Organic Chemistry

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10. Addition Reactions

Alkyne Hydration

Vinyl alcohols (alcohols directly on a double bond) undergo a process called tautomerization. Don't worry too much about it because we will devote an entire chapter to this process next semester, so you aren’t expected to fully understand it yet.

 

For now, just memorize what the enol and keto forms look like, so you can predict the products that form when you add alcohol to an alkyne

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Vinyl alcohols yield tautomers.

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back when we talked about how we could add groups. Two double bonds. We discussed how there were three different ways. Toe add water, tow a double bond to Macon 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 gonna 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. All right, 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 it a phenomenon called Todd Memorization. Now, this is a phenomenon that we're not going to fully understand the mechanism for until or go to, 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, fresh refresher on what this is. So you guys know what Todd memorization 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 gonna reversible e swap the position of a hydrogen. Okay. And a pie bond. Okay, 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 Todd More ization. So here, I'll show you here would be an alcohol that's directly attached to a double bonds is vinyl alcohol and through the top memorization process that I'm not going to show you. The mechanism for this is going to turn into ah, completely different functional group where basically my double bond is gonna move over here. My h is going to move down here. So if you're on a switch, places and what's gonna wind up happening is that you get ah, carbon Neil formed. And instead of this being a c H two now, this is gonna turn into a C H three. So what winds up happening is that this turns from a vinyl alcohol toe? A key tone. Alright, 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 gonna tell you guys to memorize that a double bond and a hydrogen switch places anytime they have a vinyl alcohol. All right, 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 Enel All right. And that makes sense because in stands for Al Keen, all stands for alcohol. So basically, whenever you have an alcohol on the AL Keen, that would be called in anal. Well, the Enel rapidly taught, um arises to the Keto form. The Keto form is just the key tone or the alga hide that's produced after Todd Memorization takes place. Now what you notice is that I didn't draw these equilibrium arrows evenly. This is 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 gonna be favored in almost all cases highly favored over the final form. So what that means is that immediately upon making any vinyl alcohol or most vinyl alcohol's, I can expect it to rapidly transformed into the Keto phase. And the Keto side of the equilibrium looks like a key tone or in Alba hide. 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 key tone or an alga hide as the product, all right, and it's through this process of Todd memorization now, exactly which ones do we get? Let's go ahead and look at each specific regent

Both acid-catalyzed hydration and oxymercuration-reduction of any alkyne leads to formation of a ketone. These reactions both yield a Markovnikov vinyl alcohol, which then tautomerizes.  

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Markovnikov addition of alcohols yields ketones.

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So there is oxi Mercure ation of all kinds. Okay. And there's hydro beret shin of all kinds. So when we do an oxy Mercure ation of Al Kind, what we're really doing is we're doing it a Markov Nankov addition of alcohol. Okay, remember that Oxy Mercure ation is the most popular way. One of the most popular ways. Toe Adam or cabinet, cough, alcohol, toe a double bond. Well, the same thing applies for a triple Bond is Well, what that means is that if I have to 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 gonna go in the more substituted position. So I would expect that after an Oxy Mercure ation, I'm gonna get alcohol right here, okay? And the more substituted position. Now notice that I put the Oxy Merck re agents down here, but I also included h over h 20 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 Markov Nicolov additions, right? Both of them favor the market nick off alcohol. So actually, I can use both of them. Even though Oxy mark is maybe more commonly used, hydration is still a great choice. Okay, so both of these re agents really leads the same intermediate structure, which is going to be this evil. All right. Are you getting that? So far? The reason I'm calling it an Enel is because now I have a more cognitive alcohol on a double bond. But we know that it's not going to stay like that because Enel's air not stable. They like to talk. Memorize. So after the top memorization process, what's the product gonna look like? Well, the product is gonna be the same ring. Okay, But now, instead of having a single bonds Oh, I'm gonna get a double 120 instead of having a double bond to the carbon, I'm not gonna have a single bonds of the carbon. So it turns out that the product of Oxy, Mercure ation or even hydration is going to be key tones. Okay, so any time that I'm hydrating market Nick aww hydrating a triple bond, I'm gonna get a key tone. Is the product okay? Now What part of this mechanism should you be able to draw the first part? The second part. You are fine. Just to say Todd Memorization. Just label it and then draw the product. Okay? Like I said, I'm not gonna teach you that full mechanism till we get to or go to. Alright. But for right now, you know at least the general idea of what's going on.
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