Organic Chemistry

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24. Carbohydrates

Monosaccharides - Forming Cyclic Hemiacetals

By definition, monosaccharides contain at least one carbonyl group and multiple alcohols. With that in mind, do you remember a reaction from the past that includes both of these groups? That's right, guys! It's the hemiacetal/acetal reaction. Let's see how this works with monosaccharides.



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in this video. What I want to do is give you guys a little bit of context for what is going to be the most important reaction of mono sack rides, and that is cycle ization. So guys, by definition, mono sacha rides always contained at least one Carbonell group, right, Kate owner Aldo hide usually and multiple alcohol's. Okay, now, remember, guys we've already learned in the past how carbon eels and alcohols react with each other. Expect in an acidic condition. So remember that in our carbon you'll section of your textbook, we learned that the nuclear Filic addition of one alcohol on a carbon Neil produces a functional group called Ah Hemi Ass. It'll remember that and remember that then a second equivalent of alcohol to the Hemi Hospital produces a functional group called in a Settle. And that's what I have drawn out here. Remember that the general structure was that your 10 H comes in and you get an R and O R groups that would be this one right here, and then your second r o H can come in. It's not exactly that mechanism, but eventually it comes in and replaces the o h. And then you get this one over here, okay? And that was the general structure of an asset. Tell Now, remember, guys that usually heavy acid towels are not stable. Remember that you usually never end with the heaviest tell you go to the all the way to the full asset tell. But there was only one exception to that. Remember that the Onley exception is a cyclic mes hotel. And it is possible for if you're ohh comes from the same molecule as your curva, Neil, that you conform a ring with your hammy as to tell, and that one is actually stable. So let's draw out. Let's try to draw. What would that look like? And the way that you could draw that out is by using the general structure. Remember that the general structure is that you have o h. Let me put on the other side. Is that you have, um Ohh. Oh, are. And then in this case, h are now Why did I do that? Because this h comes from here, right? So it's the Alba high age. Um this are is this one here and then this are here is actually the our group on this. Oh, okay. So now all you have to do is plug in the are groups. So what are the are groups? Well, remember that the way any intra molecular reaction, you count out the distance from the nuclear fall to the Electra file. So it would be one, um, 12345 So I know this would be a five member ring. So then what I would do is I would erase this are erase this are and replace it with a five member ring that looks like this. Okay, where this is going to be carbon or this is gonna be Adam one. There's gonna be Adam to There's gonna be Adam three. This is gonna be Adam four, and this is gonna be Adam five. Okay, so that is a very ugly, cyclic amines, a towel. But that is the answer. That is the correct answer to this, uh, to this nuclear Filic edition. And this one is stable. It wouldn't actually add a second cloven of alcohol because it's already stable like this. Okay, so it turns out that many mono sacha rides can undergo this reversible inner molecular ring forming Hemi as tell mechanism. And this is what we call this whole part right here. The ring for me, Henry has to tell Mechanism is what we call cycle ization. Okay, so here's an example of civilization D glucose, which you guys should know very well by now. What d glucose is it undergoes nuclear feel like addition to form a cyclic six member Khenyeza. Tell where basically this. Oh, it's always going to be the pen ultimate. Not always, but many times the penultimate ohh attacks the carbon eel and forms a ring. The size of that ring would be 123456 Now, guys, I actually didn't number this. According to nuclear fall to Elektra file, I numbered it. Based on the mono sack ride numbering that we already know. The top one is one and then it goes down from there. But regardless, it's six, right? There's a six atoms in this ring. So then this is what it would look like as a cycle ization as a ham. Yes, to tell what you would get is that this is now one, 2345 And this is Adam six. Okay, it's not carbon six. It's Adam six. And what we see is that what I labeled as the gray notice that it's gray down here in a gray box. This is that same Adam here. So everything lines up. Okay? Now, notice that what was in this gray circle before is now this guy down here, how did that happen? Because remember that. Ah, carbon Neil. After nuclear Filic edition with alcohol, it turns into a Hemi as a towel. Where now I'm gonna have, uh, our group h groups. Those are your two groups that stayed from the beginning. And then you're gonna have Ohh. And then the O R group, which is that part right there? So, basically of the four groups from a heavy hospital now, guys, by looking at this, you're not supposed to be able to draw this yet. Don't worry. I'm just exposing you to the fact that cycle ization happens through heavy ass. It'll mechanism. Is that fine? I'm going to explain how to get all those positions later, but right now, just know that it happens through and asked him. He has to tell. Okay, Great. Let's move on.
  • Recall that the only stable hemiacetals are cyclic (5 and 6-membered rings)

Provide the mechanism for the cyclic hemiacetal formation of the following hydroxycarbonyl.

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