Fischer projection introduction

- [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.