Which of the following sugars are reducing sugars? Which ones ...

Question

Which of the following sugars are reducing sugars? Which ones would undergo mutarotation?

(b) α-D-fructofuranosyl-β-D-mannopyranoside

Accepted Answer

All right. Hello everyone. So this question is asking us for part one to provide the structure of alpha D ribo Furano beta D galacto Puran. Aside part two is asking, is it a reducing sugar? And part three is asking, can it undergo muter rotation? So for a question like this one, it's important to recognize that the easiest approach to drawing out a given sugar molecule is to take it unit by unit, right? So first let's start by drawing out alpha D rival, Uran Ail, which if you recall is referring to the five member ring form of D ribose. Now here on the screen, I went ahead and pre drew the Fisher projection that is the straight chain structures of both D ribose on the left and D galactose on the right. So these are going to be our references, so to speak, we're going to take these structures into consideration when going from the fissure projection to the ring structure that we're trying to drop. So let's start with D rivals now because D ribose is going to form a five member ring, we can go ahead and start by drawing out our template. So we have a five member ring and one of the atoms in the ring is going to be oxygen depicted towards the top of the ring itself. So just to showcase dimensionality, three dimensionality, I should say I'm going to go ahead and shade shade in this front part of the ring itself. And recall also that just by convention, carbon number one is said to be the carbon on the very right side of the structure. OK. So now let's talk D Rebus. So recall first and foremost, that hydroxy groups that are shown to be on the right side of a fissure projection are shown to be pointing downwards in the ring structure itself. So carbons 23 and four, for example, have all of their hydroxy groups pointing to the right, which means that those hydroxy groups would point downwards in the ring structure. So that's what I'm going to showcase here. Now, the exception actually, now that I think about it, one of the exceptions at least is actually carbon number three because the hydroxy oxygen of carbon number three is actually the one that's contributing to the ring structure of rivals carbon three, however, is associated to carbon four. And because we're talking about ad sugar that is D ribose, right. Carbon number four, which is the primary alcohol should be pointing above the ring. So this leaves us now with carbon number one. Now, carbon number one can also be described as the stereo descriptor carbon and it's the anomic carbon. So the positioning of the hydroxy group on probably number one, when we're talking about a ring structure of a given sugar, depends on whether or not we're talking about the alpha or beta configuration of this molecule. 24, our D Rebo Furano, we're specifically told that it is in the alpha configuration. And so what that means to say is that the hydroxy group of the anomic carbon should be pointing in the opposite direction relative to carbon number four, because carbon number four is the stereo descriptor carbon outside of the ring itself. So once again, our stereo descriptor is carbon number four. And because it's pointing upwards, the hydroxy group of carbon number one should point downwards so that this molecule is in the alpha configuration. So now this completes the structure of our first sugar. So now let's go ahead and proceed with the second part. And our second part is going to be a beta D galactose. This if you recall would mean that it is the six member ring formed from the galactose. So here we can do the same thing as the first part and start by drawing out our template. Now, I'm specifically going to start by drawing out a chair confirmation and by convention, right, the oxygen of this ring should be pointing to the top right corner, which means at carbon number one, the anomic carbon should be pointing or should be located at the bottom right corner. Alright. So here we have our template, right. So first let's start with our stereo descriptor, our stereo descriptor in this case would be carbon number six. That is not part of the ring itself. So because we're talking about de galactose, that stereo descriptor should be pointing above the ring attached to carbon number five. So from here, let's talk hydroxy groups, the hydroxy group on carbon number two, for example, is to the right in the fissure projection, which means it points downwards in the ring. For carbon number three, it's pointing to the left in the fisher projection, which means it points upwards in the ring structure. And the same is true for carby number four. So now this leaves us with the stereo descriptor carbon or excuse me, the anomic carbon carbon number one. So for our de galactose, we're told that it's going to be in the beta configuration. So going back to our stereo descriptor which is carbon number six, it's pointing above the ring. So for the beta configuration, the hydroxy group of the anomic carbon that is carbon number one should be pointing upwards as well. So that these two groups are pointing in the same direction. All right. So from here, right, all we have to do is connect them. So to connect them, we would attach the anomic carbons to each other with an oxygen in between them. So here, if I scroll down a little bit to open up some space. I can move our D galactose downwards and start orienting our de ribo furino towards its anomic carbon. So to connect our ener carbons, I can erase one hydroxy group and erase the, the hydrogen of the other hydroxy group and use the remaining oxygen to connect both anomic carbons. So now that we've answered the first part of the question that is we've drawn the structure of our given. Let's talk about the remaining two parts. What does it mean for a sugar to be a reducing sugar? And can it undergo mutter rotation? Well, recall that these two questions are linked, right? If a given sugar is a reducing sugar, then by nature, it can undergo muter rotation. So here, right, what makes a reducing sugar? There are two general types of reducing sugars. Number one, any straight chain monosaccharide is going to be considered a reducing sugar. But the same cannot be said for cyclic monosaccharide or disaccharide, et cetera. For a cyclic sugar to be considered a reducing sugar, it has to have a cyclic hemi acetyl group. And so what that means if you recall is that a heavy ace towel contains one hydroxy group and one alcoy group on the same carbon. A coxie group refers to or groups. So going back to the cyclic structures of our given, right, the oxygen inside of the ring is considered an oxy group because it itself is attached to another carbon of the ring. But notice how if you look at both anomic carbons for both monosaccharide units, we don't see a hemi acetal because neither of these carbons have a hydroxy group associated to them. So therefore, right, these sugars are indeed non reducing, they're non reducing because we don't see any he acetal parts. So therefore because theyre not reducing or because our sugar here is not reducing, it cannot mule rotate. So there you have it. And so if you watch this video all the way through, thank you so very much for doing so. And I hope you found this helpful.

Which of the following sugars are reducing sugars? Which ones would undergo mutarotation?
(b) α-D-fructofuranosyl-β-D-mannopyranoside

Key Concepts

Reducing Sugars

Mutarotation

Furanose and Pyranose Forms

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