Alkyne Hydration: Study with Video Lessons, Practice Problems & Examples

Hydration of triple bonds leads to the formation of vinyl alcohols, which undergo tautomerization to yield ketones or aldehydes. This process involves the reversible swapping of a hydrogen and a pi bond, resulting in a more stable keto form. Oxymercuration and hydroboration of alkynes favor Markovnikov addition, placing the alcohol at the more substituted position. Both methods produce enols that quickly tautomerize into ketones, emphasizing the importance of understanding these reactions in organic synthesis.

Vinyl alcohols yield tautomers.

Back when we talked about how we could add groups to double bonds, we discussed how there were 3 different ways to add water to a double bond to make an alcohol. Well, it turns out that if we add water to a triple bond, we still are going to get that alcohol. The thing is that we're going to get a slightly different product. Instead of just having a single bond with an alcohol, we're now going to have a double bond with an alcohol attached to it. And even though that sounds like a very minor difference, that's actually going to translate into a huge difference in the functional group that we get afterwards. So let's go into this right now. This is going to be the hydration of triple bonds. It turns out that any time that you make a vinyl alcohol, that's the name of basically having an alcohol directly on a double bond, that is going to react very uniquely. It's not going to react like the addition reactions that we saw with double bonds. In fact, this is going to do with a phenomenon called tautomerization. Now this is a phenomenon that we're not going to fully understand the mechanism for until Organic Chemistry 2. So it's kind of unfortunate that you have to talk about it now, but I'm just going to give you guys like a really quick refresher on what this is, so that you guys know what tautomerization is. So basically, if I were to summarize it, I'm not going to teach you the full mechanism because that would be a whole separate lesson. But all you really need to know is they're going to reversibly swap the position of a hydrogen and a pi bond. This is what I'm saying. Anytime that you make a vinyl alcohol, this is something special. This is not a regular alcohol. This is an alcohol that is now subject to a phenomenon called tautomerization. So here, I'll show you. Here would be an alcohol that's directly attached to a double bond. This is vinyl alcohol. And through the tautomerization process that I'm not going to show you the mechanism for, this is going to turn into a completely different functional group where basically my double bond is going to move over here. My H is going to move down here. So these are going to switch places. And what's going to wind up happening is that you get a carbonyl formed and instead of this being a CH₂, now this is going to turn into a CH₃. So what winds up happening is that this turns from a vinyl alcohol to a ketone. How did that happen? Like I said, unfortunately, it would take me 20 minutes to explain this whole thing to you. So instead, I'm just going to tell you guys to memorize that a double bond and a hydrogen switch places anytime that you have a vinyl alcohol. Now, we do have some fancy words for this because this is its own thing. Basically, when it's in the vinyl alcohol phase, that's called the enol. And that makes sense because ene stands for alkene. O stands for alcohol. So it's basically whenever you have an alcohol on the alkene, that would be called an enol. Well, the enol rapidly tautomerizes to the keto form. The keto form is just the ketone or the aldehyde that's produced after tautomerization takes place. Now what you notice is that I didn't draw these equilibrium arrows evenly. This is a phenomenon that's constantly in equilibrium, but one of the arrows is much bigger than the other. And that's because it turns out that the keto form is going to be favored in almost all cases, highly favored over the enol form. So what that means is that immediately upon making any vinyl alcohol or most vinyl alcohols, I can expect it to rapidly transform into the keto phase and the keto side of the equilibrium looks like a ketone or an aldehyde. So, basically, the whole gist of what I'm trying to say is that any time that you hydrate a triple bond, you're actually going to get a ketone or an aldehyde as the product. Alright? And it's through this process of tautomerization. Now exactly which ones do we get? Let's go ahead and look at each specific reagent.

Markovnikov addition of alcohols yields ketones.

So there is oxymercuration of alkynes. Okay? And there's hydroboration of alkynes. So when we do an oxymercuration of an alkyne, what we're really doing is we're doing a Markovnikov addition of alcohol. Okay? Remember that oxymercuration is one of the most popular ways to add a Markovnikov alcohol to a double bond. Well, the same thing applies for a triple bond as well. What that means is that if I have 2 sites, I have let's say the blue site and the red site, and I'm trying to figure out where the alcohol is going to go, it's going to go in the more substituted position, so I would expect that after an oxymercuration, I am going to get an alcohol right here in the more substituted position. Now notice that I put the oxymerc reagents down here, but I also included over H₂O. Do you guys remember what that was? That's hydration. Okay? This would be an acid catalyzed hydration. And just so you know, both of these create Markovnikov additions. Right? Both of them favor the Markovnikov alcohol. So actually, I can use both of them. Even though oxymark is maybe more commonly used, hydration is still a great choice. Okay? So both of these reagents really lead to the same intermediate structure, which is going to be this enol. All right? Are you getting that so far?

The reason I'm calling it an enol is because now I have a Markovnikov alcohol on a double bond. But we know that it's not going to stay like that because enols are not stable. They like to tautomerize. So after the tautomerization process, what's the product going to look like? Well, the product is going to be the same ring. Okay? But now, instead of having a single bond to O, I'm going to get a double bond to O. Instead of having a double bond to the carbon, I am not going to have a single bond to the carbon. So it turns out that the product of oxymercuration or even hydration is going to be ketones. So any time that I am hydrating, Markovnikov hydrating a triple bond, I am going to get a ketone as the product. Now, what part of this mechanism should you be able to draw? The first part. The second part, you are fine just to say tautomerization. Just label it and then draw the product. Okay? Like I said, I'm not going to teach you that full mechanism until we get to Orgo 2. All right? But for right now, you know at least the general idea of what's going on.

Here’s what students ask on this topic:

What is the mechanism of alkyne hydration?

Alkyne hydration involves adding water to a triple bond, resulting in a vinyl alcohol (enol) that undergoes tautomerization to form a ketone or aldehyde. The process can be catalyzed by acid or mercury salts (oxymercuration) for Markovnikov addition, or by hydroboration for anti-Markovnikov addition. The enol intermediate is unstable and quickly converts to the more stable keto form through the reversible swapping of a hydrogen and a pi bond.

What is tautomerization in the context of alkyne hydration?

Tautomerization is the process where a vinyl alcohol (enol) formed during alkyne hydration rearranges to a more stable keto form. This involves the reversible swapping of a hydrogen atom and a pi bond. The enol form, which has an alcohol group attached to a double bond, is less stable and rapidly converts to a ketone or aldehyde, which is the keto form.

How does oxymercuration of alkynes differ from hydroboration?

Oxymercuration of alkynes involves the Markovnikov addition of water, placing the alcohol at the more substituted carbon. This process uses mercury salts and results in a ketone after tautomerization. Hydroboration, on the other hand, follows anti-Markovnikov addition, placing the alcohol at the less substituted carbon, and typically results in an aldehyde after tautomerization. Both methods produce enols that quickly tautomerize to their more stable keto forms.

What are the products of alkyne hydration?

The products of alkyne hydration are ketones or aldehydes. Initially, the hydration of an alkyne forms a vinyl alcohol (enol), which is unstable and undergoes tautomerization. This rearrangement results in the formation of a ketone if the hydration follows Markovnikov's rule, or an aldehyde if it follows anti-Markovnikov's rule.

Why is the keto form favored over the enol form in alkyne hydration?

The keto form is favored over the enol form in alkyne hydration because it is more thermodynamically stable. The keto form has a stronger C=O double bond compared to the C=C double bond in the enol form. Additionally, the keto form benefits from better electron delocalization and lower energy, making it the predominant species in equilibrium.