WEBVTT

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Fischer projections
are another way

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of visualizing molecules
in three dimensions.

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And let's use the
example of lactic acid.

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It's called lactic
acid since it has

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a carboxylic acid functional
group over here on the right.

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And this is the only chirality
center in lactic acid.

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It's an sp3 hybridized
carbon with four

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different substituents
attached to it.

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So with only one
chirality center,

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we would expect to
have two stereoisomers

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for this molecule.

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And those stereoisomers would
be enantiomers of each other.

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Over here, I've picked
one of those enantiomers.

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And I've just drawn
it in this fashion.

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Let's see which enantiomer
we have over here.

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Well, this is my
chirality center,

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the one attached to my OH.

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And if I were to assign
absolute configuration

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to that chirality center,
I look at the first atom

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connected to that
chirality center.

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Well, that's oxygen
versus carbon

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versus a carbon over
here in my carbonyl.

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So obviously,
oxygen's going to win.

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So we can assign oxygen
a number 1 priority

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since it has the
highest atomic number.

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And when I compare these
two carbons to each other,

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I know the carbon on the right
is double bonded to an oxygen.

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So that's going to give it
higher priority than the carbon

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over here on the left since
that's bonded to hydrogens.

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And then my other hydrogen
attached to my chirality center

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is going away from me in space.

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So when I'm assigning
absolute configuration,

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I look at the fact that
it's going one, two, three.

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It's going around this way.

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It's going around clockwise.

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Therefore, this is the R
enantiomer of lactic acid.

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So that's all from
a previous video.

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Now, if I want to draw a Fischer
projection of R lactic acid,

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what I would do is I would
put my eye right here.

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And I would stare down
at my chirality center.

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And I would draw
exactly what I see.

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Well, if I'm staring
down this way,

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I could draw a line
right here to represent

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my flat sheet of paper.

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And I can see that both
my hydrogen and my OH

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are above my sheet of paper,
whereas my carboxylic acid

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and my CH3 are below
my sheet of paper.

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So this carbon is my
chirality center carbon.

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And I have my OH
coming out at me.

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And this is actually going
to be on the right side.

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So if you take out your
molecular model set,

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you will see this OH will
be coming out at you.

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And it will be on the
right side of you.

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And this hydrogen will
be coming out at you,

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and it will be on
the left side of you.

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So that hydrogen would
go over here like that.

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This carboxylic acid
functional group-- this

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is the top my head right here.

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Then that would make this go at
the top of what I'm looking at.

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And so that is going
away from me in space.

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So we would use a dash
to represent that.

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And so we could go ahead and
draw our C double bond to an O.

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And then an OH
going away from me.

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And then if I look at
this CH3 group over here,

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it's also going away from me.

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It's going down in space.

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So I can represent it going
down in space like that.

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And this is the viewpoint
of a Fischer projection.

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So if I'm going to convert
this into a Fischer projection,

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a Fischer projection is just
drawing a cross like that.

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And then at the top, you
have your C double bonded

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to an O and then
an OH as just a way

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of abbreviating this carboxylic
acid functional group.

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And then I have a
hydrogen over here.

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And then I have an
OH group over here.

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And then I have a CH3 here.

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So this is a Fischer projection.

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This is the Fischer
projection for R lactic acid.

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So this is R lactic acid.

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And Fischer projections
were invented

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by Emil Fischer, who won the
Nobel Prize in chemistry.

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One of the things was for his
research in carbohydrates.

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And he drew Fischer projections
to help him draw carbohydrates.

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And so that's where you'll
see Fischer projections used

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most often, even though
some chemists don't really

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like them very much.

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So this is the Fischer
projection for R lactic acid.

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And if I wanted to draw the
Fischer projection for S

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lactic acid, I
would just reflect

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this molecule in a mirror.

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So let's see if I can fit
my mirror in over here.

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And I would have my OH
reflected in my mirror.

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And then I'd go ahead and
draw my Fischer projection.

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And then my methyl group
would be over here.

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My hydrogen would be over here.

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And my carboxylic
acid functional group

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would be right there.

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So this would be S lactic
acid on the right and R

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lactic acid on the left.

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S lactic acid is the
type of lactic acid

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you find in the buildup of
muscles after extreme exercise.

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And the type of lactic
acid that some people have

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heard of from milk is
actually a racemic mixture.

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So the bacteria in sour milk
will break down the lactose

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into a 50% mixture of R and a
50% mixture of S lactic acid.

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Let's take a look
at a carbohydrate,

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since Fischer used Fischer
projections for carbohydrates,

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specifically.

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So here I have a carbohydrate.

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And if I were to number
this carbohydrate,

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this carbonyl would
get a number 1.

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And then this would get
a number 2 over here,

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a number 4, and a number 4.

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This is a four-carbon
carbohydrate.

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How many stereoisomers does
this carbohydrate have?

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Well, this carbon number
2 is a chirality center.

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And carbon number 3
is a chirality center,

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so 2 chirality centers.

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So I use the formula
of 2 to the n,

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where n is the number
of chirality centers.

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So I would expect 2 squared,
or 4 possible stereoisomers

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for this molecule.

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So you could draw four
different stereoisomers

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for this molecule.

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We'll draw them
in a few minutes.

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For right now, I've gone
ahead and drawn one of them

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as a sawhorse projection.

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So here I have a sawhorse
projection of one

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of the possible stereoisomers.

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And what we're
going to do is we're

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going to put our
eye right up here.

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And we're going to stare
straight down at this bond

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right here.

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And we're going to see if
we can draw the Fischer

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projection for this molecule.

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So what do we see?

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Well, let's start with
this carbon right up here.

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So we'll make that
carbon this one.

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And you can see that the
OH attached to that carbon

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is going to the right.

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And it's going up at us.

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So that OH is going to the
right, and it's going up at us.

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And then if I look at
this hydrogen over here,

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it's on the left.

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And it's going up at us.

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So my hydrogen is on the
left and it's going up at us.

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And this aldehyde functional
group, this CHO, you can see

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is going down.

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So this aldehyde functional
group is going away from us.

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So we can go ahead and
represent that aldehyde

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as going away from us
in space like that.

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Well, this chirality
center carbon

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is connected to this
chirality center carbon.

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So we'll go ahead and
draw a straight line,

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since we're looking
straight down at it.

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And once again, we will
find that our OH group

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is on the right
coming out at us.

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Our hydrogen is on the
left coming out at us.

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So let's go ahead
and put those in.

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OH group is on the
right coming out at us.

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Hydrogen is on the
left coming out at us.

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And then, of course, we
have this CH2OH down here

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as going away from us in space.

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So we'll go ahead and
draw that CH2OH going away

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from us in space like that.

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So that would be the Fischer
projection translated.

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Let's go ahead and make it into
an actual Fischer projection

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where we just go ahead
and draw straight lines.

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And the intersection
of those straight lines

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are where our
chirality centers are.

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So this would be an H.
This would be an OH.

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This would be an H.

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This would be an OH.

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This would be our CH2OH.

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And then at the top, we
have our aldehyde, CHO.

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So this is one of the four
possible stereoisomers.

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And Fischer projections
just make it much easier

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when we're working
with carbohydrates.

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So this is one of the four.

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Let's go ahead and redraw
the one we just drew

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and let's get the other three to
get our total of four on here.

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So I'm going to take the one
that I just drew on the right.

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I'm going to redraw it.

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I'm going to draw it a little
bit smaller so everything

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will fit in here.

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So this is one
possible stereoisomer.

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I have my OHs on the right.

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I have my hydrogens.

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I have my CHO.

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I have my CH2OH.

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If I wanted to draw the
enantiomer to this molecule,

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I would just have to
reflect it in a mirror.

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So I could just do this.

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I could reflect the
molecule in a mirror,

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and I would have the enantiomer.

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So this would be the
enantiomer to the stereoisomer

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that I just drew.

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If I wanted to
draw the other two,

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I can just go ahead
and real quickly

00:08:39.789 --> 00:08:42.179
put in my Fischer
projections right here.

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So I have two more to go.

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And I'm going to put the OH over
here, and then the H over here,

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and then the OH over
here, and the H over here.

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So this is yet another
possible stereoisomer.

00:08:58.530 --> 00:09:01.530
And I'll draw the mirror
image over here on the right.

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So I have to have a
hydrogen right here.

00:09:04.230 --> 00:09:06.850
And then my OH must
be on this side.

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And then I must have an
OH right here, and then

00:09:11.420 --> 00:09:14.975
a hydrogen on the other side,
and then a CHO for my aldehyde,

00:09:14.975 --> 00:09:17.200
and a CH2OH.

00:09:17.200 --> 00:09:20.420
So here I have my four
possible stereoisomers

00:09:20.420 --> 00:09:22.792
for this carbohydrate.

00:09:22.792 --> 00:09:24.500
And I'm going to go
ahead and label them.

00:09:24.500 --> 00:09:27.170
I'm going to label this
first one here stereoisomer

00:09:27.170 --> 00:09:31.760
A, stereoisomer B, stereoisomer
C, and stereoisomer D.

00:09:31.760 --> 00:09:34.860
Well, C and D are mirror
images of each other.

00:09:34.860 --> 00:09:37.169
So they are enantiomers
of each other.

00:09:37.169 --> 00:09:38.210
So these are enantiomers.

00:09:38.210 --> 00:09:40.820
A and B are mirror
images, so they

00:09:40.820 --> 00:09:42.790
are enantiomers to each other.

00:09:42.790 --> 00:09:45.600
And then we talked about
in the diastereomer video,

00:09:45.600 --> 00:09:51.170
if I took one of the ones from A
and B-- so let me just go ahead

00:09:51.170 --> 00:09:53.030
and circle that--
if I just took A.

00:09:53.030 --> 00:09:55.780
If I took one of the ones from
A and B and one of the ones

00:09:55.780 --> 00:09:58.000
from C and D, and
I'll just take C. Then

00:09:58.000 --> 00:10:00.930
A and C are diastereomers
of each other.

00:10:00.930 --> 00:10:04.910
They are non-superimposable,
non-mirror images

00:10:04.910 --> 00:10:05.570
of each other.

00:10:05.570 --> 00:10:07.750
So those are enantiomers
and diastereomers,

00:10:07.750 --> 00:10:11.349
to review what we covered
in an earlier video.

00:10:11.349 --> 00:10:13.390
Let's do one more thing
with Fischer projections.

00:10:13.390 --> 00:10:16.270
Let's assign absolute
configurations

00:10:16.270 --> 00:10:18.860
to one of the stereoisomers.

00:10:18.860 --> 00:10:20.780
So let's just choose
the first one, A.

00:10:20.780 --> 00:10:23.030
So we've been talking
about A. And let's go ahead

00:10:23.030 --> 00:10:24.930
and redraw it really fast.

00:10:24.930 --> 00:10:29.410
And let's see how can we figure
out the absolute configuration

00:10:29.410 --> 00:10:34.357
at my chirality centers
from a Fischer projection.

00:10:34.357 --> 00:10:36.440
So it just makes a little
bit trickier than usual.

00:10:36.440 --> 00:10:38.330
So here I have my
Fischer projection.

00:10:38.330 --> 00:10:40.250
And your aldehyde's
going to get a 1,

00:10:40.250 --> 00:10:44.232
and then 2, 3, 4 in terms of
numbering your carbon chain.

00:10:44.232 --> 00:10:46.190
I want to figure out the
absolute configuration

00:10:46.190 --> 00:10:47.650
at carbon 2 here.

00:10:47.650 --> 00:10:49.930
So at carbon 2, what do I have?

00:10:49.930 --> 00:10:52.060
I know a Fischer
projection tells me

00:10:52.060 --> 00:10:55.760
that if it's a horizontal line,
everything is coming out at me.

00:10:55.760 --> 00:10:57.430
So my OH is coming out at me.

00:10:57.430 --> 00:10:59.950
And my hydrogen is
coming out at me.

00:10:59.950 --> 00:11:03.620
Let's go back up here and stare
down that carbon 2 chirality

00:11:03.620 --> 00:11:04.120
center.

00:11:04.120 --> 00:11:06.760
And let's see what we would
actually see if we do that.

00:11:06.760 --> 00:11:09.390
So here is carbon 2 right here.

00:11:09.390 --> 00:11:12.640
I'm going to stare down
right here this time.

00:11:12.640 --> 00:11:15.070
So I have my OH
coming out at me,

00:11:15.070 --> 00:11:17.240
my hydrogen coming out at me.

00:11:17.240 --> 00:11:19.280
That makes this
bond and this bond

00:11:19.280 --> 00:11:22.030
to actually go away
from me in space.

00:11:22.030 --> 00:11:24.050
So the aldehyde is
going to go away

00:11:24.050 --> 00:11:27.010
from me in space like that.

00:11:27.010 --> 00:11:29.629
So I'm going to go ahead
and draw my aldehyde.

00:11:29.629 --> 00:11:31.920
Now, I'm actually going to
go ahead and show the carbon

00:11:31.920 --> 00:11:33.140
bond to one hydrogen.

00:11:33.140 --> 00:11:35.519
I know the carbon's double
bonded to an oxygen,

00:11:35.519 --> 00:11:36.810
so I'm going to go and do that.

00:11:36.810 --> 00:11:38.893
That was that trick we
learned in an earlier video

00:11:38.893 --> 00:11:40.490
for assigning absolute
configuration.

00:11:40.490 --> 00:11:41.948
And then the rest
of the molecule's

00:11:41.948 --> 00:11:43.370
actually going down in space.

00:11:43.370 --> 00:11:46.290
So this would be a carbon
here bonded to a hydrogen.

00:11:46.290 --> 00:11:48.950
And this carbon is bonded
to an oxygen and a carbon.

00:11:48.950 --> 00:11:50.690
So what is the
absolute configuration

00:11:50.690 --> 00:11:51.720
of this carbon here?

00:11:51.720 --> 00:11:56.990
Well, if I think about this
is my chirality center,

00:11:56.990 --> 00:11:59.210
what are the atoms directly
attached to that carbon?

00:11:59.210 --> 00:12:01.690
Well, I have a hydrogen,
a carbon, an oxygen,

00:12:01.690 --> 00:12:02.320
and a carbon.

00:12:02.320 --> 00:12:06.670
Well, immediately I know that
my oxygen is going to win.

00:12:06.670 --> 00:12:09.980
So I can go ahead and assign
a number 1 to my oxygen right

00:12:09.980 --> 00:12:10.650
here.

00:12:10.650 --> 00:12:13.070
And then I think about
what's next priority.

00:12:13.070 --> 00:12:15.800
Well, it would be
carbon versus carbon.

00:12:15.800 --> 00:12:19.450
So at the top, I have
oxygen, oxygen, hydrogen.

00:12:19.450 --> 00:12:22.410
The bottom carbon, I have
oxygen, carbon, hydrogen.

00:12:22.410 --> 00:12:24.080
So we saw in an
earlier video, you

00:12:24.080 --> 00:12:25.455
go for first point
of difference.

00:12:25.455 --> 00:12:27.750
So oxygen versus
oxygen, no one wins.

00:12:27.750 --> 00:12:30.280
Then I go oxygen versus
carbon, and oxygen wins.

00:12:30.280 --> 00:12:32.160
So this would get
a number 2 up here.

00:12:32.160 --> 00:12:34.430
And then this would get a
number 3 for my substituent.

00:12:34.430 --> 00:12:36.380
And my hydrogen
would get a number 4.

00:12:36.380 --> 00:12:38.340
So I'm going around this way.

00:12:38.340 --> 00:12:42.500
I am going around this way,
if I ignore my hydrogen.

00:12:42.500 --> 00:12:44.460
So I'm going counterclockwise.

00:12:44.460 --> 00:12:48.400
So it looks like it's S.
But remember, the hydrogen

00:12:48.400 --> 00:12:50.070
is actually coming out at me.

00:12:50.070 --> 00:12:53.229
So in the little trick I showed
you in the earlier video,

00:12:53.229 --> 00:12:55.520
if the hydrogen is coming
out at me, all you have to do

00:12:55.520 --> 00:12:56.560
is reverse it.

00:12:56.560 --> 00:13:00.000
So it looks like it's S, but
since the hydrogen's coming out

00:13:00.000 --> 00:13:02.710
at, me, I can go ahead
and say with certainty

00:13:02.710 --> 00:13:05.560
that it is R at that
chirality center.

00:13:05.560 --> 00:13:09.730
So at carbon 2, at
this carbon, it is R.

00:13:09.730 --> 00:13:13.190
So you can do the same thing
with the chirality center

00:13:13.190 --> 00:13:14.540
at the third position.

00:13:14.540 --> 00:13:17.130
So you could do the same
thing with this one.

00:13:17.130 --> 00:13:20.380
And if you do that, you
will find that it is also R.

00:13:20.380 --> 00:13:23.090
So you could go ahead and
say for this carbohydrate,

00:13:23.090 --> 00:13:27.730
it is R at carbon 2,
and it is R at carbon 3.

00:13:27.730 --> 00:13:29.840
So it is 2R, 3R.

00:13:29.840 --> 00:13:32.140
And there's a 2R,
3R stereoisomer.

00:13:32.140 --> 00:13:35.930
And you could do that for
all four of the stereoisomers

00:13:35.930 --> 00:13:37.750
that we drew for
this carbohydrate.

00:13:37.750 --> 00:13:40.580
And you could then compare
enantiomers and diastereomers

00:13:40.580 --> 00:13:41.670
that way, as well.

00:13:41.670 --> 00:13:44.570
So that's a quick summary
of Fischer projections.

00:13:44.570 --> 00:13:45.200
Practice.

00:13:45.200 --> 00:13:47.930
And use your molecular
model set to help you

00:13:47.930 --> 00:13:50.530
with the visualization aspect.

00:13:50.530 --> 00:13:51.296