Understanding Fischer Projections and Stereochemistry

What we have now is to take and look at molecules in a different way. We've looked at zigzag structures, we've looked at Newman, and now we have what's called a Fischer projection. And Fischer projections will take a molecule right here, the zigzag, and convert it into this right here. This is a Fischer projection. And the way I keep Fischer projections and Newman projections separate and distinct is a Fischer projection does look like fish bones. That's what I'm visualizing. And Fischer projections are typically used when you're looking at sugars. So this molecule right here is a sugar. And Dr. Fischer here just wanted a quicker and easier way to look at sugars this way. So what we want to learn how to do is go from a zigzag to a fissure and then a fissure to a zigzag. Now in order to do that we need to take back up a little bit and understand what a fissure projection is. So if I have a molecule that looks like this, ethyl group, a hydrogen and a CH3 group. we can see that it's also that central carbon right there it is a carbon okay so that's the fischer projection what the fischer projection is telling us okay is that we have that central carbon and the horizontal lines okay are representing wedges h o h and the Vertical lines are representing dashes. That bit of information is super, super important. You have to remember these horizontal lines are wedges. Now, why is that so important to understand? It's because look at this molecule here. What? is the stereochemistry. Sorry, what is the configuration? Is it an R or an S? Well, when we look at it, we can prioritize things. That's going to be priority one, two, and three, and then priority four, right? So what would the, what... is the configuration. Well, we go one, two, so we're going clockwise. One, two, three. So that would be an R because we're going clockwise, right? Whoa, whoa, whoa. Four is the lowest priority and it is what? A wedge. What have we always said? The lowest priority has to be facing in the back. It has to be a dash. And so what's the trick? when we see the lowest priority is a wedge, we just take the opposite of what we calculated. So one, two, three, clockwise. So that should be an R, but our lowest priority is a wedge. So we have to take the opposite. So that is a S. But you can do that in the Fisher projection. One, two, three, four. You can still do it. But you have to realize what the Fischer projection is. Those are wedges. So one, two, three. So R, but that's a wedge. So we have to take the opposite. So that would be a S. That is the important thing. One of the important things to remember about Fischer projections is that's what it's showing us. So what I recommend you to do is you could take this molecule here and draw it. I want you to go and make a model of it just like this. And when you make that model you'll be able to see that Fischer projections can look like this. You can actually take a molecule like this, and you can grab it and spin it in such a way that you can make it look like this. So you can take that molecule and orient it in such a way to make it look like this, that you have two groups pointing out at you and then two groups pointing away from you. So you can take a very simple molecule like methane and you can position it just like that. Highly encourage you to get a model system and do that. So now what we're going to do is now take these simple principles and apply it to a sugar which is a little bit more complicated because it has what? It has six carbons there and it's going to have How many stereocenters? When we take a look at this molecule here, we see one, two, three, four. So we have four stereocenters. So if we number this zigzag, one, two, three, four, five, six, whenever we have a zigzag, you're going to find the aldehyde or the carboxylic acid or a ketone. And those are typically found on either end. It doesn't matter which end. It's just you need to find the end where that aldehyde or carboxylic acid, or just let's say carbonyl compound, carbonyl functional group. You're going to find that. And you're going to just number, hey, there's six carbons. And you're going to place that carbonyl at the very top of your Fisher projection. So that's going to be one, two, three. four, five, and six. Now, this is very, very important because carbons two, three, four, and five, they are what? Stereocenters. And if you take this OH and this hydrogen and swap it, you have now made a different sugar. You cannot swap these. They're not interchangeable. That sugar is defined by the stereochemistry. And so when you come over to the Fisher projection, the stereochemistry here has to match, has to be perfectly matched. So how do you go from the zigzag and looking at carbon two, how do I know the OH is on my left and not the right? And then vice versa, when you look at carbon four, how do you know? So now that's what we're going to do next is I'm going to show you the tricks that I use to figure out the stereo chemistry on the Fischer projection. The first thing that you're going to do is I'm going to take this molecule and just redraw it by rotating it 90 degrees. So just to save time, I'm going to pause the video, but rotate this 90 degrees and redraw it. So I've taken this molecule and I've just rotated it 90 degrees. And you see that if we number the carbons again, the stereochemistry has not changed. You can see in carbon 2, it's a wedged hydroxyl, wedged hydroxyl. Everything has to match. If I made a mistake, then you call me out on it. But double checking, carbon 3 is dashed OH. Carbon 4 is dashed OH. Carbon 5 is DAS-OH. Okay, so everything matches. Now, how you do this, how do you figure out the answer here? Well, here's how you do it. Remember that these horizontal lines means that they're wedges. They're sticking out at us. And in order to see that, we have to look at this vertical molecule here. and approach it from the carbon that is pointed. So do you see how carbon 2 is pointing this way? So what we have to do is approach that point. So I've got to move my box here. So we need to approach it from this side. So I'm going to see my pointer finger. I'm basically going to take my eyeball and look at that carbon, carbon 2. My eyeball is right here. I'm looking at that point. Now, when I'm looking at that point, I'm now going to use my handlebar analogy. And I see that the OH is a wedge. So that's sticking out of the board. So that's going to be my left hand. And then the hydrogen is a dash. So that's in the board. So that's in my right hand. I'm going to grab them like handlebars. And then I'm going to ride my bike over and then turn and... Paste it right onto the board. You see how the hydrogen was in my right hand and the OH was in my left. They match. Now if we do that same approach now. with carbon 3. Now it would be a mistake to take my eyeball and look at carbon 3 this way. Why? Because we're trying to find the point on that apex on that carbon. So that is not the approach that we have to take. We have to approach our eyeball from this direction. Now looking at carbon 3, I see that the OH is in the board, so in my left hand, and then the hydrogen is out of the board in my right hand. and then I take it, go to carbon three, boom, hydrogen in my right, OH in my left. And then it's just do it all over again. Let's take a look at carbon four. So we have a wedged hydrogen, so that's over here in my left hand. The dashed OH is in the bore, so that's in my right hand. Take it, drive it over, boom, OH, hydrogen. And you just keep doing that, all right? So that's how you go from zigzag to fissure. Now, I also want you to be able to go from the fissure to the zigzag. And so what you're going to do is just the exact opposite. The exact opposite of what I've just showed you. And so we'll get practice. We'll get some practice problems to look at that. What's another thing that we can do with Fisher projections here? We can look at these, look at those stereo centers and figure out if they're RRS, figure out the configuration. Now we can use this as a reference here. Let's figure out the stereo center here at carbon 2. So we have to prioritize things. So that's going to be, let's do a different color here. What's the configuration at carbon 2? so that's one i'm prioritizing in pink so that would be three and four okay so we're prioritizing things so we're going one two three so we're going in the counterclockwise direction the lowest priority is a dash so that's perfect so one two three counterclockwise so that makes that a s so that means This carbon has to be an S. It has to be. If we prioritize, we can prioritize right on the Fisher projection. That's one, because we're looking at this carbon, that's two, and then all of this down here is three, and that would be priority four. Okay, now we look at it, we count it. One, two, three. We're going clockwise. One, two, three. So that's an R. No, hold on. What did we say about Fisher projections? Those are wedges. So we have to say that, okay, yes, this is the numbering scheme, one, two, three, clockwise, but we have to remember that the H and OH are wedges, so that makes the lowest priority a wedge. So we just have to take the opposite. So that would be an S. Okay, so we keep doing that. And so you could do that for every single stereo center and you should be able to do that, okay? Let's see, what time do we have here? Let's see, we have a few moments. Well class, I think that's going to be a good stopping point. So let me just give you some words of advice here. Everything that we're learning in this chapter here is very visual. And it can be difficult for some people to see it in their head. And so what you can do is get your model kit. Draw, build these molecules. and make those connections to what the molecule looks like on a board or a piece of paper to what it actually looks like in three dimensions. And building those models takes time, but I promise you the time that you invest in building these models will pay out handsome rewards.

This is a Fischer projection. And the way I keep Fischer projections and Newman projections separate and distinct is a Fischer projection does look like fish bones. That's what I'm visualizing. And Fischer projections are typically used when you're looking at sugars. So this molecule right here is a sugar.

And Dr. Fischer here just wanted a quicker and easier way to look at sugars this way. So what we want to learn how to do is go from a zigzag to a fissure and then a fissure to a zigzag. Now in order to do that we need to take back up a little bit and understand what a fissure projection is.

So if I have a molecule that looks like this, ethyl group, a hydrogen and a CH3 group. we can see that it's also that central carbon right there it is a carbon okay so that's the fischer projection what the fischer projection is telling us okay is that we have that central carbon and the horizontal lines okay are representing wedges h o h and the Vertical lines are representing dashes. That bit of information is super, super important.

You have to remember these horizontal lines are wedges. Now, why is that so important to understand? It's because look at this molecule here. What? is the stereochemistry.

Sorry, what is the configuration? Is it an R or an S? Well, when we look at it, we can prioritize things.

That's going to be priority one, two, and three, and then priority four, right? So what would the, what... is the configuration.

Well, we go one, two, so we're going clockwise. One, two, three. So that would be an R because we're going clockwise, right?

Whoa, whoa, whoa. Four is the lowest priority and it is what? A wedge. What have we always said? The lowest priority has to be facing in the back.

It has to be a dash. And so what's the trick? when we see the lowest priority is a wedge, we just take the opposite of what we calculated. So one, two, three, clockwise.

So that should be an R, but our lowest priority is a wedge. So we have to take the opposite. So that is a S.

But you can do that in the Fisher projection. One, two, three, four. You can still do it. But you have to realize what the Fischer projection is. Those are wedges.

So one, two, three. So R, but that's a wedge. So we have to take the opposite.

So that would be a S. That is the important thing. One of the important things to remember about Fischer projections is that's what it's showing us.

So what I recommend you to do is you could take this molecule here and draw it. I want you to go and make a model of it just like this. And when you make that model you'll be able to see that Fischer projections can look like this.

You can actually take a molecule like this, and you can grab it and spin it in such a way that you can make it look like this. So you can take that molecule and orient it in such a way to make it look like this, that you have two groups pointing out at you and then two groups pointing away from you. So you can take a very simple molecule like methane and you can position it just like that. Highly encourage you to get a model system and do that.

So now what we're going to do is now take these simple principles and apply it to a sugar which is a little bit more complicated because it has what? It has six carbons there and it's going to have How many stereocenters? When we take a look at this molecule here, we see one, two, three, four.

So we have four stereocenters. So if we number this zigzag, one, two, three, four, five, six, whenever we have a zigzag, you're going to find the aldehyde or the carboxylic acid or a ketone. And those are typically found on either end.

It doesn't matter which end. It's just you need to find the end where that aldehyde or carboxylic acid, or just let's say carbonyl compound, carbonyl functional group. You're going to find that.

And you're going to just number, hey, there's six carbons. And you're going to place that carbonyl at the very top of your Fisher projection. So that's going to be one, two, three.

four, five, and six. Now, this is very, very important because carbons two, three, four, and five, they are what? Stereocenters. And if you take this OH and this hydrogen and swap it, you have now made a different sugar. You cannot swap these.

They're not interchangeable. That sugar is defined by the stereochemistry. And so when you come over to the Fisher projection, the stereochemistry here has to match, has to be perfectly matched. So how do you go from the zigzag and looking at carbon two, how do I know the OH is on my left and not the right?

And then vice versa, when you look at carbon four, how do you know? So now that's what we're going to do next is I'm going to show you the tricks that I use to figure out the stereo chemistry on the Fischer projection. The first thing that you're going to do is I'm going to take this molecule and just redraw it by rotating it 90 degrees.

So just to save time, I'm going to pause the video, but rotate this 90 degrees and redraw it. So I've taken this molecule and I've just rotated it 90 degrees. And you see that if we number the carbons again, the stereochemistry has not changed. You can see in carbon 2, it's a wedged hydroxyl, wedged hydroxyl. Everything has to match.

If I made a mistake, then you call me out on it. But double checking, carbon 3 is dashed OH. Carbon 4 is dashed OH. Carbon 5 is DAS-OH.

Okay, so everything matches. Now, how you do this, how do you figure out the answer here? Well, here's how you do it. Remember that these horizontal lines means that they're wedges. They're sticking out at us.

And in order to see that, we have to look at this vertical molecule here. and approach it from the carbon that is pointed. So do you see how carbon 2 is pointing this way? So what we have to do is approach that point.

So I've got to move my box here. So we need to approach it from this side. So I'm going to see my pointer finger. I'm basically going to take my eyeball and look at that carbon, carbon 2. My eyeball is right here. I'm looking at that point.

Now, when I'm looking at that point, I'm now going to use my handlebar analogy. And I see that the OH is a wedge. So that's sticking out of the board. So that's going to be my left hand.

And then the hydrogen is a dash. So that's in the board. So that's in my right hand.

I'm going to grab them like handlebars. And then I'm going to ride my bike over and then turn and... Paste it right onto the board. You see how the hydrogen was in my right hand and the OH was in my left.

They match. Now if we do that same approach now. with carbon 3. Now it would be a mistake to take my eyeball and look at carbon 3 this way. Why?

Because we're trying to find the point on that apex on that carbon. So that is not the approach that we have to take. We have to approach our eyeball from this direction.

Now looking at carbon 3, I see that the OH is in the board, so in my left hand, and then the hydrogen is out of the board in my right hand. and then I take it, go to carbon three, boom, hydrogen in my right, OH in my left. And then it's just do it all over again. Let's take a look at carbon four. So we have a wedged hydrogen, so that's over here in my left hand.

The dashed OH is in the bore, so that's in my right hand. Take it, drive it over, boom, OH, hydrogen. And you just keep doing that, all right?

So that's how you go from zigzag to fissure. Now, I also want you to be able to go from the fissure to the zigzag. And so what you're going to do is just the exact opposite. The exact opposite of what I've just showed you. And so we'll get practice.

We'll get some practice problems to look at that. What's another thing that we can do with Fisher projections here? We can look at these, look at those stereo centers and figure out if they're RRS, figure out the configuration.

Now we can use this as a reference here. Let's figure out the stereo center here at carbon 2. So we have to prioritize things. So that's going to be, let's do a different color here.

What's the configuration at carbon 2? so that's one i'm prioritizing in pink so that would be three and four okay so we're prioritizing things so we're going one two three so we're going in the counterclockwise direction the lowest priority is a dash so that's perfect so one two three counterclockwise so that makes that a s so that means This carbon has to be an S. It has to be.

If we prioritize, we can prioritize right on the Fisher projection. That's one, because we're looking at this carbon, that's two, and then all of this down here is three, and that would be priority four. Okay, now we look at it, we count it.

One, two, three. We're going clockwise. One, two, three. So that's an R.

No, hold on. What did we say about Fisher projections? Those are wedges. So we have to say that, okay, yes, this is the numbering scheme, one, two, three, clockwise, but we have to remember that the H and OH are wedges, so that makes the lowest priority a wedge.

So we just have to take the opposite. So that would be an S. Okay, so we keep doing that.

And so you could do that for every single stereo center and you should be able to do that, okay? Let's see, what time do we have here? Let's see, we have a few moments.

Well class, I think that's going to be a good stopping point. So let me just give you some words of advice here. Everything that we're learning in this chapter here is very visual. And it can be difficult for some people to see it in their head. And so what you can do is get your model kit.

Draw, build these molecules. and make those connections to what the molecule looks like on a board or a piece of paper to what it actually looks like in three dimensions. And building those models takes time, but I promise you the time that you invest in building these models will pay out handsome rewards.

Transcript for:Understanding Fischer Projections and Stereochemistry

Transcript for:
Understanding Fischer Projections and Stereochemistry