Organic Chemistry

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24. Carbohydrates

Monosaccharides - D and L Isomerism

All monosaccharides come in dextrorotary (D) and levorotary (L) forms. You may have heard these terms before when learning about optical activity. Another way to descrbise these D and L forms are enantiomers of each other.



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Now that we know some of the basics about mono sacha rides, I want to focus on a new property, which is their Chire ality, also known as their absolute configuration. Now it turns out that some parts of monos Ackroyd, Chire ality are actually much easier than Kyra ality of other molecules. But then other elements of their Chire ality are really confusing and honestly, a little bit messed up. So what I'm going to do in this section is I'm going to clear both of those sides of Cairo ality for you. Okay, so let's go ahead and get started. So mono sack rides can come in one of two forms either in the Dextre Rotary Form also knows the deform or the level rotary form L form. And if you guys remember, these terms are borrowed from your Chire ality section of your textbook where we learned about optical activity and we learned that Dexter rotary molecules rotate like clockwise and level road. Rotary molecules rotate like counter clockwise in a polar emitter. Remember that another property about optically active molecules is that opposite rotations were always in the answer mercy of each other. So the positive and the negative rotations were always going to be perfect and the answers of each other. And that's a parallel that holds true for this section. The D and anti armor or the D sugar is always gonna be in an anti more of the L sugar. So this is true. Unfortunately, this is where the parallels from optical activity end for sugars because it turns out that when sugars were discovered, it was based on a lot of guesswork by email. Fisher, back in the 18 hundreds, he didn't have ah, poll originator Thio. He didn't have all the mono sack rides. Um, a lot of this was simply guesswork. So it turns out that he made these rules of D. N L. Before he actually knew that there were a lot of exceptions to that rule. So it turns out that d doesn't specifically mean positive rotation anymore. L doesn't mean negative rotation. So MP saying, Well, Johnny, then what does it mean? Well, I mean, I'm about to explain that to you, but just so you know, there is no correlation anymore between d being a positive rotation and l being a negative rotation, you should just think of them as two categories of sugars. Okay, now the way you determine d or the way you determine L on a mono Sakhr tribe is decided by the pen Ultimate carbon. Such a fancy word. What does that mean? Penultimate is just a word that means second toe last. Okay, so the penultimate carbon is simply going to be the last Cairo carbon of your molecule, and that's going to determine if it's a D or if it's in hell. Now note to self. This penultimate carbon is really important. It's the C five carbon usually, but it depends on the length of the It depends on the length of the mono sack, right in a six carbon sugar, it's gonna be the C five, but it's very important. It's gonna be used as what we call a stereo descriptor. Hopefully spell this right stereo descriptor later in this chapter. So these two words are going to be synonyms of each other. The penultimate carbon and the stereo descriptor carbon are synonyms of each other, so just keep that in mind. Okay? I'm gonna use both of those words interchangeably. So how do we know if it's a deer. And l? Well, thankfully, this is one of the really easy parts of Monos Ackroyd Chire ality. All it is is this The D configuration is gonna be any sugar that has the penultimate or stereo descriptor. Ohh. Pointed to the right. Okay, so let's look at glucose here. Glucose, which ohh is the stereo descriptor. It's gonna be the last Cairo carbon. So it's gonna be this one right here. That is the last Carol Carbon. It's the one that's furthest down again. It happens to be C five here. We're gonna learn how to number carbon soon. Our number sugar soon, but in this case, this would be 12345 Okay, so that's my penultimate carbon or my stare descriptor. And what we know is that if it faces to the right, that's gonna be a d glucose. So we're gonna just put that right there. This is D Glucose. And then if it faces to the left, that's gonna be an l glucose. And that makes it so easy, guys. Because you could just remember that l is left. Okay. D is right. Ellis left. You have no excuse to mess that up because the Els it's a double l. Okay, now, guys, it turns out that usually that means that it's going to be the d is going to be in our configuration on that last sugar or on that last, um, Carol center. And usually l is gonna be an s at that position. Okay, but there are exceptions to that as well. So just you know, this works for most sugars, but for other molecules bio molecules like amino acids, it's not gonna work anymore. So for right now, you can use that rule. But the rule that I really want you to remember is just right and left. That always holds true. OK, so let me just kind of prove to you that the r and s works here, but then just don't worry about it for other types of molecules. So, for example, here's this Carl center And what I would see is that let me just erase these numbers so that we can pick our priorities. So our priorities would be one the carbon. I mean, the O is number one. Then this is must be number two. This must be number three, and this must be priority number four. So that means that my rotation goes from 1 to 2 from 2 to 3. I ignore for it Looks like it's an s. But since my four is on the horizontal, that means it's going to be an r. Okay, so remember, guys, what I'm doing is I'm using the rules of determining Chire ality on Fisher projections to determine that are so. What I'm doing here is I'm showing you guys that yes, the d happens to be in our but it's not always gonna work for other types of biomolecules. Okay, with one thing that you should always remember is that to the right is D and to the left is l. And that's always gonna hold true. Cool. Alright, guys. So we're done with this concept. Let's move on to a practice problem.

Provide the generic name, including stereochemistry, for the following monosaccharide: