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 to reactions that we've already learned, so that 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? What we already learned is that a reaction called bromine water, but it's not called bromine water, bromine water is the reagent and it's called weak oxidation and what it can do is it can provide high yields of altoic acid. So remember that we actually had the same exact molecules displayed when we were learning about weak oxidation and what we learned is that if you start off with an aldose and you react it with bromine in 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 has 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 test, and the Fehling's test. 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. The test 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 aldose, right? So what 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 look when you find this on the exam let's say. So let's start off with Tollens', which is the most common of the 3. So the Tollens' test has been around for a really long time and it 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 the letters of the reagents because there are actually like 5 other ways that that can be drawn. You might see it simply as just like silver oxide 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 Tollens' reagent. Okay? Now what does Tollens' do? It does the same thing as Bromine water but instead of just getting your altoic 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 is what we call a reducing sugar. Next, we have Benedict's and Fehling's test. Now Benedict's and Fehling's test 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 I oxide, Cu₂O and they form a brick red precipitate that settles to 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 II but as long as you see some kind of copper II it's going to be Benedict's or Fehling's. Okay? Now something that's interesting about this one is that it actually starts off blue and turns red at the end. So it's a 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 3. 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 3 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 we move on 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. What we're saying is 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 a 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 ketoses can tautomerize to aldoses. So what would wind up happening is that the ketose can react, it just has to tautomerize into the aldose first and then it can it can react through this reaction. So basically that's why we say that a reducing sugar is any sugar that's capable of 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 test do the same thing as weak oxidation except that they provide visual cues and now what we're going to do in the next video is I'm going to go dive 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 gonna show you guys the definition of reducing sugars and how to predict if a sugar will be a reducing sugar or not. Let's move on to the next video.