Reducing Sugars - Video Tutorials & Practice Problems
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Reducing Sugars
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In this video we're going to begin our discussion on reducing sugars. Now before we can fully understand reducing sugars, it's first helpful for us to be able to understand acetals and ketals. And so in our previous lesson videos, although we didn't really talk about acetals and ketals very much, we did talk about hemiacetals and hemiketals and so really everything that you see in this green box right here and in this blue box right here should look familiar to you guys from our previous lesson videos and so really the only new thing that we're showing you guys in this video is this yellow box that we see over here. And so to refresh our memories, of course, we know that alcohol groups can react with aldehyde groups to form hemiacetals and alcohol groups can also react with ketone groups to form hemiketals. Now, recall that the hemiacetals and hemiketals are relatively unstable structures. And so if they're unstable, then they're going to be relatively, reactive. If you're relatively unstable, you'll be relatively reactive. And so, essentially, what we're saying here is that these hemiacetals and these hemiketals that are relatively unstable and relatively reactive, they're gonna be therefore susceptible to reacting with other alcohol such as, these purple alcohol groups that we see down below. And so when the hemiacetals and hemiketals react with these other alcohols, they can form more stable acetal and ketal structures or groups and so what you can see is that the hemiacetal that we have here which is again relatively unstable and relatively reactive. Can react with another alcohol group and when it reacts with that alcohol group, it can produce this structure here which is the acetal group and so notice that it dropped this prefix hemi which means half. And so now we have a full acetal group. And so what you'll notice, about the acetal group is that it has the o r here and then o r here. And before, we only had one of these o r's, So it was half. And so here we have a full, 2, OR groups branching off. Now notice that this arrow going to the right is actually a dehydration synthesis reaction since a water molecule, molecule, h two o, is being lost. Now, if we take a look down on the bottom half, notice that the hemiketal is also going to be relatively unstable and relatively reactive and so it can react with another alcohol group as well and that is of course going to generate this structure over here which is the ketal group and again, notice that it dropped the fix hemi because it's no longer half of a ketal. Notice over here, we have 2 OR groups branching off. Whereas, over here we only have 1 OR group branching off. And so, ultimately what we're seeing is that through this reaction that we see here, we're able to replace the alcohol groups with OR groups, that are branching off. And so, another thing to note here is that the reaction arrow going from left to right for both of these is larger than the reaction arrow going backwards from right to left. And so what this means is that the acetal and the ketal groups are going to be more stable and predominate over the hemiacetal and hemiketal groups which are again relatively unstable. And so, keeping this in mind is going to be very important and very helpful in understanding reducing sugars. The difference between Acetal and ketal groups and, hemiacetal and hemiketal groups. Now, one thing, important to note here is that, notice that the only difference between an aldehyde and a ketone is this hydrogen right here in this R group. Notice that the only difference between a hemiacetal and a hemiketal is again the same exact hydrogen and the same exact r group. And notice that the only difference between an acetal group and a ketal group is again the same hydrogen and the same R group. And so over time, you'll be able to get much better at distinguishing between aldehyde, hemiacetals, and acetal groups, and distinguishing them from ketone, hemiketal, and ketal groups just by looking for either a hydrogen atom branching off of the anomeric carbon or an additional R group branching off of the anomeric carbon. And so, now that we understand, the differences between Hemiacetals and Hemiketals and Acetals and Ketals, we can better be we will be better prepared to understand reducing sugars, which we will talk more about in our next video. So I'll see you guys there.
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concept
Reducing Sugars
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6m
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Video transcript
So in addition to being able to distinguish between acetalketal, hemiacetal, and hemiketal groups, there's also one more note that's going to be helpful in distinguishing between reducing sugars and non reducing sugars. And, so this note is that aldoses and ketosis can actually reversibly rearrange into each other and they do so via the enediol rearrangement mechanism, which is a mechanism that you guys may have covered before in your old organic chemistry courses. But really the main takeaway here of this note is that aldoses can reversibly rearrange into ketosis, and ketosis can reversibly rearrange into aldoses. So now keeping that in mind as we move forward, we can go ahead and define reducing sugars. So a reducing sugar is just any sugar that's capable of being oxidized. And so in other words, we can say that a reducing sugar is just a sugar that's capable of acting as a reducing agent. And of course reducing agents get oxidized. Now reducing sugars must either have their own free aldehyde group or they must be able to rearrange to generate their own free aldehyde group. And so reducing sugars, are going to include the following two things. It's going to include all simple linear sugars, aldoses, and ketosis since they're capable of reversibly rearranging into each other, and it's also going to include cyclic sugars, but only cyclic sugars that have a free hemiacetal or hemiketal group and this is because the hemiacetal and hemiketal groups are in equilibrium with their linear forms and if they are going to be in their linear forms that means that they're capable of either having or generating their own free aldehyde group. And so non reducing sugars on the other hand are going to be the complete opposite of a reducing sugar and so that means that non reducing sugars are not capable of act of being oxidized and they're not capable of acting as a reducing agent, which is why they are non reducing. And so non reducing sugars are going to include sugars that have full acetal and ketal groups. And so this is why it's so important to be able to distinguish between acetal and ketal groups as well as hemiacetal and hemiketal groups because it will affect our ability to identify reducing sugars and non reducing sugars. And so if we take a look at our example down below, we can appropriately label each of the following carbohydrates as either reducing sugars or non reducing sugars. And so taking a look at this sugar over here, what we can see is that we have a linear monosaccharide. And of course, if it is a linear monosaccharide or sugar, that means that it's going to be a reducing sugar. And so here, we have a linear monosaccharide that has its own aldehyde group, its own free aldehyde group and that is more evidence, to suggest that this is indeed a reducing sugar. So, we can write 'reducing' here. Here. Now, taking a look at this next sugar here, notice that we have a cyclic sugar and so note that reducing sugars can be cyclic sugars but only if they have their own free hemiacetal or hemiketal group. And so notice looking at the anomeric carbon over here, it is present in a hemiacetal group And so you can see that it has its hydroxyl group, it has its OR group right here and it has another R group and then it has the Hydrogen that's not being shown and so this is a hemiacetal and so because this has its own hemiacetal group, this is going to be a reducing sugar as well. So, down below, we can write reducing. Now, if we take a look at this final sugar over here, notice that we actually have a disaccharide. We have 2 sugars that are linked together covalently. And so what you'll notice is that the anomeric carbon of this first sugar right here is right here. Since this is the only carbon atom bonded to 2 oxygen atoms and the anomeric carbon of this sugar over here is actually right here. And so, what you'll note is that these neither of these sugars, this one or this one over here, have an anomeric carbon that's part of a free hemiacetal or hemiketal group. Instead, when we look at these anomeric carbons, we'll see that they are present in Acetal groups and Ketal groups. And so notice that, right here with this anomeric carbon, we have 2 OR groups branching off. We have this 1 and then we have this one branching off and, already we are at an acetal. And so we know that it's an acetal because we have a hydrogen coming off at this position. However, notice over here with this anomeric carbon, we have a ketal because we have an O, an OR group branching off here. We have another OR group branching off here And, instead of having, a hydrogen atom coming up, we have another R group, another carbon atom. And so here we have a ketal and, over here we have an acetal. And so again, if ketals and, acetals are then this is going to be a non reducing sugar and so down below, we can write non reducing. And so as we move forward in our course, we're going to be able to get more and more practice identifying, reducing, and non reducing sugars and really the best way to get good at this is to get practice. So, I'll see you guys in our next video.
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Problem
Problem
Which of the following disaccharides are reducing sugars?
A
Sugars A & B.
B
Sugars B & C.
C
Sugars C & D.
D
Sugars A & C.
E
>2 sugars.
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Problem
Problem
Which chemical group below indicates that the sugar is a reducing disaccharide? Is 'C' an acetal or ketal?