DIBAL - Online Tutor, Practice Problems & Exam Prep

Aldehydes can be synthesized through the reduction of carbonyl compounds using milder reducing agents. Two key reagents are a sterically hindered version of lithium aluminum hydride, which converts acid chlorides to aldehydes, and DIBAL-H, which reduces esters and nitriles to aldehydes. Both reagents add one equivalent of hydrogen, preventing full reduction to alcohols. DIBAL-H requires a hydrolysis step to yield the final aldehyde product. Understanding these reagents is crucial for effective organic synthesis involving carbonyl compounds.

LiAl(ot-Bu)3H on Acid Chlorides

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. Li Al. But then this is where it gets weird. Instead of having 4 hydrogens,3 of those hydrogens are replaced with what we call tert-butoxy groups. Then we draw parenthesis. Parenthesis, OtBut3H. As you can see, tert-butoxy groups because I basically have ethers made out of these Otbutyl groups and then I have only 1 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 in this blank up here is just that what's the thing that's making it hindered? That we've got these Otbutyl groups times 3. We're adding those to the LAH 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 with. It specifically works with acid chlorides. That's it for that one. Let's move on to the next free agent.

DIBAL-H on Esters and Nitriles

This next reagent is also kind of random, guys. It works with esters and it works with nitriles. Let's draw that. What it's going to do is it's going to turn esters and nitriles into aldehydes by adding only one equivalent of H. The name of this reagent is weird. It's called Dival H. What the heck does that stand for? Dival H, the long name, I will give you the long name. It's called diisobutylaluminum hydride. Hence the name Dival H. This reagent is a pretty big sterically hindered reducing agent that's going to work with these 2 functional groups and it gives us aldehyde. The only thing I was going to do extra, I mean really we 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. It's just going to be an aluminum. I'm sorry, not a nitrogen, an aluminum with diisobutyl groups attached to an H. Again, you can see what I mean by sterically hindered. Isobutyl groups are pretty big. It's got 2 of those around. That's going to influence its reactivity. One last thing about this reagent. It does also require a hydrolysis step. Typically, you'll see it written as Dival H and then water. Water is required to hydrolyze the intermediate and make it into the final aldehyde. Now you guys know the 2 new reducing agents for this section. We've got the sterically hindered version of lithium amide hydride is one of them and we've got DIBAL. Let's move on to the next topic.

Here’s what students ask on this topic:

What is DIBAL-H and how is it used in organic synthesis?

DIBAL-H, or diisobutylaluminum hydride, is a sterically hindered reducing agent used in organic synthesis to selectively reduce esters and nitriles to aldehydes. Unlike stronger reducing agents like lithium aluminum hydride (LAH), DIBAL-H adds only one equivalent of hydrogen, preventing the full reduction to alcohols. The reaction typically requires a hydrolysis step, where water is added to convert the intermediate to the final aldehyde product. This selective reduction is crucial for synthesizing aldehydes without over-reducing them to alcohols.

How does DIBAL-H reduce esters to aldehydes?

DIBAL-H reduces esters to aldehydes by adding one equivalent of hydrogen to the carbonyl carbon. The bulky diisobutyl groups attached to the aluminum atom create steric hindrance, which limits the reducing power of DIBAL-H, ensuring that only one hydrogen is added. The reaction proceeds through the formation of an intermediate, which is then hydrolyzed with water to yield the final aldehyde product. This selective reduction is useful for synthesizing aldehydes without further reducing them to alcohols.

What are the key differences between DIBAL-H and lithium aluminum hydride (LAH)?

The key differences between DIBAL-H and lithium aluminum hydride (LAH) lie in their reducing power and selectivity. LAH is a strong reducing agent that typically reduces carbonyl compounds all the way to alcohols by adding two equivalents of hydrogen. In contrast, DIBAL-H is a milder, sterically hindered reducing agent that adds only one equivalent of hydrogen, making it suitable for reducing esters and nitriles to aldehydes without further reduction to alcohols. Additionally, DIBAL-H requires a hydrolysis step to yield the final aldehyde product.

Why is DIBAL-H considered a sterically hindered reducing agent?

DIBAL-H is considered a sterically hindered reducing agent because it has two bulky diisobutyl groups attached to the aluminum atom. These large groups create significant steric hindrance around the reactive site, limiting the reducing power of the reagent. This steric hindrance ensures that DIBAL-H adds only one equivalent of hydrogen to the carbonyl compound, making it selective for reducing esters and nitriles to aldehydes without further reduction to alcohols. The steric hindrance is crucial for achieving selective reductions in organic synthesis.

What is the role of the hydrolysis step in DIBAL-H reductions?

The hydrolysis step in DIBAL-H reductions is essential for converting the intermediate formed during the reduction process into the final aldehyde product. After DIBAL-H adds one equivalent of hydrogen to the carbonyl compound, an intermediate is formed. Adding water (H₂O) during the hydrolysis step breaks down this intermediate, releasing the aldehyde. This step is crucial for completing the reduction process and obtaining the desired aldehyde product in organic synthesis.