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Organic chemistry
Course: Organic chemistry > Unit 3
Lesson 3: Conformations of alkanesNewman projections 2
Newman Projections 2. Created by Sal Khan.
Want to join the conversation?
- what is a dihedral angle?(6 votes)
- In the context of the video, a dihedral angle is the angle between two points on two different parallel circles or planes. For example, imagine a clock. The angle between the hands is a dihedral angle because the minute hand has to pass over the hour hand (indicating that they are not on the same plane, if they were they would've collided).(15 votes)
- Why wouldn't there be another confirmation where the back CH3 group is eclipsed by a hydrogen and the front CH3 group is eclipsing a hydrogen?(3 votes)
- There is. He just didn't show them all in the video.
Just as there are three staggered conformations, there are three eclipsed conformations.
One has the two CH₃ groups eclipsed. The other two have CH₃ eclipsed with H.(6 votes)
- The angles between the hydrogen atoms and the carbon atom in ethane are 109.25 degrees since it is sp3 hybridised .then dihedral angles should have been 109.25 divided by 2 i.e 54.6 degrees .then how is the dihedral angle 60 degrees???(3 votes)
- In the case of the sp3 you are looking at 4 atoms in a tetrahedral (3D) configuration, which it turns out the farthest apart they can all be in 109.25 (think a triangular pyramid) On the Newman projection you are projecting the object into a 2D plane which will give you a 360 degree circle with 3 atoms in view, which will put them 120 degrees apart, add 3 more atoms and they are now 60 degrees apart (the 4th atom in the tetrahedrons, C, is behind the other C so it is not in the "circle" of the projection)(5 votes)
- I was hoping this video would touch on this, but it didn't:
If you have an electronegative atom, like Cl or F, would the eclipsed conformation of two Cl or two F have a higher potential energy or a lower potential energy then the eclipsed conformation of two methyl groups? And if so why?
Thank you!(5 votes) - At, Sal mentions the Gauche conformation to be 2nd most stable. I was wondering which conformation would be more stable: 1. Gauche where there is a staggered state, but the methyl groups are close to each other and hence more electronic repulsions, or 2. Dihedral angle between the methyl groups is 120 degrees, there is interaction b/w methyl and hydrogen twice but hydrogen has very less electron density as compared to methyl. Thanks! 09:08(2 votes)
- The energies of interaction are approximately
• 3.3 kJ/mol for each CH₃-CH₃ gauche
• 4.2 kJ/mol for each H-H eclipsed
• 5.0 kJ/mol for each CH₃-H eclipsed
• 23 kJ/mol for each CH₃-CH₃ eclipsed
The major interactions are those between CH₃ and CH₃.
The CH₃-CH₃ eclipsed conformer is the least stable.
That is the case as long as the CH₃ groups are within about 30 ° of each other.
Then comes the CH₃-H eclipsed conformer (CH₃ groups at 120 °). It is still high-energy but more stable than the CH₃-CH eclipsed conformer.(3 votes)
- At, when draws the 3d structure of butane, how does he decide that H will go up and CH3 would go down? 2:25(1 vote)
- It's an arbitrary choice.
But the CH₃ looked "down" in the original structure, so he put it down in his saw-horse projection.
It is convenient to draw the biggest groups either up or down in saw-horse and Newman projections, so most chemists do that automatically without even thinking.(4 votes)
- If we rotated the back methyl group by 60 degrees as Sal was initially saying, wouldn't that be more stable than the Gauche conformation ?? And would that be a Gauche conformation ?? Thanks !! :)(2 votes)
- In that conformation, it is still eclipsed. However, the back methyl group (in this context) is only eclipsed by a single hydrogen, which has a much smaller electron cloud than a methyl group.(2 votes)
- When he first draws the butane molecule at around the 1 minute mark, does the drawing have to be exactly like that? For example, when he draws the hydrogens for the middle 2 carbons, do they have to be both at the top or both at the bottom? If so, why? I was just thinking that they would be as far away as possible because the electrons want to repel each other.(1 vote)
- They don't have to be at any place, no. Think of them as in constant rotation but spending the majority of the rotational time in the more stable positions.(3 votes)
- How do you know when to rotate from the front or the back?(1 vote)
- You can rotate either one. The important point is the anti/gauche/staggered/eclipsed relationship of the groups to each other.(2 votes)
- How would you go the other way around? For example, if you had that newman projection, how would you know how to name it?(1 vote)
- The front carbon has a CH₃ group and two H atoms. That part of the molecule is CH₃CH₂—.
The hidden carbon has two H atoms and a CH₃. That part of the molecule must be –CH₂CH₃.
Put them together, and you have CH₃CH₂—CH₂CH₃.
There are four carbons in a row with no substituents, so the name is butane.(2 votes)
Video transcript
In the last video, we visualized
an ethane molecule with a Newman Projection. What I want to do in this video
is show that you can really visualize longer chains,
or even, we'll see in future videos, even cyclical,
ring-based carbon molecules with Newman Projections
as well. And I guess the next most
complex molecule to study would be butane. We could do propane, but butane
will be interesting. This was ethane right here,
butane will have four carbons. And if I were to draw it in
kind of a ball and stick model, it would look like
something like this. So this would be one
carbon right there. Then you would have another
carbon right over there, and another carbon right
over there. And then you'd have your
fourth carbon. And then your hydrogens. You would have a hydrogen coming
out like this, like that, and then up like that. This guy would have two
hydrogens that would stick out like that. This guy would have
two hydrogens that stick out like that. And then finally this guy will
also have three hydrogens, the ch3, just like that. Now, if we try to draw a Newman
Projection, here's like, well, what do you consider
the front, or the back, carbon in all of that. And you actually can pick. And what's interesting in a
butane molecule is, if you pick this guy, so this is a one,
two, three, four carbon, if you pick the two carbon as
our front, and our three carbon as our back, and then we
viewed this carbon, the ch3 as kind of one of the add-ons
on to that carbon, you can then do a Newman Projection. So let's try to do this. So this'll be the front one. So we'll put this carbon in the
front, and we'll put this carbon over here, we'll
put this carbon over here in the back. And before I even draw the
Newman Projection, let me redraw this. But I'm just going to draw
this, instead of with the hydrogens, the bonds, explicitly
defined, I'm just going to call this a ch3. So let me redraw this. So I'll do this in orange. So you have this carbon. I'll do this as kind of as a
modified ball and stick. So that carbon, it has a
hydrogen, it has that hydrogen and that hydrogen. And instead of drawing this out,
I'm going to just draw this whole thing right here and
I'll do it in, I'll do it in magenta. I'm going to draw this whole
thing as just a ch3. So I'm going to draw this whole
thing as just a ch3. So I'll draw it really big,
because it's not just one atom, it's four atoms.
So this is our ch3. And, well, to do ball and stick
everything really should be a ball, so I'll draw a ball
there, a ball there. So that's our carbon number two,
and then it has this bond over here to this carbon
number three. Which, when we do our
Newman Projection, we'll put in the back. So our carbon number
three is like that. And then the carbon number
three, it has two hydrogens, and then it has this. You can kind of view it as this
methyl group attached to it, if you want. It has this ch3 attached
to it right there. So I'll do the ch3, I'll do
it in this blue color. And so we could draw
it like this. So the ch3 is coming off-- so
I'll draw it really big because it's not
just one atom. So it's a ch3. And then you have your two
hydrogens and they're-- sorry, you have your two hydrogens
down here. So let me be very clear here. This hydrogen and that hydrogen,
that's that hydrogen and that hydrogen, this thing
here is that thing there. That big ball right there
is this whole ball. And then let me find
a-- I'll do green. This hydrogen and this hydrogen
is this hydrogen and this hydrogen. And when you look at it this
way, now you say, oh, now I can see how I would draw
a Newman Projection. I put this in the front,
that in the back. I treat this whole part of
the molecule as just a group, if you will. It is a group. So lets do the Newman
Projection here. And we can think about where
it's most stable. So the way I've drawn it right
here-- I'll do this as the front, this carbon
two is going to be the front molecule. So you have the ch3
group going down. And then you have these
two hydrogens, hydrogen, and hydrogen. That's the front. And in the back you have
this blue one. You can imagine in the front, if
we want to, maybe I'll do a little small orange
thing to show this is the orange carbon. And then the blue carbon is
going to be in the back. The blue carbon is in the back,
I'll draw it like this. So that's my blue carbon. The way we've done it here, we
have a ch3 pointing straight up, and then we have
our two hydrogens. Now, just like we talked about
in the first video on Newman Projections, all of these groups
have-- these hydrogens have electron clouds
around them. This whole ch3 group has
a larger electron cloud around it. It's a carbon atom
plus hydrogens. They all want to get away
from each other. The ch3 is even a
bigger molecule. So to some degree, it's going
to play a bigger role in whether something has a higher
or lower potential energy or whether it's wound or not. So I guess the most obvious, or
maybe it's not obvious, but the ch3 groups, since they
have the biggest electron crowd, they're kind of crowding
the molecule. This ch3 group and this ch3
group, they're going to want to get as far away from each
other as possible. So the way we did this, it
looks kind of like our staggered conformation, but when
we're dealing with actual methyl groups that are separated
as far as they can from each other, we
call this the anti-conformation right here. And if we think about dihedral
angles between the two methyl groups, the dihedral angle
here is 180 degrees. 180 degrees dihedral angle. And this is the lowest
potential energy or the most stable. And if that confuses you when I
talk about lowest potential energy, just think about it. A rock on the ground has a lower
potential energy than a rock that is 50 feet
in the air. A rock on the ground is
also more stable. It's less likely to
do something. Something 50 feet in the air,
maybe if you nudge it a little bit, it'll fall off the cliff
or wherever it is. Or maybe it's already falling. Who knows? It's going to move when you have
higher potential energy. Or it takes very little for
it to release energy. But when you have
lower potential energy, you're more stable. So this is the most stable
conformation. Now what are the other
situations you could do here? Well, you could keep rotating
these-- let's say we rotated the back carbon around
clockwise, what are the other conformations we could get? And so let me just draw the
front portion right here. So you have your ch3, and then
you have your two hydrogens, hydrogen, and hydrogen. And let me copy and
paste this. So there's two other real-- I
mean, there's everything in between, but these are the ones
that are interesting. Control copy, and then let me
copy it, copy, and then paste. So I'll actually draw
three of these. So then you have that. Then let me paste it one more
time, and then you have that. So obviously this would
be the front carbon in every situation. If I want I could make it a
little orange dot to show that that's the front carbon. And then let me draw
the back carbon. I should've copied and
pasted this as well. So you have your back carbon
in every situation. Now, if we were to rotate this
character by 60 degrees-- actually if we were to rotate
the back by 60 degrees, what would it look like? Well, then we would have-- this
hydrogen would move up there-- so then you would
have this hydrogen. Actually if we were to move it
by 120 degrees, I should say. This would be 60, and then
another 120 degrees. So this hydrogen would
go up there. This methyl group would now be
over here, and then this hydrogen would go over here. So we've just rotated to this
whole thing by 120 degrees. Now, this conformation,
this was called the anti-conformation. It's the most stable because
the carbon-- the methyl groups, are as far away from
each other as possible. This right here is called
the Gauche conformation. Let me do this in a different--
and you can view this as the second
most stable. At least the methyls
are staggered. They're not directly
behind each other. So here the methyls are
as far apart from each other as possible. If you look at the ball and
stick model, I actually drew it in that conformation
right here. They're as far apart
from each other. If you were to flip this
molecule, if you were to flip it, this methyl would get closer
to this methyl, and their electron clouds would
start to crowd each other. So in this situation this
is anti, most stable. If you rotate a little bit
they'll get a little bit closer but they'll still
be staggered. You get the Gauche
conformation. Now, if we rotate this, if we
rotate the back guy now 60 degrees clockwise, what's
going to happen? Well, then you're going to have
an eclipsed conformation, where the carbons are directly,
but where the methyl groups are directly
behind each other. And that's going to be your
least stable situation. Right? So you'd have this guy-- and
I'll draw it slightly-- so you'd have this guy, ch3 there,
and then you would have your hydrogens that are right
behind each other. So a hydrogen and a hydrogen. So in this situation where
eclipsed-- this is the least stable, and also the most
potential energy. And then if we were to go
another 60 degrees from this, then we'd go to another
Gauche conformation. If you rotate this another 60
degrees, then you'd have a ch3 here, and then you would have--
this hydrogen would be up here, and then this
hydrogen here. So this is staggered. The methyl groups are, at least
they're not directly behind each other, but they're
not as far as they could be if we were to rotate another 120
degrees and get to the anti-conformation. So this one right here is also
a Gauche conformation. So hopefully you understand
now that, you just have to pick two carbons and then you
can, if there's, kind of, big things attached to each of those
carbons, you can just represent them as groups. And when you do that-- you'd
use a Newman Projection for any part of a molecule. And when you do that, you can
start to think about how it can rotate and what parts, or
what versions of it, will be--