Leah here from leah4sci.com/MCAT and in this video, we'll look at how to convert between Newman
Projections to Fischer Projections and then back from Fischer to Newman. If you missed
video one make sure you go back and watch that first before you continue this video to make sure you understand
and can follow along. You can find the entire series along with my practice quiz and cheat
sheet by visiting my website leah4sci.com/Fischer.
Recall that the Newman Projection is a method of analyzing a molecule that has pre rotation
between two sp3 carbons. So that you have for example carbon 2 in a molecule, here but
carbon 3 is hiding behind carbon 2. For this specific molecule both carbons 2 and carbons
3 are chiral. So if we convert it to a Fischer Projection we're going to see a 2-dimensional
flat structure that not only represents the molecule but also tells us about the chirality
from both carbon 2 and carbon 3. You had the option to convert this so that carbon 2 is
on top, carbon 3 is on bottom or the other way around. I like to take that fourth carbon
and put it down making that carbon 2. The rear carbon up making that carbon 3. But ultimately
as long as you do it correctly it doesn't matter which way you do it because molecules
can rotate in space so what's up now will be down later. If you're not confident with
Newman Projections, go back and watch that video series first on my website leah4sci.com/Newman-Projections.
So let's look at how to convert this Newman to a Fischer. We're starting with a Newman
Projection in the staggered conformation. Which means the carbon in the front has an
opposite orientation to the carbon behind it. On the Fischer Projection, the horizontal
lines represent the groups coming up and out of the page, which means they're facing in
the same direction. So we'll start with the long and tedious process for conversion to
make sure you understand every step. And then I'll show you shortcut which when you understand
it will save you a lot of time on exams.
The first thing we want to do is eclipse the newman projection because we want to see it
in a pre Fischer confirmation so that it sorts of start to look like a Fischer and then you
just rewrite the groups. What I mean by that is if we have a Newman Projection with the
blue carbon in the front and the green carbon eclipse behind it, you can almost imagine
that as a Fischer Projection that looks like this where the blue carbon in the front has
the 2 substituents up and 1 down. The green carbon behind it has that same orientation,
that's what we want to keep in mind. And that easily converts into a regular Fischer the
way we're used to seeing it. When I look at the Fischer Projection I looked down from
the top and see 2 substituents coming up. I wanna rotate the Newman so I have 2 substituents
up and one down for both the forward and the rear carbon. That means we keep the forward
carbon exactly as we see it with an OH up right, Hydrogen up left and a CH3 going down.
And then we'll rotate the rear carbon a hundred and eighty degrees. So if the methyl is up
on the rotated molecule, it'll be down. We'll show it slightly offset from the purple substituents
to we can make up what's going on.
The Bromine is down to the right so we rotate it up to the left. Hydrogen is down to the
left, so we rotate it up to right. And now if you put your eye on top of this molecule,
you can see exactly what goes on the right side and what goes on the left side so all
we do is superimpose it. We take the forward carbon and bring it to the bottom and the
rear carbon to the top. The forward carbon has an OH to the right, Hydrogen to the left
and a CH3 going down. The rear carbon has a Hydrogen to the right, Bromine to the left
and a CH3 going up, and there we have our Fischer Projection.
That wasn't too bad. Now let's try to do it a little faster. But before we do, I wanna
point out something critical on the starting molecule. Notice that the OH and the Br are
on the same side of the molecule on the Newman but they're on opposite sides on the Fischer
and that's because the starting Newman is staggered and we have to rotate it so that
the two extremes are both facing down. That's the two methyl group which will wind up on
opposite ends of the Fischer Projection. So the key here, the trick to recognize is that
when they start out on the same side in a staggered conformation, they have to wind
up on opposite sides in an eclipsed conformation assuming we're doing a hundred and eighty
degree rotation. You can do other degrees to get different molecules, but why would
you wanna give yourself that headache if you can keep it simple?
Ones again, here's our starting molecule and we'll draw a Fischer Skeleton. I like to start
with a carbon that has the two upper substituents. For this Newman it happens to be the forward
carbon in your example it could be the rear carbon, it doesn't matter. I like to put the
forward carbon on the bottom and since this is the one that has the two upper substituents,
I put the OH where I see on the right, the Hydrogen where I see on the left and the CH3
going down. The next step, the shortcut is to recognize that we need the rotation. The
CH3 is up, it will get rotated down and that means it's on the extreme of the molecule,
or recognize that it starts out by itself on a Newman meaning it's not paired with something
so it winds up by itself on the Fischer. The Bromine and Hydrogen are oriented right and
left, but because they have to undergo a 180 rotation, they're going to go left and right.
So the Bromine instead of right, goes left and the Hydrogen goes to the right. How do
I know? If they are on the same side when the Newman is staggered, they have to be on
opposite side when we turn it into a Fischer because they have to first undergo that eclipsed
formation. Now what happens if you're given a Fischer Projection and ask for the Newman?
This one is much easier. Start up by drawing a circle for your Newman Projection and then
choose where you want to place your carbons. I want the green carbon forward and the purple
carbon to the rear. You can draw the Newman as Staggered or Eclipsed and this will depend
on what your professor asks for or the choices that are are .. represent from your exam.
Let's start with Eclipsed because that's much easier.
Just draw a circle in between the green and purple carbons and that's exactly the circle
that you see on the Newman Projection. For the forward green carbon I have a Hydrogen
on the right, NH2 to the left and a CH3 going down. And for the rear carbon, don't forget,
this represents down on a Fischer Projection so I have a CH3 going down, a chlorine up
to the right and a Hydrogen up to the left. That was very easy. Now what happens if you
want the staggered conformations? You have two options. The first option is to draw the
eclipsed conformation and then rotate one of your carbons a hundred and eighty degrees
or you can do the shortcut by starting with one carbon exactly as you see it and using
that trick of when you rotate at the group switch to the second carbon. So we'll start
out with the green carbon as we see it. Hydrogen up right, NH2 up left and a CH3 going down.
The rear carbon will be rotated which means that if the Hydrogen and Cl are together on
a Fischer, they have to be opposite on the Newman. So if we draw the staggered, chlorine
will now be on the same side as the NH2, so we'll put the chlorine down towards the left.
Hydrogen will be on the same side as the other Hydrogen and the CH3 goes on top.
Be sure to join me in the next video where I show you how to convert a sugar that's in
a Fischer Projection to the Hayworth or Chair conformation. You can find my entire series
on Fischer Projections alongwith my practice quiz and cheat sheets by visiting my website
leah4sci.com/Fischer.