Indicate if the following pairs of sugars are enantiomers, ano...

Question

Indicate if the following pairs of sugars are enantiomers, anomers, epimers, or an aldose-ketose pair:

a) α-D-galactopyranose and β-D-galactopyranose. ____________________

b) D-glucose and D-mannose. ____________________

c) D-glucose and D-fructose. ____________________

d) α-D-glucopyranose and β-D-glucopyranose. ____________________

e) D-galactose and D-glucose. ____________________

f) α-D-mannopyranose and α-L-mannopyranose. ____________________

Accepted Answer

Alright, so this practice problem wants us to indicate if the following pairs of sugars are enantiomers, anomers, epimers, or an aldoseketose pair. And so taking a look at our first pair here, we have alpha d galactopyranose and beta d galacto pyranose. And so notice the only difference between these two sugars is that one has alpha and the other has beta. And so this means that they differ at the configuration of the anomeric carbon. And so these 2 are going to be anomers of each other. So we can go ahead and write anomers here. Now looking at the next pair, we have d glucose and d mannose. And recall from our previous lesson videos, we said that because D mannose has these two ends in its name, that reminds us that D mannose is going to be the C two epimer of D glucose. And so, of course, D glucose and D mannose are going to be epimers of each other. So we can go ahead and write epimers here. Now, moving on to the 3rd pair, we have D glucose and D fructose. Now, recall in the list of 6 monosaccharides that we had talked about in our previous lesson videos that D Fructose was the only ketose on that list and so that reminds us that D Fructose is the ketose form of d Glucose. And so d Glucose and d Fructose are going to be an Aldose Ketose pair. And so we can go ahead and write that in right here. Now, another way we could have figured that out, Aldose Ketose pair. Another way that we could have figured out that option c here was an aldose ketose pair. Is that recall that, D glucose is an aldohexose and d fructose is an aldo ketose. And so, they are not going to be enantiomers because that would make them mirror images of each other. They're not gonna be anomers because anomers only relate to cyclic sugars whereas these 2, don't necessarily indicate that they're in their cyclic forms. And, of course, epimers only differ in the configuration of just one single chiral carbon. And so, these 2, they differ because one has an aldehyde and one has a ketose. And so that's more than just differing by 1 chiral carbon configuration. So, we should have been able to, through process of elimination, figure out that it wasn't any of these 3 and so that would have only left the Aldose Ketose pair, for option c. Now, moving on to the 4th pair, we have alpha d glucopyranose and Beta D Glucopyranose and so again, they only differ in the Alpha and Beta configurations which makes them Animers. Now, moving on to this pair right here, we have D galactose and D glucose and of course, recall from our previous lesson videos that what helps us remember the relationship between these two sugars is that D galactose has a total of 4 vowels in it And so these 4 vowels, reminds us that d galactose is the c 4 epimer of d glucose. And so these 2 are going to be epimers of each other. And so the last pair that we have is alpha d mannopyranose and alpha L mannopyrenose. And so because we have D and L here and we have the same exact sugar otherwise, this, of course, is telling us that these 2 must be mirror images of each other. And if they're going to be mirror images, then that means that they're going to be, enantiomers because, they have, opposite, complete opposite configurations. And so, what we can do is write that these are going to be enantiomers. And so, these are the answers to, this practice problem and that concludes this practice problem. So I'll see you guys in our next video.

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