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How to Draw Fischer Projections
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Go from 3D to Fischer and Fischer to 3D. Even for something this complex without a model Kit
Fischer projections are great for easily representing chiral molecules on paper,
such as D-Glyceraldehyde, L-Alanine, or D-Glucose. But what exactly are we
looking at? Mr. Organic Chemistry is here to help us interpret Fischer projections.
Like Mr. Organic Chemistry's bowtie, the horizontal lines are wedges coming out of
the page directly at you. And like Mr. Organic Chemistry's spine, the vertical lines are dashes
going down away from you and into the page. That means for a molecule like L-alanine,
what we're really looking at is the carboxy and methyl group on dashes down and into the page,
and the amine and hydrogen on wedges coming out of the page.
Wait a minute. This isn't how you typically draw a 3D molecule. What if I gave you D-glyceraldehyde
like this? Option one is tricky, and that's the mental visualization. In my last video,
I showed you to open this like a book, grabbing this purple group, all the way to the left until
it resembles Mr. Organic Chemistry's bowtie. But students found it confusing, so let's try this
option instead. Option two is learning how to look at the molecule. As you're watching this video
directly facing the screen, if someone looks from behind you, all that's visible is the back of your
head. What you want to do is look at this molecule head on right here, so that these two groups are
coming at you just like Mr. Organic Chemistry's bow tie. Turn your head to the side. Yes,
actually do this with me. So it's almost like you're looking into the screen from the side
like this. Looking at it from the side, what do we see? First look for the spine. That's the top and
the bottom that are going away from you, where the carboxy at the top lines up with your forehead.
And the purple CH2OH on the bottom lines up with your chin. Now for the tricky part, the bow tie,
as you're looking at this molecule from the side, notice how the blue hydrogen,
and green OH, line up with your eyes. Since the blue hydrogen is going down and into the page,
it's closer to your right eye, and the green OH is on a wedge coming out of your page.
It's closer to your left eye. This is how I know to put the H on the right and OH on
the left. One more time. Let's do another one looking at S-alanine from the side. Looking at
it like Mr. Organic Chemistry, what do you see? Closer to your forehead and away from you is the
carboxy. Closer to your chin and away from you is the methyl group, giving me the top and bottom.
Now for the horizontal groups. The amine comes out of the page and lines up with your left
eye. The hydrogen goes into your page, lines up with your right eye, giving me left and right.
What if I want my nitrogen at the top? Can I just rotate it 90 degrees? We know it works
for the 3D version because I have S-alanine on the left, and again, S-alanine on the right.
What about the Fischer? Are they also the same? Mr. Organic Chemistry tells me that
I have these two structures. Are they the same? We can find the R and S, but I want to show it to you
with option 2. Since we know we can rotate 3D, let's see what happens. Nope, we can't
superimpose them. In fact, because everything is reversed, They happen to be enantiomers.
Since the Fischer projection already implies what goes into and out of the page,
by rotating the molecule, we're essentially breaking every bond and reversing it. What
about 180? If 90 is the enantiomer, meaning we swapped all bonds once,
another 90 is swapping all of the bonds again or swapping them back.
The enantiomer of an enantiomer is the same thing. If you're done with time wasting model
kits for Fischer projections, give this video a thumbs up. But what if you have more than one
chiral center? Given 3-bromo-2-butanol with 2 chiral centers, can you quickly
turn it into a Fischer projection? Because it's more complicated, We have more options.
Option one requires rotating the molecule, because if you move your face to the side as before,
you'll notice bromine and the invisible hydrogen are coming at you like a bowtie,
but the OH and its invisible hydrogen are facing away from you like an inverted bowtie. Since the
side view does not match up with Mr. Organic Chemistry's bowtie, we're going to rotate
this bond 180 degrees. Nothing changed on the left, but look at what happened on the right,
on the green side of the molecule. OH down and out of the page, rotated 180, will be
up and into the page. Hydrogen down and into the page, rotates 180 to be up and out of the page.
Now you have to stand up to line your face up with the molecule,
but if you're looking down at it, You're simply looking at Mr. Organic Chemistry
with two bow ties. To turn this into a Fischer, we start with a skeleton that
has two lines for both chiral centers. We'll start with the top and the bottom.
Lining up with your forehead, we have the blue methyl group going up and away
from you. Lining up with your chin, we have the green methyl down and away from you. So
we'll put a blue CH3 at the top and a green one at the bottom. Now for the bow ties,
notice that the bromine on a wedge out of the page lines up with your
left eye and the hydrogen going into the page lines up with your right eye.
This puts bromine on the left and the gray hydrogen on the right. Now for the green
chiral center, notice how your left eye lines up with a green hydrogen up and out of the page,
and your right eye lines up with a green OH going into the page,
giving me a green hydrogen on the left and the OH on the right.
By rotating for option one, it takes extra time, but it's less confusing.
After doing this a couple of times, you can move on to option two,
which is just looking at the molecule without rotation. We start by looking at the molecule
exactly as before where the blue chiral center is already oriented the way we want.
That's a methyl lining up with my forehead, bromine with my left eye, and hydrogen with my
right. The lower chiral center is where it gets tricky. The groups still line up with my eye,
but because they're going away from me, they're opposite of what I'm looking for.
Understanding that to rotate means to switch everything,
you can simply line it up as you see it, but draw it on the other side.
The OH away from me lines up with my left eye, rotated,
it goes on the right. The hydrogen away from me lines up with my right eye,
rotated, it goes on the left. And the methyl group lining up with my chin coming at me,
rotated, will go into the page like Mr. Organic Chemistry's spine. And so I put it on the bottom.
One more. Since we can look at it from any direction, this time, put your face on the
bottom of the screen, and look directly up. Just imagine the entire screen is tilted,
but you're looking at it directly from the side. Since we have two chiral centers, the lower chiral
carbon is like Mr. Organic Chemistry's bow tie, and so we draw it as we see it.
The methyl group is away from me and lines up with my chin,
going down. The NH2 lines up with my left eye, so it goes left. The invisible hydrogen with my
right eye goes right. For the green portion of the molecule, we're still going to line it up,
but remember, because the groups are away from you, you have to rotate.
The ethyl group lines up with my forehead, but it's towards me, away from me puts it at the top,
the methyl group lines up with my left eye, rotated it goes on the right,
the invisible hydrogen lines up with my right eye, rotated it goes on the left. The same
trick works in the reverse. When looking at it head on, I envision Mr. Organic Chemistry with
two bow ties. Looking at this from this side, I have the top and bottom away from me. Lining up
with my right eye are the groups going down and into the page, giving me an OCH3 and bromine.
Lining up with my left eye, I have the groups coming out of the page, that's the 2 hydrogen.
To draw it in the more proper format, Start with your skeleton. Pick one
chiral center that will stay as you see it. In this case, I take the top,
where the OCH3 lines up with my right eye going down and into the page. Hydrogen with my left eye,
well, it's invisible, so I don't have to draw it on the structure.
For the lower chiral carbon, because I rotated on the molecule, I'm going to look at it backwards.
If bromine lines up with my right eye, Rotated, it comes out of the page. The invisible hydrogen
would go into the page, but it's invisible. But even this method becomes overwhelming
when you're dealing with larger molecules, like glucose, with four chiral centers.
Option one, you can rotate all the chiral centers so that Mr. Organic
Chemistry has four bow ties. Then you place your head to the side,
see what lines up with your right and left eye, and draw it accordingly. Once you pass
three chiral centers, this method is not only tedious, but it gets very confusing.
Option two, you can take your starting molecule and put your head above or below, remembering
that any chiral carbon away from you has to be rotated. But even this can get confusing,
which is why, with so many chiral centers, I like to take a few more minutes for option
three. The guess and check method. It's tedious, but it is guaranteed.
For step one, draw the Fischer skeleton and randomly guess a location for each
substituent. Because I have an OH in each chiral center,
let's randomly put them on the left. For step two, we'll find R and S for each chiral center
on the 3D structure. Here I get R, S, R, R. And the Fischer projection, I got S, S, S, and S.
If you guessed right, you keep it. If you guessed wrong,
you simply swap. And that is the correct Fischer Projection for D-glucose. Wait,
why are we swapping? And how did I find the R and S so easily on the Fischer
Projection? That's exactly what I teach in this video, which you can find on my website,
along with the Fischer Projections practice quiz and cheat sheet at leah4sci.com/Fischer
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