Hey guys. So now let's talk about protecting groups. Protecting groups are reactions that are used to shield certain types of functional groups. In this case, I'm using the word moieties. Moieties just means some kind of reactive region of the molecule from a reaction that's going to happen on another part of the molecule. I know that sounds complicated, but basically what we're going to try to do is we're going to try to shield vulnerable functional groups from certain types of strong reagents. 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 the reaction is complete. So if you're not able to regenerate that vulnerable functional group at the end, that's not really a great protecting group. So let me give you an example of why I might need something like this. Let's go ahead and look at this reaction. We've got an alcohol and an alkyl halide on the same molecule. So first of all, that brings up our first point. You're only going to use a protecting group if you have more than 1 functional group on a molecule. If you only have 1 functional group, we don't care. You don't need to protect anything. But if you have more than 1, then there may be some instances where you want to react with 1 and not the other and that's when you use a protecting group. Let's look at this reagent. Our reagent is an alkene. As you guys might remember, alkenes are good nucleophiles, but they're also strong bases. So is there anything that an oh, that's supposed to be erasing. Is there anything that the alkanide could do to those functional groups? Well, in this case, what I'm trying to do is as you can see my end product, I'm trying to make this alkynide perform a substitution reaction on the alkyl halide. Alright? In this case, this would be an SN2 reaction. 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 alcohols react with alkenes? Actually, yes. And they react through a different mechanism. They react through an acid base mechanism because we know that alcohols have an acidic proton and alkenides are very strong bases. So it turns out that this reaction will not proceed to completion. In fact, the alkynide will almost exclusively react with the Oh and it will almost not it will pretty much not react at all with the alkyl halide. So if I do want this reaction to happen, is there any way to make it only react with the alkyl halide and not the alcohol? Well, scientists determine, hey, you know what? Alcohols are messing up a lot of different reactions. So if 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 that alcohol, that would be really helpful. And that's exactly what we're going to do with our protecting group.
12. Alcohols, Ethers, Epoxides and Thiols
Alcohol Protecting Groups
12. Alcohols, Ethers, Epoxides and Thiols
Alcohol Protecting Groups - Online Tutor, Practice Problems & Exam Prep
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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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.
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