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Organic Chemistry
Learn the toughest concepts covered in Organic Chemistry with step-by-step video tutorials and practice problems by world-class tutors.
13. Alcohols and Carbonyl Compounds
Protecting Alcohols from Organometallics
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Use of Protecting Groups
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Now that we understand organa metallics a little bit better, Hopefully you're aware of one of the major limitations that they have. And that limitation is that they tend to cross react with acidic hydrogen. So how do you prevent that from happening? Well, let's go ahead and talk about a strategy for that which is called protecting groups. Okay. So as I've explained before, Organa metallics are very powerful bases, so they can obviously reactor's nuclear files and attack Electra files. But they can also react as bases and deep protein. Eight things. Okay, what kind of things can they d protein eat? Well, they tend to just react with any acidic protons available. And that ruins the re agent. And we've talked about already. Is that those acidic protons air Usually stuff like Carl Oxalic acid. Very acidic alcohol. Pretty acidic. And water thes air. All examples of protons that air so acidic that they're gonna mess with the re agent instead of letting it react with an electric file. Okay, so let me give you an example. Let's say that I was trying to do a substitution reaction on this molecule. Now, notice that it has an alcohol. Hey, light on one side and an alcohol on the other. Now notice that my re agent here is a green yard. Okay, so I got my green yard Regent, we know that the way that we like to draw Green Yard regents is as a c H three Negative. Okay, that tells me that I've got a negative charge. Is gonna attack some positive. Okay, Alcohol. Hey, let's do ch three negatives or green eared react within. Al Kyohei lied. Absolutely. Remember that the Al Kyohei lied happens to have a partial positive there, so I would expect to get an s and two reaction on that Al Kyohei lied. But wait, we've got a problem. We've also got a alcohol on this molecule that alcohol is going to be very acidic. So what that means is notice? I put p A k f 16 that's acidic enough to react with my green yard. So what winds up happening is that instead of reacting in s and two, that doesn't happen. It winds up reacting with the proton as an acid based reaction. And what we wind up getting iss ch four because now you've got your ch three plus the H, and then you wind up getting a negative charge on the Oh. Now, by the way, the MGB are positive is just a spectator ion that happens to associate with Oh, okay. This is considered ruined. This is not good. No bueno. Because now the Green yard can't react with the electrical that I intended it to. Which was this carbon right here? Okay, so how do we prevent this from happening? Well, it turns out that alcohol's can be protected. Okay? And there are some strategies that we use before to protect alcohols, and this part is gonna require a little bit of prior knowledge. Okay, If you've already learned about using protecting groups for alcohol, then we're gonna review that right now. If you don't know how to use protecting groups for alcohol, then I would recommend going back to my lessons on protecting groups in the alcohol section. And then that will give you a better understanding. So you can understand this better. Okay, but basically, there's two different ways. There's two different types of re agents that we can use to protect alcohol's. Either we can use T beetle ethers or silly little ethers. Okay, Both of them are two different ways to protect an alcohol from reaction. Okay. And when you protect it, that means it's not gonna be deep protein aided by the organo metallic. Instead, it's gonna be locked up in that either. Okay, just years. No. A really common acid that's used for protection is para toll. Ewing Selfie. Olynyk. I'm sorry. So phonic acid. Okay, which is kind of a long name, but it's actually a very common acid in your book, but it's usually abbreviated instead of spelling it all like that. They usually say it that it's t S O h or that's Pts A Okay. Either way, both of them just know that there are source of h plus because this is an acid, okay, and acid is used in a protection and a protection reaction. We always use one of these. We make an ether, and the way we make it is through the reaction of an acid like H plus and a double wand. Okay, so for this problem, I'm gonna go ahead and ask us to try to solve it on your own and try to get the final product. Now If you already have reviewed protecting groups from alcohol, then go ahead and just go ahead and try to solve it Now. If not, if you have no clue, I'm talking about with protecting groups. Then don't worry to sit back and I'll explain the whole reaction in a second, so go ahead and try it out.
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Provide mechanism and final product of transformation.
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Alright, guys. So you can see that I haven't organa metallic here, but I've got an issue. My issue is that this organo metallic if it gets exposed, that alcohol, it's gonna react with it. And that's gonna be a really bad cross traction. It's gonna mess up my organo metallic. So what can we do? We can use a protecting group. How does a protecting group work? Well, in this case, what I would do is I would take my ice. Oh, beauty lean, which is a molecule. It looks like this a double bond with, you know, on four carbon chain. And what I would do is I would use P t s A, which is a source of h plus. Okay, so what that's going to do is it's going to protein E, and I'm gonna wind up getting a car broke out. And it looks like this, according to this would be a Markov knockoff edition in case you're familiar with that term. Okay, now that Carvell cat ion winds up getting reacted by the alcohol, okay? The alcohol grabs that carbon cat ion. So what we now get is an alcohol that looks like this Oh, h. But now it's got a Turk. Beautiful ether on one side and a positive charge. Okay, Cool. Okay, that positive winds up getting deep throat nated. Okay, so I would use the conjugate of that, which is on the conduct of P. T s. A. But I could also just in this case, I'll just use water. Okay, so I'm gonna use water, too. Deep protein ate that. And what I wind up getting is my t beautiful either. Okay, Now this alcohol, it used to be an alcohol. Now, this is considered a protected alcohol. The reason is protected is because ethers don't really react with anything. Okay, so now when I introduce my organa lithium re agent, I'm gonna write a negative there, because this is that's the best way toe synthetically draw the product. I'm gonna take that negative, and I'm gonna attack the carbon deal, Okay? We're gonna attack the carbon. He'll push the electrons up and notice that I don't have to worry about the organa lithium reacting with the T beauty either or reacting with the alcohol. Why? Because this entire thing is non reactive. It's protected right now. That's what protective means it's nonreactive because it hasn't. It's an ether now, and ethers really don't react with anything. So our direct with very little. Okay, so now I'm gonna go ahead and draw my Tetra Hydro Intermediate, which is O negative. Now, I've got a ch two ch three coming off of that, and I still have my tea beetle either. Okay, In my last step, I'm gonna use acid. And what does acid do? Well, what acid is going to do over water, it's gonna do two things. First of all, it's gonna protein eight. Okay, so now this becomes an alcohol. Second of all, it's gonna deep protect this TV will, either. What I mean by D protect is it takes it completely off and goes in the reverse direction. Okay, so now, instead of having a t butor eat betel, either you're gonna wind up getting the alcohol again, okay? And that just has to do with the fact that this whole thing that we've been drawing is a reversible reaction. Okay, so this actually has an arrow going back. This part here, actually, as an arrow going backwards Now, the organo metallic is a forward reaction. Once you get to the Organon Too talented for words, but I'm trying to say is that if you expose this toe acid, it could go back and re generate that alcohol, which is the whole definition off a protecting group. That's why we use protection groups because we can easily remove them afterwards. So now my final product for this model for this reaction is gonna be O. H here, Ethel group here and alcohol here and notice that I'm sorry, I'll get out of the way So you guys can see that Notice that now I have this alcohol back, and I never reacted with it. So the whole point of using protecting groups is that we can avoid ruining our organic metallic and react somewhere else on the reaction instead. And then we can go back and be protected and get that alcohol back. All right. Just you know, the one we just used is called a T beautiful, either. But there's also another type of protecting group called a sill. Either Some professors require that you know it. Some professors don't, so if you don't know about it, it's fine. But just letting you know that's an alternative method to protect and you should know about it. If I covered it in your in your lessons, then you should know about it. If I didn't cover it, then don't worry about it. Alright guys. So I hope that makes sense. Kind of an intro to protecting groups. Um, and a nap. Location of it. Inorganic metallics. Alright. So hopeful. That makes sense. Let's go ahead and move on.
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