Alcohol Protecting Groups - Video Tutorials & Practice Problems
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Alcohols are highly reactive. This can be a problem if we want to react on some other part of the molecule. How can we guarantee that a reaction won’t take place at the alcohol?
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General features of Alcohol Protecting Groups.
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Video transcript
Hey, guys. And let's talk about protecting groups. Protecting groups are reactions that air used to shield certain types of functional groups. Okay, in this case, I'm using the word moy. Eighties movies just means some kind of reactive region of the molecule from a reaction that's gonna happen on another part of the molecule. All right, so now I know that sounds complicated, but basically, we're going to try to do is we're gonna try to shield vulnerable, functional groups from certain types of strong re agents. Okay. And by definition, this has to be a completely reversible, easily reversible reaction. The reason for that is that you're supposed to be able to take the molecule off after direction is complete. So if you're not able to regenerate that vulnerable function group at the end, that's not really a great protecting group. Okay, let me give you an example. Why Might need something like this. Let's go ahead and look at this reaction we've got in alcohol and an alcohol. Hey, lied on the same molecule. Okay, so, first of all that brings up our first point. Your Onley gonna use a protecting group if you have more than one functional group on a molecule. If you only have one functional group, we don't care. You don't need to protect anything. But if you have more than one, then there may be some instances where you want to react with one and not the other. And that's when you when you use a protecting group. Okay, so let's look at this re agent are re agent is an Alka Nayd. And as you guys might remember, Alka, nights are good nuclear files, but they're also strong bases. So is there anything that that's supposed to be erasing? Okay, Is there anything that the Alcon I could do to those functional groups? Well, in this case, what I'm trying to do is, as you can see, my and product, I'm trying to make this falcon I perform. Ah, substitution reaction on the alcohol. Hey, lied. All right. In this case, this would be an s and two reaction. Okay, so that's what I'm trying to make happen. But notice that there's that other functional group of the molecule. The alcohol can alcohol's react with Alcon IEDs, Actually, yes. And they react through a different mechanism. They react through an acid based mechanism because we know that alcohols have an acidic proton and Alcon eyes are very strong bases. Okay, so it turns out that this reaction will not proceed to completion. Okay. In fact, the Alcon I'd will almost exclusively react with the O. H. And it will almost not. It will pretty much not react at all with the alcohol. Hey, lied. So if I do want distraction toe happen, is there any way to make it on Lee? React with the alcohol? Hey, lied and not the alcohol. Okay, well, scientists determined. Hey, you know what? Alcohols are messing up a lot of different reactions so we can figure out a way to get rid of the alcohol for a few minutes, then run the rest of the reaction and then regenerate the alcohol. That would be really helpful. And that's exactly we're going to do with our protecting group.
For example, the following substitution reaction has a major problem as drawn. Can you spot the issue?
Protecting groups are reversible reactions that temporarily block groups from reacting, so that we can transform other parts of the molecule.