Let's talk about how to properly draw monosaccharides using a representation that was designed specifically for sugars, and that representation is called the Fischer Projection. So guys, in 1891, Emil Fischer, who's one of the most prolific organic chemists of the 19th century, essentially the guy that discovered carbohydrates and carbohydrate chemistry, devised a representation called Fischer Projections specifically for the purpose of depicting carbohydrates. Even though you've learned about Fischer projections prior to this topic, this is really the reason he designed them. It was so that he could better understand and better illustrate the chirality of sugars, okay. Now it's going to be really important because of this, it's going to be very important that we understand how to go from bond line to Fischer with sugars and also how to go from Fischer to Bond line.
Okay, now one thing that you need to know right away is that the way that you're supposed to align your monosaccharide, your Fischer projection, is with the most oxidized carbon atom on the top of your Fischer Projection. So here is what you see; I have this monosaccharide that's represented on the left as just a bond line. And it's much more difficult to deal with the monosaccharide in that form, it's much more difficult to illustrate the chirality to compare it to another one. So Emil Fischer said, why don't I draw them like this, that's going to make them so much easier compared to each other because you just see all the OHs, you can compare OHs much easier this way. And what you see is that this top carbon would be considered more oxidized because it has 2 bonds to O and this bottom one would be considered less oxidized because it only has one bond to O. Okay, so you always put the most oxidized part on the top. Okay, so this is D-ribose, we've talked about ribose already before, but now you know that that's actually what the bond line looks like. Okay? So, bond line to Fischer. This is the first essential tool that you need. Okay?
So similar to our lessons in Orgo 1, remember that whenever we wanted to turn a bond line into a Fischer, we used this Johnny patented method called the caterpillar method, right? And what the caterpillar method taught us is that what we want to do is we want to rotate all the bonds so that you have essentially like the back of the caterpillar. So where does the caterpillar come from? I think of this as being a caterpillar and this is like its face, it's just like happy chewing on a leaf or something, and then those things at the top are like the hairs on the back of the caterpillar. So you can go back and learn about the caterpillar method there, but I'm just going to remind you guys that what the caterpillar method says is that every substituent that's already facing up stays the same.
So what we notice is that carbon 2 stays exactly the same, the OH is faced toward the back here and it's still faced towards the back. But, in order to make the other substituents face up, in order to rotate them to the up position, you also need to rotate the wedge and dash information. So that means that, to rotate this OH to face up, I need to face it towards the back on carbon 1, and on carbon 3 I need to face it towards the back as well because it rotated. Then once you have it lined up that way, you can easily draw your Fischer projection because then all you need to do is look at it, think okay in the front what do I see? I see HHH, so that would be this one, this one, this one, and that would be on this side. And then on the back what do I see? OH, OH, OH and that would be back here. Okay? So that is the most surefire way to get it right.
Now, alternatively, once you get good at this method, what you'll notice is that all you really have to do as a shortcut is swap the stereochemistry of all downward facing alcohols. Okay? So what that means is that if I wanted to kind of take a shortcut and bypass the caterpillar and go straight to the Fischer, how could we do that? Let's see. Can we actually do this? Well, what you'd say is, here's my eyeball, right? I'm looking at it this way. Okay. And what I see is that on the carbon 2, I have an OH faced away from me. This one right here, I'm going to make it yellow. So I would already put it faced away from me on this side, as if my eyeball was looking this way. Okay? So let's say well it's faced away, those 2 red eyeballs are in the same position just with different representations, so it's faced away. Now on the red eyeball on the left, it looks like those OH's are faced towards me. So it looks like I should put them on this side of the Fischer projection, but no they're faced downwards. So if it's down, you have to flip it. So those 2 OH's on carbon 1 and 3, they look like they're going to face this way, but I actually because they're faced downwards I have to swap them to this side, meaning that this one here and this one here even though they're both towards me, towards the eyeball, I have to draw them away from me because they swapped. Okay, so that's just a little shortcut. The downward facing positions you would swap.
Now let's go from Fischer to Bond line. So from Fischer to Bond line, the most again surefire way, the most reliable way to convert it would be to use a reverse caterpillar. So that would say hey, we've got this molecule, this is what it looks like from my eyeball here, what would the caterpillar look like? Well, the caterpillar would be a straight back with positions 1, 2, 3. And everything that's closest to the eyeballs should be on a wedge. So that would be this group here, this group here, should be here and here. Right? And everything that's far from the eyeball should be on a dash, so that should be this one here. Okay? So that is the reverse caterpillar, I just made a caterpillar. Now how do I turn this into a bond line? Well it turns out that when you do that, you're going to get 2 possible answers. They're both the same molecule, but they're just rotated differently because it just depends if I'm starting off with a zigzag that starts going up or a zigzag that starts going down. You're going to get 2 different possibilities. Okay, so let's start off with the one that is going up, okay. So the one that's going up, what I would do is I would just draw this aldehyde exactly the same. And I would say that this is position 1, 2, 3, and then this is your CH2OH is this guy right here. Okay. Similarly, over here my aldehyde would now face up. So this would be position 1, 2, 3 and then this is my CH2OH where it's just face down now. Okay? Now how would we actually draw in the OHs? Well notice you would only need to flip the ones that are face down now. So in the first one, which positions can stay exactly the same? Which positions are still facing up? 1 and 3. So what I could do is I could draw 1 as a wedged OH, and I could draw 3 as a dashed OH. Why? Because those are the same exact positions that they were in on the caterpillar. By the way, I'm not going to draw H's because they can be implied. Right? Now on 2, 2 is facing downwards so now I need to flip it. Meaning that 2 should actually have a dash OH, and now I'm done with that molecule, it's done. Cool? Awesome. Now let's see how it would differ with the second one. So with the second one, which position stayed the same, in terms of they're still facing up? Now it's 2. So 2 I can face the same direction. It should be an OH that's wedged. Now 1 and 3 because of the way I drew my zigzag, now 1 and 3 have to be changed. So now 1 would be a dash OH, and 3 would be a wedged OH. Okay? So guys, even though these two molecules look very different, they are the same exact molecule and both of these answers would be correct answers on a test. Your professor would give you full credit for both of them. The reason I drew both of them is that if I drew the first one and you drew the second one, I'm going to get so many questions of Johnny is mine the same as yours? So I want to make sure to draw it both ways just so that you know, you're always going to get 2 different looking answers, but both of them are the same if you just use the correct rules. Alright? So now we know how to draw Fischer projections and how to also kind of undraw them and go back to the bond line, let's go ahead and do some practice related to Fischer projections.
