Fischer projections are another way
of visualizing molecules in three dimensions.
And let's use the example of lactic acid.
It's called lactic acid since it has
a carboxylic acid functional group over here on the right.
And this is the only chirality center in lactic acid.
It's an sp3 hybridized carbon with four
different substituents attached to it.
So with only one chirality center,
we would expect to have two stereoisomers
for this molecule.
And those stereoisomers would be enantiomers of each other.
Over here, I've picked one of those enantiomers.
And I've just drawn it in this fashion.
Let's see which enantiomer we have over here.
Well, this is my chirality center,
the one attached to my OH.
And if I were to assign absolute configuration
to that chirality center, I look at the first atom
connected to that chirality center.
Well, that's oxygen versus carbon
versus a carbon over here in my carbonyl.
So obviously, oxygen's going to win.
So we can assign oxygen a number 1 priority
since it has the highest atomic number.
And when I compare these two carbons to each other,
I know the carbon on the right is double bonded to an oxygen.
So that's going to give it higher priority than the carbon
over here on the left since that's bonded to hydrogens.
And then my other hydrogen attached to my chirality center
is going away from me in space.
So when I'm assigning absolute configuration,
I look at the fact that it's going one, two, three.
It's going around this way.
It's going around clockwise.
Therefore, this is the R enantiomer of lactic acid.
So that's all from a previous video.
Now, if I want to draw a Fischer projection of R lactic acid,
what I would do is I would put my eye right here.
And I would stare down at my chirality center.
And I would draw exactly what I see.
Well, if I'm staring down this way,
I could draw a line right here to represent
my flat sheet of paper.
And I can see that both my hydrogen and my OH
are above my sheet of paper, whereas my carboxylic acid
and my CH3 are below my sheet of paper.
So this carbon is my chirality center carbon.
And I have my OH coming out at me.
And this is actually going to be on the right side.
So if you take out your molecular model set,
you will see this OH will be coming out at you.
And it will be on the right side of you.
And this hydrogen will be coming out at you,
and it will be on the left side of you.
So that hydrogen would go over here like that.
This carboxylic acid functional group-- this
is the top my head right here.
Then that would make this go at the top of what I'm looking at.
And so that is going away from me in space.
So we would use a dash to represent that.
And so we could go ahead and draw our C double bond to an O.
And then an OH going away from me.
And then if I look at this CH3 group over here,
it's also going away from me.
It's going down in space.
So I can represent it going down in space like that.
And this is the viewpoint of a Fischer projection.
So if I'm going to convert this into a Fischer projection,
a Fischer projection is just drawing a cross like that.
And then at the top, you have your C double bonded
to an O and then an OH as just a way
of abbreviating this carboxylic acid functional group.
And then I have a hydrogen over here.
And then I have an OH group over here.
And then I have a CH3 here.
So this is a Fischer projection.
This is the Fischer projection for R lactic acid.
So this is R lactic acid.
And Fischer projections were invented
by Emil Fischer, who won the Nobel Prize in chemistry.
One of the things was for his research in carbohydrates.
And he drew Fischer projections to help him draw carbohydrates.
And so that's where you'll see Fischer projections used
most often, even though some chemists don't really
like them very much.
So this is the Fischer projection for R lactic acid.
And if I wanted to draw the Fischer projection for S
lactic acid, I would just reflect
this molecule in a mirror.
So let's see if I can fit my mirror in over here.
And I would have my OH reflected in my mirror.
And then I'd go ahead and draw my Fischer projection.
And then my methyl group would be over here.
My hydrogen would be over here.
And my carboxylic acid functional group
would be right there.
So this would be S lactic acid on the right and R
lactic acid on the left.
S lactic acid is the type of lactic acid
you find in the buildup of muscles after extreme exercise.
And the type of lactic acid that some people have
heard of from milk is actually a racemic mixture.
So the bacteria in sour milk will break down the lactose
into a 50% mixture of R and a 50% mixture of S lactic acid.
Let's take a look at a carbohydrate,
since Fischer used Fischer projections for carbohydrates,
specifically.
So here I have a carbohydrate.
And if I were to number this carbohydrate,
this carbonyl would get a number 1.
And then this would get a number 2 over here,
a number 4, and a number 4.
This is a four-carbon carbohydrate.
How many stereoisomers does this carbohydrate have?
Well, this carbon number 2 is a chirality center.
And carbon number 3 is a chirality center,
so 2 chirality centers.
So I use the formula of 2 to the n,
where n is the number of chirality centers.
So I would expect 2 squared, or 4 possible stereoisomers
for this molecule.
So you could draw four different stereoisomers
for this molecule.
We'll draw them in a few minutes.
For right now, I've gone ahead and drawn one of them
as a sawhorse projection.
So here I have a sawhorse projection of one
of the possible stereoisomers.
And what we're going to do is we're
going to put our eye right up here.
And we're going to stare straight down at this bond
right here.
And we're going to see if we can draw the Fischer
projection for this molecule.
So what do we see?
Well, let's start with this carbon right up here.
So we'll make that carbon this one.
And you can see that the OH attached to that carbon
is going to the right.
And it's going up at us.
So that OH is going to the right, and it's going up at us.
And then if I look at this hydrogen over here,
it's on the left.
And it's going up at us.
So my hydrogen is on the left and it's going up at us.
And this aldehyde functional group, this CHO, you can see
is going down.
So this aldehyde functional group is going away from us.
So we can go ahead and represent that aldehyde
as going away from us in space like that.
Well, this chirality center carbon
is connected to this chirality center carbon.
So we'll go ahead and draw a straight line,
since we're looking straight down at it.
And once again, we will find that our OH group
is on the right coming out at us.
Our hydrogen is on the left coming out at us.
So let's go ahead and put those in.
OH group is on the right coming out at us.
Hydrogen is on the left coming out at us.
And then, of course, we have this CH2OH down here
as going away from us in space.
So we'll go ahead and draw that CH2OH going away
from us in space like that.
So that would be the Fischer projection translated.
Let's go ahead and make it into an actual Fischer projection
where we just go ahead and draw straight lines.
And the intersection of those straight lines
are where our chirality centers are.
So this would be an H. This would be an OH.
This would be an H.
This would be an OH.
This would be our CH2OH.
And then at the top, we have our aldehyde, CHO.
So this is one of the four possible stereoisomers.
And Fischer projections just make it much easier
when we're working with carbohydrates.
So this is one of the four.
Let's go ahead and redraw the one we just drew
and let's get the other three to get our total of four on here.
So I'm going to take the one that I just drew on the right.
I'm going to redraw it.
I'm going to draw it a little bit smaller so everything
will fit in here.
So this is one possible stereoisomer.
I have my OHs on the right.
I have my hydrogens.
I have my CHO.
I have my CH2OH.
If I wanted to draw the enantiomer to this molecule,
I would just have to reflect it in a mirror.
So I could just do this.
I could reflect the molecule in a mirror,
and I would have the enantiomer.
So this would be the enantiomer to the stereoisomer
that I just drew.
If I wanted to draw the other two,
I can just go ahead and real quickly
put in my Fischer projections right here.
So I have two more to go.
And I'm going to put the OH over here, and then the H over here,
and then the OH over here, and the H over here.
So this is yet another possible stereoisomer.
And I'll draw the mirror image over here on the right.
So I have to have a hydrogen right here.
And then my OH must be on this side.
And then I must have an OH right here, and then
a hydrogen on the other side, and then a CHO for my aldehyde,
and a CH2OH.
So here I have my four possible stereoisomers
for this carbohydrate.
And I'm going to go ahead and label them.
I'm going to label this first one here stereoisomer
A, stereoisomer B, stereoisomer C, and stereoisomer D.
Well, C and D are mirror images of each other.
So they are enantiomers of each other.
So these are enantiomers.
A and B are mirror images, so they
are enantiomers to each other.
And then we talked about in the diastereomer video,
if I took one of the ones from A and B-- so let me just go ahead
and circle that-- if I just took A.
If I took one of the ones from A and B and one of the ones
from C and D, and I'll just take C. Then
A and C are diastereomers of each other.
They are non-superimposable, non-mirror images
of each other.
So those are enantiomers and diastereomers,
to review what we covered in an earlier video.
Let's do one more thing with Fischer projections.
Let's assign absolute configurations
to one of the stereoisomers.
So let's just choose the first one, A.
So we've been talking about A. And let's go ahead
and redraw it really fast.
And let's see how can we figure out the absolute configuration
at my chirality centers from a Fischer projection.
So it just makes a little bit trickier than usual.
So here I have my Fischer projection.
And your aldehyde's going to get a 1,
and then 2, 3, 4 in terms of numbering your carbon chain.
I want to figure out the absolute configuration
at carbon 2 here.
So at carbon 2, what do I have?
I know a Fischer projection tells me
that if it's a horizontal line, everything is coming out at me.
So my OH is coming out at me.
And my hydrogen is coming out at me.
Let's go back up here and stare down that carbon 2 chirality
center.
And let's see what we would actually see if we do that.
So here is carbon 2 right here.
I'm going to stare down right here this time.
So I have my OH coming out at me,
my hydrogen coming out at me.
That makes this bond and this bond
to actually go away from me in space.
So the aldehyde is going to go away
from me in space like that.
So I'm going to go ahead and draw my aldehyde.
Now, I'm actually going to go ahead and show the carbon
bond to one hydrogen.
I know the carbon's double bonded to an oxygen,
so I'm going to go and do that.
That was that trick we learned in an earlier video
for assigning absolute configuration.
And then the rest of the molecule's
actually going down in space.
So this would be a carbon here bonded to a hydrogen.
And this carbon is bonded to an oxygen and a carbon.
So what is the absolute configuration
of this carbon here?
Well, if I think about this is my chirality center,
what are the atoms directly attached to that carbon?
Well, I have a hydrogen, a carbon, an oxygen,
and a carbon.
Well, immediately I know that my oxygen is going to win.
So I can go ahead and assign a number 1 to my oxygen right
here.
And then I think about what's next priority.
Well, it would be carbon versus carbon.
So at the top, I have oxygen, oxygen, hydrogen.
The bottom carbon, I have oxygen, carbon, hydrogen.
So we saw in an earlier video, you
go for first point of difference.
So oxygen versus oxygen, no one wins.
Then I go oxygen versus carbon, and oxygen wins.
So this would get a number 2 up here.
And then this would get a number 3 for my substituent.
And my hydrogen would get a number 4.
So I'm going around this way.
I am going around this way, if I ignore my hydrogen.
So I'm going counterclockwise.
So it looks like it's S. But remember, the hydrogen
is actually coming out at me.
So in the little trick I showed you in the earlier video,
if the hydrogen is coming out at me, all you have to do
is reverse it.
So it looks like it's S, but since the hydrogen's coming out
at, me, I can go ahead and say with certainty
that it is R at that chirality center.
So at carbon 2, at this carbon, it is R.
So you can do the same thing with the chirality center
at the third position.
So you could do the same thing with this one.
And if you do that, you will find that it is also R.
So you could go ahead and say for this carbohydrate,
it is R at carbon 2, and it is R at carbon 3.
So it is 2R, 3R.
And there's a 2R, 3R stereoisomer.
And you could do that for all four of the stereoisomers
that we drew for this carbohydrate.
And you could then compare enantiomers and diastereomers
that way, as well.
So that's a quick summary of Fischer projections.
Practice.
And use your molecular model set to help you
with the visualization aspect.