Alcohols are so reactive that sometimes we want to react with another part of the molecule without actually interfering with the alcohol, and this is a situation where you want to use a protecting group. So now I want to talk about a type of protecting group called a silyl ether protecting group. There are two main ways to protect alcohols. You can use a t-butyl ether protecting group or a silyl ether protecting group. You may have to learn one or both of these for your professor. I'm only going to include the ones that your professor needs. If you're watching this video and it fits in your playlist, that means your professor wants you to know this one. So silyl ethers, I know they sound weird, but it just means that it's something made with silicon. It's a molecule, made of silicon. It looks a lot like an ether except instead of an O, it's going to have an Si, a silicon. These are used to protect. Remember that the definition of a protecting group is something that you can put on the molecule, protect and then take it off later.
There's this weird reagent called TBDMS. TBDMS, you absolutely do not need to know what it stands for. What you do need to know is that it's the most common silyl chloride used in Organic Chemistry 1 to make a silyl ether. This is the structure: TBDMS. And I would advise knowing what the structure looks like. Why? Because you need to know the mechanism for this. So how does this work? Let's say that I'm trying to react with this alkyl halide. But we know that some reagents that react with alkyl halides also react with alcohols. For example, strong bases. Strong bases can do elimination on an alkyl halide, but they could also deprotonate an alcohol. So how do we protect that? You could expose the alcohol to a silyl chloride. What's going to wind up happening is that this silicon has a pretty strong dipole pulling away from it, so there's going to be a partial positive charge right there. You can use your alcohol to attack that silicon, but now silicon is just like carbon. It's actually right under carbon in the periodic table, so silicon wants to have four bonds. Right now, by adding that bond to the silicon, we're making five. So if we make a bond, we have to break a bond. Is there an easy bond here to break? Yes, we can break off chlorine and cause it to kick out as a leaving group. What that's going to give us is a molecule that looks like this where nothing's happened to my alkyl halide yet. But now I have OSi with the two methyl groups and the tertiary butyl group. Cool so far? We've also got the H that's still present and positive charge. What we can now do is we can use the chlorine that got kicked off to deprotonate. What we're going to wind up getting is HCl due to the fact that we are deprotonating with the Cl and we're going to get our protected alcohol. Now the reason that this is helpful is because now if I were to expose this to a reagent that reacts normally with alcohol, this thing will not react with alcohol.
This thing is unreactive, which is the entire point of a silyl ether. Now, in the next step, there's another step that I'm going to skip here, we could do whatever we want to this part of the molecule. We could react this with a strong base, or maybe a nucleophile, whatever. And it would only react with the target functional groups and it would not react with the silyl ether. So now that's great. We can do whatever we want to the alkyl halide, but what happens when we want to get that alcohol back because that's the whole point of a protecting group? You want to make sure you can take the silicon off and get that alcohol back. Then we're going to use another reagent and that reagent is kind of weird. It's a nitrogen with four butyl groups. What's going to happen is that the fluorine from that molecule can wind up k
