Monosaccharides - Reducing Sugars

Concept: Concept 1

9m
Video Transcript

Hey fam, in this video I want to do a comprehensive review of one of the most commonly tested topics in carbohydrate chemistry and that is the topic of reducing sugars, let's go ahead and get started. So guys, whereas a lot of different textbooks or maybe even videos online would start at the definition of reducing sugars for this topic, I think it's going to be easier for you to understand if I first relate the reactions, we're looking at two reactions that we've already learned. So, you understand the reactions first and then I'm going to go into the definition of reducing sugars a little bit more, okay? So, just kind of hold on tight, it's all going to make sense at the end I'm just going to mix things up a little bit. Alright, guys. So, what do we already know? Well, we already learned is that a reaction called bromine water but it's not called bromine water but bromine water is the reagent and there it's called weak oxidation and what it can do is it can provide high yield of aldonic acid. So, remember that we actually had the same exact molecules displayed when we're learning about weak oxidation and what we learned is that if you start off with an aldose and you react it with bromine and water, what you're going to get through a mechanism that you don't really need to know is the formation of an aldonic acid. So, you already know this, this reaction is called weak oxidation, wonderful, but this reaction has a limitation, the limitation is that even though it works really well it does not undergo a visible transformation. Now, that actually isn't a big deal in modern days because we have NMR and a lot of different analytical techniques that we can use to verify if a reaction is completed or not but thinking back to the 1800s, when these reactions were first developed, it was kind of nice to have reagents that would turn red or turn blue or green once they reacted, so the reactions we're going to learn about are reactions that accomplish the same exact transformation while giving us visual cues and those reactions are going to be the Tollens' test the Benedict's tests and the Fehling's tests and these tests are going to react with what we call reducing sugars, which I'm going to define very soon, into aldonic acid, but they're not just going to do it the way that bromine water did it they're going to do it while also providing specific visual indications that the reaction took place, okay? So, the reaction proceeds forward by oxidizing, I should put oxidize, the tests will oxidize any sugar capable of forming a straight chain aldose or ketose and this is going to be our definition of reducing sugars for right now. So, when I said that it reacts with reducing sugars, we're going to say the reducing sugar is any sugar that's capable of forming a straight chain aldose or ketose. So, d-mannose is that a reducing sugar? yeah because it's a straight chain almost, right? So, we're saying is that these reactions Tollens' Benedict's Fehling's they're all going to do the same thing, they're going to turn that d-mannose into, they're going to oxidize it and turn it into a carboxylic acid and then they're going to give us specific visual indications, okay? So, let's go really a little bit more into what the reagents are for each of these that you can know what to expect, when you, when you find this on the exam, let's say. So, let's start off with Tollens', which is the most common of the three, so the Tollens' test has been around for a really long time and it seems like every book or every professor has their own unique cocktail of reagents that they like to use for the Tollens' test. So, sometimes you might see it drawn out like this, sometimes you might see it drawn out like this they look very different from each other but there are commonalities between them and what the commonalities are is that there's always going to be some elemental, silver, there's always going to be some ammonia and there's always going to be some base. So, what I want you guys to think about is I don't want you to memorize the exact order of those letters or the reagents because they're actually like five other ways that can be drawn, you might see it simply as just like silver oxide and ammonia like nh3. So, I don't really want you to focus too much on the exact letters as much as the general picture, that if you're reacting some kind of sugar with silver ammonia, a combination of silver ammonia and base, that's going to be a Tollen's reagent, okay? Now, what does Tollens do? it does the same thing as bromine water but instead of just getting your aldonic acid you're also going to get a silver mirror on the outside of your test tube, so that it looks silver and you're going to know oh there was a reducing sugar present, okay? By the way, we're assuming like I said, this is a reducing sugar. Now, this will be called a reducing sugar.

Next, we have Benedict's and selling tests. Now, Benedict's and selling tests actually do the same exact thing, they react with your reducing sugar they turn it into an aldonic acid, they just have a different visual indication and the visual indication they have is that they turn into copper one oxide cu2o and they form a brick red precipitate that settles at the bottom of the test tube, okay? Now, you might ask? Well, what's the difference between Benedict's and Fehling's, guys it doesn't matter they're just slightly different complexes of copper two-plus but as long as you see some kind of copper two-plus it's going to be Benedict's or Fehling's, okay? Now, that's a new single dis one is that it actually starts off blue and turns red at the end. So, that's kind of cool transformation that would, you would know for sure that a reducing sugar was present, okay? Now, slight disclaimer guys, not every professor, not every textbook, not every homework will require you to know all these three, some professors just care about Tollens' some professors just care about Benedict's but I just decided to include them all in this lesson because it's like why not, they're all so easy they're all so similar it's like the more you know. Now, you know that there are three different tests that all do very similar things you should just know the difference in their visual indication, okay? So, one last thing I want to point out before you want to kind of the bottom of the page, which is that one of the biggest misconceptions of reducing sugars is that students hear the term reducing sugar and they think that that means that the sugar is going to get reduced, right? But what are we saying? we're actually not saying that at all, we're saying it as a reducing sugar is a sugar that can be oxidized, it's a sugar that has an aldose present like this one that can be oxidized. So, it's important for you guys to know that that's a very easy trick that a lot of people mess up on, a lot of people don't understand it but are reducing sugar is actually a sugar that can eventually be oxidized, okay? And then I guess one last disclaimer, which is that if you're paying really close attention you might notice that ketones usually can't be oxidized very well. So, you might be thinking Johnny why would a ketose react in an oxidation reaction? Guys because usually these reactions are in a base solvent or a base catalyst. So, in base we actually know that ketosis can tautomerize aldoses so what would end up happening is that the ketose can react, it just has to tautomerize into the aldose first and then it can react to this reaction. So, basically, that's why we say that a reducing sugar is any sugar that's people are forming an aldose or a ketose because if it's a ketose it can always just tautomerize back to the aldose and then react, okay? Guys, so we're done with this part. Now, just to really briefly recap Tollens' Benedict's and Fehling's tests do the same thing as weak oxidation except that they provide visual cues and now we're going to do in the next video is I'm going to go way deeper into the definition of reducing sugars because my definition right now is any sugar capable of forming a straight chain aldose or ketose, but there are actually a lot of different situations and a lot of tricky situations that you need to be aware of to know if it can form an aldose or a ketose. So, in the next video, I'm going to show you guys the definition of reducing sugars and how to predict if a sugar will be reducing sugar or not, let's move on to the next video.

Concept: Concept 2

3m
Video Transcript

So, what is a reducing sugar. Alright, guys. So, I made it sound really complicated in my last video, it's not, there are just specific rules you need to make sure that you can be aware, the first one is easy a reducing sugar is any straight chain monosaccharides. So, literally it doesn't matter, if it is an aldose or a ketose, if you see it in straight chain it for sure is a reducing sugar, okay? So, that takes through a lot of your examples but it could also be any cyclic monosaccharide, it's a ring, disaccharide or sugar derivative that has at least one cyclic hemiacetal group present, okay? So, hemiacetal, you guys should know that but here's a reminder, a cyclic acetal would have OR groups on both sides. So, you have OR ,OR. Remember, that it's cyclic hemiacetal would have an OR on one side but then a free alcohol on the other and OH on the other side, this is basically present on a lot of different monosaccharides sugars and if you have a hemiacetal present that is a reducing sugar, okay? Now, you might be saying why? what's the big difference? why does it matter? because guys, hemiacetal groups can be hydrolyzed to straight chain saccharides whereas acetals cannot once you're in the acetal, this is actually what we call a glycoside. Remember, glycosides guys are like a it's specifically like an o-glycoside. Remember, that you put the carbon on there and then it's pretty much locked in place unless you react it with a lot of acid and hydrolyze it off but a cyclic hemiacetal is constantly hydrolyzing back to the straight chain whereas an o-glycoside is not. So, that's why cyclic hemiacetal will be a reducing sugar because we you'd expect that in solution is going to slowly hydrolyze back into the straight chain structure, does that make sense? whereas the other one has an o-glycoside it's lost into place unless, we specifically want to hide hydrolyze it with acid, okay? So, guys, let's go ahead and do the next question as an example.

Example: Example: Identify the following sugars

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Problem: Identify the following sugar as reducing sugars (RS) or non-reducing sugars (NS)

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Problem: Identify the following sugar as reducing sugar (RS) or non-reducing sugar (NS)

2m

Problem: Identify the following sugar as reducing sugar (RS) or non-reducing sugar (NS)

3m

Problem: Identify the following sugar as reducing sugar (RS) or non-reducing sugar (NS)

3m