Concept: Concept: General Features of Alkyne Hydration10m
Back when we talked about how we could add groups to double bonds, we discussed how there were three different ways to add water to a double bond to make an alcohol. 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. 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. Let's go into this right now. This is going to be the hydration of triple bonds.
It turns out that anytime 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. This is a phenomenon that we're not going to fully understand the mechanism for until Orgo 2. It's kind of unfortunate that we have to talk about it now. But I’m just going to give you guys a really quick refresher on what this is so that you guys know what tautomerization is.
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. 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.
Here, I'll show you. Here would be an alcohol that’s directly attached to a double bond. This is vinyl alcohol. 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. They’re just going to switch places. What’s going to wind up happening is that you get a carbonyl formed. Instead of this being a CH2, now this is going to turn into a CH3. 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. 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.
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. That makes sense because ene stands for alkene, ol stands for alcohol. Basically, whenever you have an alcohol on the alkene, that would be called an enol. 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.
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. 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. 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.
Basically, the whole gist of what I’m trying to say is that anytime that you hydrate a triple bond, you're actually going to get a ketone or an aldehyde as the product. It's through this process of tautomerization. Exactly which ones do we get? Let's go ahead and look at each specific reagent.
There is oxymercuration of alkynes and there’s hydroboration of alkynes. When we do an oxymercuration of an alkyne, what we’re really doing is we're doing a Markovnikov addition of alcohol. Remember that oxymercuration is one of the most popular ways to add a Markovnikov alcohol to a double bond. The same thing applies for a triple bond as well. What that means is that if I have two 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. I would expect that after an oxymercuration, I'm going to get an alcohol right here in the more substituted position.
Notice that I put the oxymerc reagents down here but I also included HA over H2O. do you guys remember what that was? That’s hydration. This would be an acid-catalyzed hydration. Just so you know, both of these create Markovnikov additions. Both of them favor the Markovnikov alcohol. Actually, I can use both of them. Even though oxymerc is maybe more commonly used, hydration is still a great choice. 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. After the tautomerization process, what’s the product going to look like? 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 now going to have a single bond to the carbon. It turns out that the product of oxymercuration or even hydration is going to be ketones. Anytime that I'm Markovnikov hydrating a triple bond, I’m going to get a ketone as the product.
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 till we get to Orgo 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 have one more and that was hydroboration. Remember what was kind of interesting about hydroboration was that it did everything opposite. Hydroboration is actually going to be 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? It would actually add to the less substituted position. It would add right here.
Once again, notice that I'm getting an enol. I'm still getting a vinyl alcohol. The difference is that it’s just in a different position. Notice my agents really quick. Just notice that I said BH3, or another boron source that's because hydroboration, there's a lot of different boron sources that your professors could use. You just have to be aware of the one that your professor likes to use.
Obviously the bottom part was the oxidation step. Now I’ve got the enol. How do I figure out what the product looks like? Same process. I’m going to switch the position of the double bond and the H. What that means is that in my final product, what I’m going to get is now instead of a double bond there, I’m just going to get a single bond. Instead of a single bond to the O, I’m going to get a double bond to the O.
Where did the extra H go? The extra H went here because right now there was only one H her and now there's going to be two H’s. There's that original H and then there's that extra H. That extra blue H or whatever was this H right here and it transferred.
Notice what kind of molecule this is. I just drew a terminal carbonyl. This actually has a hydrogen at the and. This is actually going to lead to the formation at aldehydes. When you do a Markovnikov addition of water, that's going to be a ketone product. When you do an anti-Markovnikov addition of water, what you're going to lead to is an anti-Markovnikov alcohol or enol which then turns into aldehyde. Notice that in general this is still called the keto form. If you were to say just in terms of tautomers, this is the enol tautomer, this is the keto tautomer. But this specific molecule happens to be an aldehyde because the carbonyl is right at the edge and it has one H on it. Just something that you guys need to be aware of. It’s something that could definitely come up. You just need to know what's going. I hope that made sense. Let me know if you have any questions.
Predict the product for the reaction below.
What is the final product of the reaction drawn below?
Which of the following is the enol intermediate in the acid-catalyzed addition of water to propyne?
Which one of the following alkynes gives a single ketone in the acid-catalyzed hydration of each?
For the transformation shown, select the most appropriate reagent(s) to effect the change.
b) H2, Pd
c) 1. Disiamylborane, 2. HO –, H2O, H2O2
d) K2Cr2O7, H+
e) H2SO4, HgSO4