Concept: Concept: Why LiAlH4 doesn't work3m
In this video, we're going to learn how to make aldehydes through reduction.
At this point of the course, you've definitely seen a carbonyl coupled with a reducing agent. What we're used to seeing is the product of a carbonyl and reducing agent is alcohols. We have been dealing with very strong reducing agents up until this point. The two that come to mind for me when it comes to carbonyls are lithium aluminum hydride and sodium borohydride. These are two reducing agents that you’ve definitely seen by now and they're both really good at making alcohols from carbonyls.
In this specific example, we would actually want to use LiAlH4 or lithium aluminum hydride. You might remember that this one is the one that works better with carboxylic acids. What winds up happening is that it adds two equivalents of hydrogen to the carbonyl to get an alcohol. Now, this is great.
But what if we want to stop with one equivalent of hydrogen? What if we don't want to go all the way to the alcohol? Instead we want to just go to the aldehyde first and stay there. If that’s our goal, we're going to have to use some milder reducing agents because we’re going to have to use some reducing agents that don't want to go all the way to the alcohol that don't want to add two equivalents and just are okay with adding one equivalent of hydrogen instead.
What we're going to do now is we’re going to learn two such reagents that would qualify as milder reducing agent that actually make aldehydes, not alcohols. We're going to see if they have some things in common in terms of the fact that they use steric hindrance to reduce the power of the reducing agent, and they’re both going to add only one equivalent of hydrogen at each time. These reagents do seem kind of random. They’ve got some weird names. We’re just going to try our best and I'm going to hope to help make it flow. Let’s go ahead and start off with the first one.
Concept: Concept: LiAl(ot-Bu)3H on Acid Chlorides2m
The first new reagent that we're going to learn specifically works with acid chlorides. It turns acid chlorides into aldehydes.
This reagent has a really long name but all it is is it’s a sterically-hindered version of lithium aluminum hydride. Let me draw it out for you. It actually starts off exactly the same. LiAl, but then this is where it gets weird. Instead of having four hydrogens, three of those hydrogens are replaced with what we call tert-butoxy group. Then we draw a parenthesis, (ot-Bu)3H. As you can see, tert-butoxy groups because they basically have ethers made out of these ot-butyl groups. Then I have only one hydrogen left over that’s actually going to react. The other ones don’t. The other groups don’t. This molecule, even though it looks really complicated, it's just going to be a version of lithium aluminum hydride that adds one equivalent. I’m going to get aldehyde instead.
What I want you guys to draw on this blank up here is just that what’s the thing that’s making it hindered? That we've got these o t-butyl groups times three. We’re adding those to the LiAlH to make it a lot less reactive. That's the first one. Unfortunately, you do have to memorize this. You have to memorize how to draw it and what it works. It specifically works with acid chlorides. That’s it for that one. Let’s move on to the next reagent.
Concept: Concept: DIBAL-H on Esters and Nitriles3m
So this next reagent is also kind of random guys. It works with esters and it works with nitriles, let's draw that, and what it's going to do it it's going to turn esters and nitriles into aldehydes by adding only one equivalent of each. The name of this reagent is weird, it's called DIBAL-H and what the heck does that stand for? So DIBAL-H, the long name I will give you the long name, it's called Diisobutyl aluminum hydride hence the name DIBAL-H. Alright and this reagent is a pretty big sterically hindered reducing agent that's going to work with these two functional groups and it gives us aldehyde.
The only thing I was going to do extra I mean really you just have to memorize it I'm not going to show you the mechanism but I do want to show you the way it looks so it's just going to be a nitrogen, I'm sorry not a nitrogen, an aluminum with diisobutyl groups attached to an H. So again you can see what I mean by sterically hindered. Isobutyl groups are pretty big. It's got two of those around and that's going to influence its reactivity. Now one second, one last thing about this reagent, it does also require a hydrolysis step.
So typically you'll see it written as DIBAL-H and then 2, water. Water's required to hydrolyse the intermediate and make it into the final aldehyde. So now you guys know the two new reducing agents for this section. We've got the sterically hindered version of lithium aluminum hydride is one of them and we've got DIBAL. Alright, so let's move on to the next topic.
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