Fischer Projection - Online Tutor, Practice Problems & Exam Prep

Fischer projections are essential for representing sugars and carbohydrates, where vertical bonds indicate atoms going into the page and horizontal bonds come out. To convert a Fischer projection to a bond line structure, first use wedge and dash notation, then visualize the structure as a "caterpillar" to determine atom orientation. Finally, rotate every other bond to achieve a zigzag pattern typical of bond line structures. This method is crucial for analyzing stereochemistry and understanding molecular configurations in organic chemistry.

Introduction to different projections.

So like I said, like Newman projections, there's actually a lot of different types of projections. As you can see, this one right here is called a Fischer Projection. It's used mostly for sugars. So later on, if we get into sugar chemistry and carbohydrates, we'll be using Fischer Projections a lot. But they're also used in this chapter as well. Two other common ways that are used are the Haworth projection. The Haworth projection is just that 3D projection of a ring. That's the actual name, and it's usually used for rings. So it's used to highlight what's at the top and what's at the bottom. And then finally, we have this one called a Sawhorse Projection, and this one is usually used for stereochemistry to basically say how these atoms are related to each other in terms of their orientation, their shape, their configuration. Well, in all of these cases, whichever projection we're using, also remember there's Newman as well, in all of these cases, we're going to have to convert them into bond line before analyzing them completely. What that means is that these projections are really good for analyzing certain types of things. But if we want to compare them against other normal molecules, we're going to have to convert them into bond line first because that's really like our metric system. That's our standardization.

How to convert Fischer projections into bondline structures

So basically, you can see I have made a caterpillar. What am I talking about? Well, really, a Fischer projection, even though it looks 2D, it's not really 2D. The way it's really supposed to be interpreted is that every vertical bond is going into the page on a wedge. Okay? So these would be vertical and vertical. And then every horizontal bond coming off of it is a wedge. I'm sorry, I might have said wedge. Those are dashes and these are supposed to be wedges. So even though it looks 2D, it's really not 2D. That's just the way it's drawn, to make it easier. But really the way it's supposed to be interpreted is with wedges and dashes. What I asked you guys to do is, okay, if we're going to convert this into a bond line, we need to use this wedge and dash notation first. So, if you're given a bond line, first do what I just did and make it wedge and dash, like I just did. Then we're going to use an eyeball and we're going to pretend like we're looking at it from the side. And what we're going to see is that it's going to make what I call a caterpillar. Basically, what that means is that this CHO here would be right here, CHO. This CH₂OH would be here, CH₂OH, and what we would notice is that we have 3 different junctions, 1, 2, 3, and these are the places where bonds come off the top. And if you think about it, maybe draw a line down here, this actually kind of looks like a caterpillar. Right? Now this just got really goofy, but it kind of looks like a caterpillar with it's like it's like on a leaf and it's like eating away and it's like munching away, and it's got like its little hair sticking up. That's why I call it a caterpillar. Alright? So basically, I don't want you guys to necessarily draw the whole caterpillar. You don't need to draw a face or anything. I'm not going to be strict about that. But what you should do is realize that this bond here, 1, relates to one right here. So what that means is then I look at the eyeball and I say okay, according to that eyeball, what should be in the front and what should be in the back? What does it seem closest to itself? And what it's seeing is that there should be an H in the front, because that's the closest one to the eyeball, and there should be an OH in the back, because that's the furthest one from the eyeball on the one carbon. Is that making sense? So I'm looking at the one carbon and saying what's in the front, what's in the back. In the same way I would work with the other ones. Then I'd say 2 should have an H in the front and an OH in the back, and then 3 should have an OH in the front and an H in the back. Once I have my caterpillar, then I have to do my last step and that's going to give me my bond line. So we're actually really close to the bond line. The thing is that bond line structures, are they ever like that where all the bonds are in a straight line? Usually not. Usually there's a zigzag pattern. Right? So we need to restore this back to a zigzag pattern. How do we do that? By rotating every other bond. And another thing I like to say is that by rotating every even bond. So what we're going to do here is I'm going to show you guys how to do this. Basically, what we would do is we have 1, 2, and what we would do is we would rotate every other bond. So we would rotate 2 as my atom. I'm going to want 2 to face down. And if we rotate 2 to face down, that's going to restore my zigzag. So now what I'm going to do is I'm going to draw this like this, where I have 1 is here, 2 is here, 3 is here, and then it goes down like this. So then the CHO is in the same exact place, The CH₂OH is in the same exact place. In fact, 1 and 3 are in the same exact place. Notice that they were both pointing up before. Before, 1 and 3 are both pointing up. So that means that the groups that are on 1 and 3 should look exactly the same. That means that 1 should have an OH at the back and it means that 3 should have an OH at the front. Do I have to draw the H's? No, because this is bond line. Remember in bond line, H's are omitted. The only thing that's changed is that now I'm rotating 2 down. That means that whatever I had on 2 has to flip. Where should the OH go? Should it go on the back, the front? Since it's rotating, the OH should now go on the front. Because of the fact that it rotated down, that means that that bond that was in the back is now going to rotate to the front. So now what I've just done is I've just made my bond line. That is a bond line structure right there. And all I did was I rotated every other bond, so meaning that I rotated this one. This one doesn't get rotated. See, like this one is fine. That one doesn't get rotated, But then this one got rotated and then this one didn't. So notice that every other one I rotate and if this is a longer chain, then I would have also rotated atom 4 to go down. So I would have rotated atom 2, atom 4, atom 6, until my Fischer projection is done. And that would make the zigzag pattern. What I want you guys to do is just as a free response, convert the following Fischer projection into a bond line structure, go ahead and try to solve it yourself, and then I'll go ahead and step in and show you guys how.

Convert the following Fischer projection into bondline structure.

Alright, guys. Let's go ahead and go step by step. The first step would be to redraw this with the wedge and the dash. So I would put this on a dash. I'll put this on a dash. I would put these guys on wedges. Br, Br, H, H. Is that cool so far? Now what I want to do is I'm going to draw my eyeball because that's going to help me remember what things look like. Okay? Notice that I have carbon 1 here, carbon 2 here, these are the back of the caterpillar. So now when I convert this into a caterpillar, what it's going to look like is like this, where I have atom 1, atom 2, that's the back of the caterpillar. What it's going to have here is COOH. What it's going to have on over here is CH₂NH₂. Now I just have to figure out what's on the top, what's on the hairs. Okay? So it should actually be really simple. I should just have for my wedges, I should have H and H. Okay? Because those are the ones that are closest to the eyeball. For the back, what I should have is BrBr. Is that making sense so far? Cool. Now, I think a question that some of you guys might have is that notice that before up here, I was drawing the H's on the right side and the front, the wedge on the right and the dash on the left. And then here I was drawing the wedge on the left and the dash on the right. It does not matter. You can draw them however you want, as long as the thing that's in the front is still in the front, the things that's in the back is still in the back. Alright? So now we have our caterpillar. So now how do we convert this into a bond line? All I do is I rotate every other bond or what I want to make sure is every other atom is face down. Every even atom is face down. So that means that's going to be atom 2. Okay? So atom 2 is going to be the one that has to face down. Okay? So let's go ahead and convert this. What that means is that now this is going to turn into this, this and that, where this is now atom 1.

Here’s what students ask on this topic:

What is a Fischer projection and how is it used in organic chemistry?

A Fischer projection is a two-dimensional representation of a three-dimensional organic molecule, primarily used for sugars and carbohydrates. In this projection, vertical lines represent bonds going into the page (dashes), and horizontal lines represent bonds coming out of the page (wedges). This method simplifies the visualization of stereochemistry, making it easier to compare different molecules and their configurations. Fischer projections are particularly useful in carbohydrate chemistry to depict the orientation of hydroxyl groups and other substituents around chiral centers.

How do you convert a Fischer projection to a bond line structure?

To convert a Fischer projection to a bond line structure, follow these steps: First, redraw the Fischer projection using wedge and dash notation, where vertical bonds are dashes (into the page) and horizontal bonds are wedges (out of the page). Next, visualize the structure as a 'caterpillar' to determine the orientation of atoms. Finally, rotate every other bond to achieve the typical zigzag pattern of bond line structures. This method ensures that the stereochemistry is accurately represented in the bond line structure.

What is the significance of the 'caterpillar' method in converting Fischer projections?

The 'caterpillar' method is a visualization technique used to convert Fischer projections into bond line structures. By imagining the molecule as a caterpillar, you can determine the relative positions of atoms and groups. This method helps in understanding which atoms are in the front and which are in the back, making it easier to rotate every other bond to achieve the correct zigzag pattern in the bond line structure. This step is crucial for maintaining the correct stereochemistry of the molecule.

Why is it important to rotate every other bond when converting Fischer projections to bond line structures?

Rotating every other bond when converting Fischer projections to bond line structures is important to achieve the correct zigzag pattern typical of bond line structures. This rotation ensures that the stereochemistry of the molecule is accurately represented. Without this step, the resulting bond line structure may not correctly depict the spatial arrangement of atoms, leading to incorrect interpretations of the molecule's properties and reactivity.

What are the common types of projections used in organic chemistry besides Fischer projections?

Besides Fischer projections, other common types of projections used in organic chemistry include Newman projections, Haworth projections, and Sawhorse projections. Newman projections are used to visualize the conformation of molecules by looking down the axis of a bond. Haworth projections are used for cyclic structures, particularly sugars, to show the three-dimensional arrangement of atoms in a ring. Sawhorse projections are used to depict the stereochemistry of molecules, showing the spatial relationship between atoms. Each type of projection has its specific use and helps in understanding different aspects of molecular structure and stereochemistry.