Identify the following sugars as either reducing sugars — you can just write RS — or non-reducing sugars — you can just write NS. Okay? Guys, if you haven't taken the time to already solve this, I would encourage you to pause the video and try to solve it. But if you already have, I'm going to go ahead and give you the answers now.
Okay. So let's look at the very first one. The very first one actually appears to be glucose. You don't have to necessarily memorize that, but just it looks like glucose to me. So is that a reducing sugar or not? What do you guys think? Well, I do see a hemiacetal. It's definitely not a straight chain, so that's important. It kind of fails the first test. It doesn't have a straight chain. But, does it have a free hemiacetal? Yes, it does. That hemiacetal would be right here. Okay. See how I have a carbon that has an O and an OH? Guys, it's always going to be the anomeric carbon by the way. The anomeric carbon has a hemiacetal that's present, so then this would be a reducing sugar, I'm going to put RS, okay.
By the way, you might be saying, “Johnny, aren't there lots of hemiacetals on this molecule?” Actually, no, there are no other hemiacetals. There are alcohols; even this is an alcohol but it's not close enough to the O here to be a hemiacetal. So all these other OHs are just alcohols. This is the only alcohol that's actually considered to be a hemiacetal. Cool. So let's move on to the next one.
This next one is tricky. So this next one is a disaccharide and disaccharides, you don't need to know a lot about them yet, but we do need to know if it's a reducing sugar or not. So is this disaccharide a reducing sugar? Does it have any free hemiacetal groups? Well, let's look at the left side of the sugar first. What I see is that I have this carbon, that's the anomeric carbon, and that anomeric carbon is attached to OR, OR and then OR, right? This is an O-glycoside. This is a sugar that has formed an acetal linkage. So is this a reducing sugar? What do you guys think? Do you see an acetal, right? So this would look more like this.
So very commonly a lot of students will say this is not a reducing sugar because it has an acetal present. But that's actually the wrong answer. This is a reducing sugar. And why is that? Because guys we have two functional groups. Remember I said you have to have at least one hemiacetal. This is not a hemiacetal, so that part can't react with the Tollens' reagent or whatever. But what do we have over here? Over here, we have a free hemiacetal. So since we have at least one free hemiacetal on the entire structure, this would still be a reducing sugar. So we would still write RS for this one. Tricky, right?
This is why I wanted to go more in-depth with you guys because reducing sugars are very commonly tested and you need to know what the tricks are to know whether one is reducing or not. So this once again is a reducing sugar not because of the first acetal linkage but because of the second hemiacetal on this side.
Now I'm going to take myself out of the screen for the last one. So guys, this is a straight chain monosaccharide. So is it a reducing sugar or not? By the way, I want to point out this is a ketose. Is that a big deal? Can you reduce, can you oxidize a ketose? Totally. Remember I said it's any straight chain monosaccharide, it doesn't matter whether it is a ketose or an aldose because I assume that in base, in the solution that's provided, it's going to tautomerize back to the aldose and then it's going to react with the oxidizing reaction. So I would also say that this is a reducing sugar. So far, all three of our answer choices here were reducing sugars, but they were all reducing sugars for slightly different reasons and there were some tricks associated with them. Cool?
Alright guys, so I hope this made sense. Let's move on to the next video.
