Hemiacetals

Concept: Concept: General Features

4m
Video Transcript

In this next page, we’re going to discuss one of the products that happens when a neutral alcohol attacks a carbonyl and that's called hemiacetals. Let's just start off with one big disclaimer. That disclaimer is that technically the word acetal is used to describe the product of an alcohol and an aldehyde while the word ketal is used to describe an alcohol and a ketone. However, it turns out that professors are lazy, even textbook are lazy. They prefer, since aldehydes and ketones are essentially the same molecule in terms of their reactivity in nucleophilic addition is identical. Instead of using the distinction of acetal ketal, hemiacetal hemiacetal, instead of saying all that, we're just going to use the acetal version.
Whenever you see one of these gem-diether products, because notice that the N part of an acetal reaction is they have two ether groups. They're in a germinal position. They’re germinal. Whenever you have this, we're not going to worry about the R group so much. We’re not going to worry was it originally an aldehyde or a ketone. I don't really care. I’m just going to call it an acetal even though technically it might be a ketal. But it's really an industry standard thing where professors are not specific about the difference between an acetal and a ketal. If your professor specifically always makes that distinction, then by all means go with what they’re saying. But I’m just letting you know that even like online home works and a lot of textbooks don't really care about the difference between an acetal and a ketal. Awesome. Disclaimer over.
You should know this by now. Hemiacetals are only stable when they are cyclic or when they’re a ring form. Here I have another picture of a cyclic hemiacetal. Notice that I have a central carbon that has the four groups that I’m looking for. What is a hemiacetal? A hemiacetal is a functional group with either two Rs or two Hs or a mix. It doesn't matter. And an OH and an OR in a germinal position, so an alcohol and an ether in a germinal position. That’s a hemiacetal. Notice that this molecule is also a hemiacetal because I’ve got my H, I’ve got my R, I’ve got my OH, my alcohol and my ether, my OR.
When it’s a cyclic hemiacetal, you're stable. But if it's not cyclic, then you're not going to be able to end up at the hemiacetal. let me show you guys the general overview of this reaction. It turns out that when you react a carbonyl with one equivalent of alcohol, you're going to get what we call a hemiacetal. When you react it with the second equivalent of alcohol, it's going to be called an acetal and it’s going to make that germinal diether. The mechanism for the first step and for the second step is almost identical. The only way to really get it to stop at the hemiacetal is to make that cyclic version because if it’s not cyclic, it’s just going to pass straight through hemiacetal stage and go straight to acetal.
I hope that’s making sense so far. Now what we’re going to do in this next video is I’m going to show you the exact acid-catalyzed mechanism to create a hemiacetal.