Alkyne Hydration - Online Tutor, Practice Problems & Exam Prep

Hydration of triple bonds leads to unique products through tautomerization, where a vinyl alcohol (enol) converts to a more stable ketone or aldehyde. Oxymercuration and hydroboration are two methods for adding water, with oxymercuration favoring Markovnikov addition, resulting in ketones, while hydroboration leads to anti-Markovnikov addition, yielding aldehydes. Understanding these reactions is crucial for grasping the behavior of functional groups in organic synthesis.

Alkyne Hydration

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. 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 afterward. So let's go into this right now. This is going to be the hydration of triple bonds. So 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 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, you know, 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 that they're going to reversibly swap the position of a hydrogen and a pi bond. So this is what I'm saying. Any time 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 any time 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. Alright? And that makes sense because 'ene' stands for alkene, 'ol' 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 in 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. 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 an Markovnikov addition of alcohol. 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'm 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 with H₂O. Can you guys remember what that was? That's hydration. This would be 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 oxymerc is maybe more commonly used, hydration is still a great choice. So both of these reagents really lead to the same intermediate structure, which is going to be this enol. 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. 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'm not going to carbon. So it turns out that the product of oxymercuration or even hydration is going to be ketones. So any time that I'm hydrating, Markovnikov hydrating a triple bond, I'm 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. Like I said, I'm not going to teach you that full mechanism until we get to Organic Chemistry 2. But for right now, you know at least the general idea of what's going on. Remember that we had three different ways to add alcohol. We had hydration. We had oxymerc. We had one more, and that was hydroboration. Remember what was kind of interesting about hydroboration was that it did everything the opposite. Hydroboration is actually going to be an anti-Markovnikov addition of alcohol. What that means is that if once again I have the blue site and I have the red site, which one is it going to add to? Well, it would actually add to the l

Here’s what students ask on this topic:

What is tautomerization in the context of alkyne hydration?

Tautomerization is a chemical process where a vinyl alcohol (enol) converts to a more stable ketone or aldehyde. In the context of alkyne hydration, when water is added to a triple bond, it initially forms a vinyl alcohol. This enol is unstable and rapidly undergoes tautomerization, where the position of a hydrogen atom and a double bond switch places, resulting in the formation of a ketone or aldehyde. This process is crucial for understanding the final products of alkyne hydration reactions.

How does oxymercuration of alkynes lead to ketone formation?

Oxymercuration of alkynes involves the Markovnikov addition of water to the triple bond. This means the hydroxyl group (OH) attaches to the more substituted carbon atom. The initial product is a vinyl alcohol (enol), which is unstable. Through tautomerization, the enol converts to a ketone. The general reaction can be summarized as follows: the triple bond reacts with mercuric sulfate (HgSO₄) in the presence of water (H₂O) and sulfuric acid (H₂SO₄), leading to the formation of a ketone after tautomerization.

What is the difference between Markovnikov and anti-Markovnikov addition in alkyne hydration?

In alkyne hydration, Markovnikov addition refers to the addition of water where the hydroxyl group (OH) attaches to the more substituted carbon atom of the triple bond. This typically results in the formation of a ketone after tautomerization. Oxymercuration is an example of this. Anti-Markovnikov addition, on the other hand, involves the hydroxyl group attaching to the less substituted carbon atom. This usually leads to the formation of an aldehyde after tautomerization. Hydroboration is an example of anti-Markovnikov addition.

Why is the keto form favored over the enol form in tautomerization?

The keto form is favored over the enol form in tautomerization because it is generally more stable. The stability arises from the stronger carbon-oxygen double bond (C=O) in the keto form compared to the carbon-carbon double bond (C=C) in the enol form. Additionally, the keto form often has lower energy due to better electron delocalization and less steric hindrance. As a result, the equilibrium between the enol and keto forms heavily favors the keto form, making it the predominant species in most cases.

What reagents are used in the hydroboration of alkynes?

In the hydroboration of alkynes, boron-containing reagents such as borane (BH₃) or other boron sources are used. The reaction typically involves two steps: the addition of the boron reagent to the alkyne, followed by oxidation. A common set of reagents for this process includes BH₃ or B₂H₆ for the hydroboration step, and hydrogen peroxide (H₂O₂) with a base like sodium hydroxide (NaOH) for the oxidation step. This results in the anti-Markovnikov addition of water, ultimately forming an aldehyde after tautomerization.