fischer projections

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.