Fischer projection introduction | Stereochemistry | Organic chemistry | Khan Academy

Summary

The video script explains the use of Fischer projections to represent molecules, particularly for chirality centers. It details how to assign configurations to chiral centers using priority rules and visual tricks, emphasizing the importance of hydrogen's position. The instructor demonstrates two methods for determining R or S configurations and shows how to draw the enantiomer of a compound using a mirror image technique, highlighting the reversal of configurations at chiral centers.

Takeaways

• 🔬 Fischer projections are a method for representing molecules, particularly useful for depicting chirality centers.
• 🏅 Emil Fischer, the creator of Fischer projections, won the Nobel Prize in Chemistry for his work on carbohydrates.
• 🧬 A chirality center in a molecule is a carbon atom bonded to four different groups, which can lead to different spatial configurations.
• 🌐 In Fischer projections, horizontal lines represent bonds coming out of the page, which are depicted as wedges in three-dimensional drawings.
• 🔽 Vertical lines in Fischer projections indicate bonds going into the page, represented as dashes in three-dimensional drawings.
• 📉 Assigning configuration to a chirality center involves determining the spatial arrangement of the four attached groups, with hydrogen typically having the lowest priority.
• 🔄 The process of assigning R or S configuration to a chirality center involves visualizing the molecule from a specific perspective and following the clockwise or counterclockwise direction of the highest priority groups.
• 🔄 A second method for assigning configuration involves ignoring the hydrogen and determining the direction (clockwise or counterclockwise) of the remaining highest priority groups.
• 🔍 A model set can be a helpful tool for visualizing and practicing the assignment of configurations to chirality centers.
• 🪞 To draw the enantiomer of a compound, the mirror method can be used, reflecting the molecule across a hypothetical mirror plane to reverse the configuration at each chirality center.

Q & A

What is a Fischer projection?

A Fischer projection is a method of representing the structure of chiral molecules, particularly carbohydrates, in two dimensions. It was developed by Emil Fischer and is used to depict the spatial arrangement of atoms around a chirality center.

Why is the carbon atom in the Fischer projection considered a chirality center?

The carbon atom is considered a chirality center because it is bonded to four different groups, which gives it a tetrahedral geometry and the ability to exist in non-superimposable mirror-image configurations known as enantiomers.

What do the horizontal and vertical lines in a Fischer projection represent?

In a Fischer projection, a horizontal line represents a bond that is coming out of the page towards the viewer, while a vertical line represents a bond that is going away from the viewer into the page.

How is the spatial orientation of bonds represented in a Fischer projection?

The spatial orientation of bonds in a Fischer projection is represented by using wedges for bonds coming out of the page and dashes for bonds going into the page, with the carbon atom at the center of the projection.

What is the significance of assigning priorities to the groups attached to a chirality center?

Assigning priorities to the groups attached to a chirality center is crucial for determining the absolute configuration of the molecule. This is done by ranking the groups based on their atomic numbers and using the Cahn-Ingold-Prelog priority rules.

How does one determine the R or S configuration of a chirality center from a Fischer projection?

The R or S configuration is determined by visualizing the molecule from a perspective where the lowest priority group (usually hydrogen) is pointing away from the viewer. The direction of the remaining groups (from highest to lowest priority) is then followed to decide if the configuration is clockwise (R) or counterclockwise (S).

What is the trick mentioned in the script for determining the R or S configuration when the hydrogen is coming out at you in a Fischer projection?

The trick is to ignore the hydrogen and look at the other three groups. If the direction of the groups from highest to lowest priority appears counterclockwise, the actual configuration is R, and if it appears clockwise, it is S.

How can one draw the enantiomer of a compound using a Fischer projection?

To draw the enantiomer of a compound, one can use the mirror method. This involves reflecting the compound across a mirror plane, reversing the spatial orientation of all groups, and ensuring that the configuration at each chirality center is inverted.

Why is it important to use a model set when learning to assign configurations to chirality centers?

Using a model set helps in visualizing the three-dimensional structure of the molecule and understanding the spatial relationships between the groups attached to the chirality center. This hands-on approach can make it easier to grasp the concept and apply it to different molecules.

What is the difference between a wedge and a dash in a Fischer projection?

In a Fischer projection, a wedge represents a bond that is coming out of the page towards the viewer, while a dash represents a bond that is going into the page away from the viewer. These notations help to indicate the stereochemistry of the molecule.

Outlines

00:00

🔬 Fischer Projections and Chirality Center Configuration

The paragraph introduces Fischer projections, a method devised by Nobel laureate Emil Fischer for representing molecules, particularly carbohydrates. It focuses on a chirality center, a carbon atom with four distinct groups attached: hydrogen, hydroxyl (OH), aldehyde, and CH2OH. The instructor explains how to visualize these groups in three-dimensional space using a Fischer projection, where horizontal lines represent bonds coming out of the page (wedged) and vertical lines represent bonds going into the page (dashed). The paragraph also discusses methods for assigning the configuration (R or S) to the chirality center, emphasizing the importance of prioritizing the four groups attached to the carbon and visualizing the molecule from a perspective where the lowest priority group (hydrogen) points away from the viewer.

05:00

🧪 Assigning R or S Configuration and Drawing Enantiomers

This paragraph delves into the process of assigning the R or S configuration to a chirality center, using the priority rules for the four groups attached to the carbon. It explains how to determine the priority order by comparing the atomic numbers of the atoms directly bonded to the chiral center and those of the next atoms in the chain. The instructor demonstrates two methods for assigning the configuration: one involves visualizing the molecule from a specific perspective and the other uses a trick to quickly determine the configuration based on the appearance of the groups in the Fischer projection. The paragraph concludes with a discussion on how to draw the enantiomer of the compound using a mirror image technique, emphasizing the need to reverse the configuration at each chirality center and the importance of reflecting all groups accurately.

Keywords

💡Fischer projection

A Fischer projection is a method used to represent the three-dimensional structure of chiral molecules, particularly in the context of organic chemistry. It was developed by Emil Fischer and is particularly useful for depicting the spatial arrangement of atoms in carbohydrates. In the video, the instructor uses a Fischer projection to illustrate the chirality center of a molecule, showing how different groups are oriented in space relative to the central carbon atom. The script describes how to interpret the lines in the projection, with horizontal lines indicating bonds coming out of the page and vertical lines indicating bonds going into the page.

💡Chirality center

A chirality center, also known as a stereocenter, is an atom in a molecule that is bonded to four different groups. This creates a stereoisomerism, where the molecule can exist in different spatial arrangements that are not superimposable. The video script discusses a carbon atom as a chirality center, with four distinct groups attached to it: hydrogen, OH, aldehyde, and CH2OH. The instructor explains how to determine the configuration of this chirality center using different methods, emphasizing the importance of understanding the spatial arrangement of these groups.

💡Wedge and dash

In the context of molecular drawings, a wedge and a dash are conventions used to represent the three-dimensional orientation of bonds. A wedge indicates a bond that is coming out of the plane of the paper towards the viewer, while a dash indicates a bond that is going into the plane of the paper away from the viewer. The script uses these terms to explain how to interpret the Fischer projection and to draw the molecule in a more three-dimensional representation, such as when describing the OH and hydrogen groups coming out of the page.

💡Configuration

Configuration refers to the specific arrangement of atoms or groups around a chirality center. In the video, the instructor discusses how to assign a configuration to the chirality center using the Cahn-Ingold-Prelog priority rules. This involves determining the priority of the four groups attached to the carbon and then assigning the R (rectus) or S (sinister) configuration based on the observed direction of the groups when looking at the molecule from a specific perspective.

💡R and S configuration

R and S configurations are descriptors used in the Cahn-Ingold-Prelog priority rules to denote the absolute configuration of a chirality center. The script explains how to assign these configurations by considering the priority of the groups around the chirality center and the direction in which they are arranged when viewed from a perspective where the lowest priority group (hydrogen) is pointing away from the viewer. The instructor demonstrates this through the example of the molecule in the video.

💡Priority of groups

The priority of groups refers to the order of precedence assigned to the different atoms or groups attached to a chirality center based on their atomic numbers. In the video, the instructor explains how to determine the priority of the groups (OH, aldehyde, CH2OH, and hydrogen) by comparing the atomic numbers of the atoms directly bonded to the chirality center. This priority is crucial for assigning the R or S configuration to the chirality center.

💡Enantiomer

An enantiomer is one of two stereoisomers that are mirror images of each other but are not identical, much like left and right hands. The video script includes a demonstration of how to draw the enantiomer of the given molecule by using a mirror image technique. The instructor shows that reflecting the molecule across a mirror line will result in a molecule with the opposite configuration at the chirality center.

💡Stereoisomerism

Stereoisomerism is a type of isomerism where molecules have the same molecular formula and connectivity but differ in the three-dimensional arrangement of their atoms. The video focuses on enantiomers, a specific type of stereoisomerism where the molecules are non-superimposable mirror images. The instructor discusses this concept in the context of the chirality center and how different configurations can lead to different properties of the molecules.

💡Cahn-Ingold-Prelog priority rules

The Cahn-Ingold-Prelog priority rules are a set of rules used in chemistry to determine the absolute configuration of chiral centers. These rules assign a priority to each group attached to the chirality center based on the atomic numbers of the atoms. The script explains how to apply these rules to the molecule in the video, demonstrating how to assign priorities and subsequently determine the R or S configuration.

💡Mirror image

A mirror image is a reflection of an object across a mirror plane, resulting in an image that is reversed in spatial orientation. In the context of the video, the instructor uses the concept of a mirror image to demonstrate how to draw the enantiomer of a chiral molecule. By reflecting the molecule across a mirror plane, the instructor shows how to reverse the configuration at the chirality center, resulting in the enantiomer.

Highlights

Fischer projection is a method for representing molecules, particularly useful for carbohydrates.

Fischer won the Nobel Prize in Chemistry for his work, including the development of Fischer projections.

A chirality center is identified by four different groups attached to a carbon atom.

The groups around the chirality center include hydrogen, OH, aldehyde, and CH2OH.

In Fischer projections, horizontal lines represent bonds coming out of the page, indicated by wedges in 3D representations.

Vertical lines in Fischer projections signify bonds going into the page, depicted by dashes in 3D models.

Assigning a configuration to a chirality center involves determining the priority of the attached groups.

Hydrogen is assigned the lowest priority in the configuration of chirality centers.

A method to visualize chirality involves staring straight down at the carbon atom with the lowest priority group pointing away.

The priority of groups is determined by atomic numbers, with oxygen typically having the highest.

In case of a tie in group priority, the atoms bonded to the carbons are examined to break the tie.

The R or S configuration is assigned based on the clockwise or counterclockwise direction from the highest to lowest priority group.

A practical trick for Fischer projections is to ignore the hydrogen when determining the configuration.

The mirror method is an effective way to draw the enantiomer of a compound from its Fischer projection.

When reflecting in a mirror, the configuration at each chirality center is reversed.

A model set can be a helpful tool for visualizing and understanding chirality and enantiomers.

Transcripts

- [Instructor] On the left we have a Fischer projection,

which is just another way of representing a molecule.

And Fischer came up with these when he was working

with carbohydrates, and he actually won the Nobel Prize

for his chemistry.

At the center here at the intersection of these lines,

we have a carbon, and this carbon is a chirality center.

There are four different groups attached to this carbon.

There's a hydrogen, there's an OH, there's an aldehyde

and there's a CH2OH.

So in this picture, you can see I've drawn in

the carbon here.

And on the right is a picture of the actual molecules.

So this is our carbon, this is our chirality center.

We're actually staring straight down at it.

A horizontal line means a bond that's coming out

of the page.

So this line right here indicates a bond

that's coming out of the page.

So we would represent that with a wedge.

So this hydrogen is coming out at us in space.

And hopefully the picture, it's a little bit easier to see

this hydrogen is actually going up, it's coming out

at us in space.

Same thing with this horizontal line right here to the OH.

That means a bond that's coming out of the page.

So we represent that with a wedge.

The OH here is coming out at us in space.

A vertical line means a bond going away from us in space.

So this line right here means a bond to an aldehyde.

It's going away from us.

So that is represented by a dash.

And in the picture, this bond here is going away from us.

It's going into the page.

Same thing with this vertical line here to the CH2OH.

That means going away from us in space.

We draw a dash here.

The bond is going into the page.

So this line right here is showing up on,

going away from us in space.

If we wanted to assign a configuration

to our chirality center, there are several methods

that you can use.

I'll show you the two that I like to use.

The first way that I like to do it is to think about

the priority of these four groups.

And we know from earlier videos that hydrogen

is gonna have the lowest priority.

And we want the lowest priority group

pointing away from us in space.

And so the only way to do that would be to put our eye

right here and to stare at our chiral center this way.

And from that perspective, the hydrogen is going away

from us in space.

So let me go to a video where it's much easier to visualize

what's going on here.

So here we are staring down at our chiral center,

and you can see there's an OH coming out at us.

There's a hydrogen coming out at us,

there's an aldehyde going away from us in space,

and a CH2OH going away from us.

If we stare at our chiral center from this direction,

let me go ahead and rotate the molecule,

now we can see that there's an OH coming out at us in space

and a hydrogen going away from us,

and then we have our aldehyde going down and to the right,

and our CH2OH is going down and to the left.

So let's draw what we saw in the video.

So here's our picture, and we'll start with

our chiral center.

So right here, let's draw in our carbon.

And then our OH is coming out at us in space,

so we put that on a wedge.

So let me put an OH here.

The hydrogen is now going away from us in space.

So now we would represent that with a dash.

The aldehyde is going down into the right

with a bond that's in the plane of how we're

viewing it anyway, and let's put the carbon double bond

to an oxygen here.

And then we have our CH2OH going down and to the left.

So let's put in our, I'll go ahead and draw in

the carbon with two hydrogens and then our OH down here.

So let's assign priority to our four groups.

So here is our chiral center.

We look at the atoms directly bonded to our chiral center,

and that would be a hydrogen, an oxygen,

a carbon and a carbon.

We know that oxygen has the highest atomic number

out of those atoms, so the OH group gets a number one.

Hydrogen has the lowest atomic number,

so hydrogen gets the lowest priority,

and we say that's group four.

We have a tie between our two carbons

because carbon has the same atomic number.

So to break the tie, we need to look at what those carbons

are bonded to.

The carbon on the left is directly bonded to an oxygen

and two hydrogens.

So we write down here, oxygen, a hydrogen, a hydrogen.

So in order of decreasing atomic number.

This carbon on the right is double bonded to this oxygen,

and we saw in an earlier video how to handle that.

We treat that like a carbon bonded to two different oxygens,

even though it's not really, it has a double bond to one,

but this helps us when we are assigning priority;

and this carbon is also bonded to a hydrogen.

So this one.

So that would be oxygen, oxygen, hydrogen.

So we write oxygen, oxygen, hydrogen.

Next we compare and look for the first point of difference.

So this is an oxygen versus an oxygen,

so that's a tie.

We go to the next atom, and we have an oxygen

versus a hydrogen.

Obviously, oxygen wins.

So this group wins, the aldehyde is higher priority

than the CH2OH.

So the aldehyde must get a number two

and the CH2OH should get a number three.

So for assigning R or S, we know that the hydrogen

is going away from us in space.

So we don't have to worry about that,

we're done with step one and step two

from the earlier videos.

Next, we go around in a circle from one to two to three.

So we're going from one to two to three.

So we're going around this way, and that is clockwise.

And we know that clockwise is R.

So the configuration at our chirality center is R.

I wanted to take a minute to show how to go from

this drawing to this picture.

So if the hydrogen is on this side,

we wanna put our eye on this side so the hydrogen

is going away from us and stare down at our chiral center.

So this carbon.

I like to imagine this carbon as being in the plane

of the page.

So here is our chirality center.

Imagine a flat sheet of paper right here,

and that sheet of paper is passing through

your chiral center.

The OH, we know, is up in space.

There's a wedge here.

So when we're looking at it from this perspective,

the OH should be up relative to that flat sheet of paper.

And we can see it is, this is going up.

The aldehyde here would be going down,

because this is a dash, and this would be your right side

if your eye is right here.

So the aldehyde is going down relative to

the sheet of paper, and it's to the right.

So here's our aldehyde going down into the right,

and then this would be your left side.

The CH2OH is also going down, but it would be going down

and to the left.

So here we can see the CH2OH going down and to the left.

Now once you have this picture, it's easy to assign

a configuration to your chiral center.

So that was the first method.

The second method is, in my opinion, even easier.

This is the way that I usually use.

We already know that the OH group gets the highest priority.

So that's the number one.

The aldehyde got a number two, the CH2OH got a number three

and the hydrogen got a number four.

So the trick I showed you in earlier videos

is to ignore, ignore the fact that the hydrogen

is actually coming out at you in space.

And we know that because this horizontal line here

in the Fischer projection means a wedge.

So just ignore the hydrogen, look at one, two and three.

And one to two to three is going around in this direction,

which we know is counterclockwise.

So it looks like it's S.

So it looks like it's S for this chiral center.

However, since the hydrogen is actually coming out

at us in space, we saw in an earlier video,

the trick is just to take the opposite of how it looks.

So if it looks S, it's actually R.

And this trick should always work when you're working

with Fischer projections.

So there are many ways to do this.

In my opinion, you should get a model set

and figure out a method that works the best for you.

Finally, let's draw the enantiomer of this compound.

So the mirror method works the best

when you're working with Fischer projections.

So on the left is a model of our compound,

on the right is its mirror image.

We can see that this OH is reflected in our mirror,

so let's go down here, let's draw a line to represent

our mirror and let's reflect this OH in our mirror.

Then we need to draw a horizontal line right here,

which represents these two bonds.

And we have a hydrogen on the right side,

so we draw in our hydrogen.

Next we have a vertical line like that,

so we put in the vertical line; and then we have

an aldehyde at the top.

So I'll draw in our aldehyde.

And finally, a CH2OH at the bottom here.

So a CH2OH.

Our starting compound had only one chiral center.

So this one right here, and here's the chiral center

in the enantiomer.

We don't have any more chiral centers in our compounds.

So you don't have to worry much about the aldehyde

or the CH2OH when you're talking about reflecting them

in the mirror.

Make sure to get this switched.

So if this OH is on the right, then it'd be on the left

for the enantiomer.

So your goal is to reverse the configuration

at each chirality center.