enol form more prevalent in these cases.

Why do beta dicarbonyl compounds favor the enol tautomer?

Beta dicarbonyl compounds favor the enol tautomer due to their structural ability to stabilize the enol form through resonance and hydrogen bonding. The enol form allows for overlapping resonance structures, which increases stability. Additionally, hydrogen bonding between the enol and keto groups further stabilizes the molecule. This makes the enol form more stable and thus more favored in beta dicarbonyl compounds compared to the keto form.

What is the pKa range of beta dicarbonyl compounds and why is it significant?

The pKa range of beta dicarbonyl compounds is around 9-13. This is significant because it indicates a high level of acidity, comparable to that of ammonium (NH₄⁺). The increased acidity is due to the stability of the enol tautomer and the conjugate base formed after deprotonation. This high acidity makes beta dicarbonyl compounds particularly useful in organic synthesis, as they can easily participate in various chemical reactions.

How does tautomerization affect chiral centers in beta dicarbonyl compounds?

Tautomerization in beta dicarbonyl compounds can lead to racemization at chiral centers. When the compound shifts to the enol form, the chiral center becomes trigonal planar, losing its stereochemistry. Upon reverting to the keto form, the proton can add from either side, resulting in a 50/50 mixture of enantiomers. This racemization means that it is impossible to isolate a single enantiomer, as the process will always produce a racemic mixture.

What role do beta dicarbonyl compounds play in organic synthesis?

Beta dicarbonyl compounds play a crucial role in organic synthesis due to their high acidity and ability to easily undergo tautomerization. Their enol form is highly stabilized, making them excellent intermediates in various chemical reactions. This property allows for the formation of complex molecules and facilitates reactions such as aldol condensations, Michael additions, and other carbon-carbon bond-forming processes. Their unique reactivity makes them valuable tools in the synthesis of pharmaceuticals, natural products, and other organic compounds.