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Organic chemistry
Course: Organic chemistry > Unit 3
Lesson 3: Conformations of alkanesNewman projections
Newman Projections. Created by Sal Khan.
Want to join the conversation?
- Is it possible to show double and triple bonds in the newman projection?(16 votes)
- no because only the C-C sigma bonds have free rotation. When a pi bond is added, it's no longer able to rotate, therefore making the need for newman projections obsolete(40 votes)
- why do we learn this?? is there any practical use of these projections?? because itz just the way you view it. i mean how do you particularly predict its projection??(4 votes)
- Structure = function. You need to know the shape of each molecule in order to predict its behavior and how it will likely react with other molecules or atoms (eg. nucleophiles).(12 votes)
- It is eclisped from- 11:26, not staggered right? 11:29(2 votes)
- Yes, the hydrogens on the far carbon in the newman projection are, in reality, right behind the front carbons hydrogens making it an eclipsed conformation which is higher in energy.(3 votes)
- Isn't the bond angle of the 2 C-h bonds 109 degrees. Then why does sal say the angle is 60 degrees.
i think it should be 109/2 degrees..(2 votes)- When you’re considering it from a Newman projection that’s not the case.
When you look down the C-C bond the 3 hydrogen atoms are at 120 degrees from one another. Remember this is NOT the bond angle, this is the three of them arranged essentially around a circle in a Y or ⅄ shape.
If a circle has 360 degrees then 360/3 = 120. So in this orientation each H is 120 degrees around a circle.
Then if you consider how offset the front hydrogens are from the back hydrogens in that specific configuration he mentioned, they are 120/2 = 60 degrees offset.
Remember this is not the bond angles, it’s a geometry thing.
This image may help: https://en.wikipedia.org/wiki/Conformational_isomerism#/media/File:Butane_conformations.jpg
These things can be really hard to explain through text, it’s much easier to show with models.(3 votes)
- the eclipse will come to staggered, fine. A slightly staggered will become optimized such that the dihedral angle is 60 degree, fine. But.... If its in eclipse, you need "Torque" to put it into staggered, and two linear forces (in this case the repulsion between the co-linear hydrogens) can only push them away, (or at worst, attract), but if there is no initial staggering, how can you generate torque? you need d (dist b/w point of application of force and rotation point) > 0, but here its not so(1 vote)
- The conformation changes constantly because of thermal motion. The staggered conformation is not locked in only energetically favored. So if you are analyzing groups molecules for conformation you will find mostly staggered. However if you followed the evolution of a particular molecule over time you would see it "cog" between the different staggered conformations. So once it gets enough energy to push it into an eclipsed conformation, it will quickly snap back into one of the two neighboring staggered conformations to minimize the repulsion. So the only way you can ever get them to stay in the perfect alignment required for the repulsion to be axial would be to cool the molecule to absolute zero(0K).
@jayantkumarz48
It can hit any conformation naturally. But if the molecule is in eclipsed confirmation and is even slightly above 0k then it will soon wiggle, and fall back to a staggered conformation because the forces are no longer axial. A perfect eclipsed formation is an unstable equilibria like a needle standing on it's point. The slightest disturbance will cause it to tip over, and fall toward the lower energy equlibria of the staggered conformation. But even if it's in a "perfect" staggered confirmation it can get enough energy to wiggle into an eclipsed confirmation. It's just spends less time there for the reasons listed.(5 votes)
- How do you draw methane by itself?(1 vote)
- In skeletal form, the methane molecule would just be a dot. The 4 hydrogens are implied because the octet of the carbon atom must be complete. Just as ethane was represented by a single line, which indicated the presence of 2 carbons. The line implied 3 additional hydrogens attached to each carbon. There wouldn't be a Newman Projection for methane as Newman Projections discuss molecules of at least two carbons. There's no need to discuss methane in a Newman Projection because there isn't any strain energy associated with the molecule.(2 votes)
- Might be a little off topic but how do you draw methane with the line method because there is only 1 carbon? Do you just draw a dot?(1 vote)
- With the line method, there must be a C atom at the end of each line. Since methane has only one carbon, you have only a point, not a line. So you can't use the line method to draw methane. The line method starts with ethane.(2 votes)
- when we talk about molecules rotating about a s bond we know that they do so but how often do they do it?is there a measure for that.? and if it occurs too often than why don't we observe things moving as as everything is ultimately made up of molecules. Just curious!!(1 vote)
- At room temperature, most sigma bonds will be rotating many many times per second. Many things can affect this rate of rotation: steric hindrance, lower temperatures, and intermolecular H-bonding, for example, would all slow down sigma bond rotation. Despite the fact that molecules are constantly rotating and vibrating at room temperature, we can't see these movements because they are much too small for our eyes to distinguish.(2 votes)
- Why do the H's want to be as far apart from each other as possible?(1 vote)
- The electron clouds around them repel each other.
Having the H atoms as fare apart as possible minimizes the repulsions.(2 votes)
- I was always under the impression that sawhorse models included wedges so the structures could be viewed in space. Is Sal's sawhorse at the beginning acceptable also?(1 vote)
Video transcript
In the video on sp3 hybridized
orbitals, we went in pretty good detail about how a methane
molecule looks. But just as a bit of a review,
it's the tetrahedral shape. You have a carbon in the middle,
and then you would have a hydrogen-- you can
imagine I'm drawing it like this because this hydrogen is
poking out of the page. Then maybe you have another
hydrogen that's in the page. You have one above the carbon,
and you have one that's behind the page. You could imagine it's like a
tripod with a pole sticking out of the top of the tripod. Or if you were to imagine the
shape another way, if you were to connect the hydrogens, you
would have a four-sided pyramid with a triangle
as each of the sides. So it would to look something
like this. I'm trying my best to draw--
the pyramid would look something like this if you
could see through it. So this would be one side,
another side would be over here, and then the backside
would be over here, and then the fourth side is actually
the side that's transparent out front. So the fourth side would be the
actual kind of thing that we're looking through when
we look at this pyramid. It would be this front
side right over here. So you can imagine it different
ways, this was the case with methane. Now let's extend this into
a slightly more complex molecule, and that's ethane. So the way we've been drawing
it so far-- I guess the simplest way to draw ethane,
is just like that. By implication this is ethane. By implication you have a carbon
there, and a carbon there, and they'll each have
three hydrogens bonded to it. And we've drawn it something
like this. Three hydrogens bonded to
each of these guys. But now we know that carbon
has these sp3 hybridized orbitals, that it likes to form
more of a tetrahedral shape when it bonds. So an ethane molecule would
actually look more like this. Let me draw the carbons. So I'll do the carbons
in orange. So if that's the carbon
and that's the carbon. So you can imagine you have
a carbon molecule here. I'll draw it as this
little circle. And then if we have some
perspective, so the carbon-carbon bond is going
to look like that. And then you have
another carbon molecule right over there. So that's that bond over here. And we want both the carbons,
all of their bonds to be kind of in a tetrahedral shape. So then you could imagine
this bond over here going like this. This bond going like that. And you have your hydrogen
at the end. Let's make the green circles
the hydrogens. So you have that hydrogen, and
then-- or actually just the circles, we'll call them
hydrogen-- and then you can imagine this one, maybe it's
coming out of the page a little bit. That is that hydrogen. Let me label the hydrogens,
actually. I'm doing it in all different
colors so you can see what I'm talking about. And then this hydrogen is going
right below it, maybe pointed back a little bit. So that hydrogen is
right over there. So you can see this carbon,
its bonds have a tetrahedral shape. Or, if you just looked at this
part of it, these would be the base of the tripod and
this would be the thing sticking up. Now for this carbon it would
be very similar idea. This hydrogen right
here, might be sticking down like this. And I'll stop switching
colors soon enough. It takes a lot of time. That hydrogen over there, it's
pointing out in that direction like that. And then you would have,
let's see what colors do I have left? Well, I'll just do yellow. That hydrogen right
there, maybe it's pointing out like that. So this is a possible
configuration for ethane. And the way that I've drawn this
right now, and you can actually have a model that has
this, where you have little wooden sticks with balls and
this, the balls to represent the actual atoms, this is called
a ball and stick model. And this is a ball and stick
model for ethane. Now a simpler way we could
have drawn this, this is called a horse shoe projection--
or actually it's a sawhorse production. I always say horse shoe. A sawhorse projection. It would look like this. This exact same configuration
of ethane in a sawhorse projection. And you know what a sawhorse
looks like, well, it looks like what I'm about to draw. It looks like this. So you could, well I could
draw it exactly the way I drew it here. So you have the carbon, carbon,
and then you have the hydrogen, hydrogen, hydrogen,
and then you have a-- well, the way I've drawn it up
here is more like this. Just so we see the parallel
and then we can rotate things around. The way I drew it up here,
you have a hydrogen, hydrogen, and hydrogen. And then over here you
have a hydrogen, hydrogen, and hydrogen. This is a sawhorse projection. Now, either way you depict
it, I mean, these are really the same way. This is kind of just like
the lazier way of doing it on some level. You're not drawing all of these
circles and all of that, and you're putting a little
less care into actually showing the angle, how things
are angled away from the carbon and showing the
tetrahedral shape. But in either case, when you
start visualizing the molecule in this way, you start to
realize there's, well, there's actually infinite ways that
these things can be configured. And it all comes from the notion
that this is just a sigma bond right here. We learned that in the video on
sp3 hybridization and sigma and pi bonds. This is just a sigma bond. And so we can rotate
around the bonds. One of these carbons could
rotate around the axis of that bond without the other
carbon having to necessarily rotate with it. If this was a double bond, if
this was a pi bond, they would have to rotate together. So you could have a situation
like I've drawn here or you could have a situation where
they're kind of rotated the inverse of each other. And this is what I mean. So I'll do a ball and stick. So let's say this is our front
carbon, that is our back carbon, and we'll compare it
to this one over here. So let me draw this guy
the exact same way. So he's got a hydrogen down
here, he's got a hydrogen up there, and then he's got
a hydrogen up here. So that part of the ethane
looks identical. Now what I'm going to do is I'm
going to flip the other side of the ethane. I want you to pay close
attention, because hopefully you'll see the difference
between the two. So instead of doing this blue
ethane down here-- this blue hydrogen down here-- I'm
going to do it on top. So this blue one, I want to
do that in blue, this blue hydrogen, put it on top. So I'm just rotating
this around. So the blue hydrogen's on top. So I've rotated it so the
blue one's on top now. And now the green one is
going to go over here. So now the green hydrogen
is now over here. And then this purple, or this
magenta hydrogen, the way I've rotated it, is now going
to go over here. So what's the difference between
this configuration and this configuration right here? And we could have
had every other configuration in between. But what's the real
difference here? Well, here the hydrogen, you
could imagine that hydrogen is kind of, if you were looking
from that direction, that hydrogen is directly behind that
hydrogen, that hydrogen is directly behind that
hydrogen, that hydrogen is directly behind that. This is called an eclipsed
configuration, or eclipsed conformation. So eclipsed conformation. And this right here, nothing
is behind anything. If you went straight
back from this guy, you'd get to this point. And no one's behind it. And if you went straight forward
from this guy you'll get-- So in no way are any of
the guys in the back-- once again, if your viewing from
this direction-- are they blocked by any of
the people here. So we call this a staggered
conformation. Now, why do we even care? OK, I can twist around
this back molecule. What's that even going to do for
our actual-- why does it even matter? Well, one, it's just interesting
that you can actually change, that this thing
can twist around without changing the front-- without the
front molecule having to twist with it. But even more important, these
have different energy levels, so you can kind of think of
them as you're kind of twisting a spring and the spring
might want to go back to one conformation
or another. And to visualize it a little
better, I'll draw what's called a Newman Projection. So I'm going to draw this exact
thing, but with a Newman Projection, you draw the carbon
molecules directly in front or directly behind
each other. So in this situation, you would
draw the carbon molecule in front, would just be the
intersection of these bonds. So for a Newman Projection--
let me draw that out. So it's a Newman Projection and
I'll start with the Newman Projection for the staggered
conformation. So in the front-- we'll consider
this carbon the front carbon-- we have our
hydrogen pointing straight down like that. And we have a hydrogen
coming out to the top left, like that. And then we have this
hydrogen over here coming up to the right. I want to do that in
that same color. So the front carbon is
implicitly at the intersection of the bonds of these
three hydrogens. And then the back carbon-- I
said the front carbon is the intersection of the bonds of
these three hydrogens-- the back carbon you represented
as a circle. So the circle represents
the back of carbon. The front carbon is kind
of that point there. This is a way of
visualizing it. But if we were to draw it this
way, the back carbon now has that blue hydrogen popping
off of it. So it has that blue hydrogen,
this green hydrogen, and then this magenta hydrogen. And when you look at it like
this, it is more clear that it's staggered. We're just looking straight on
to this ethane molecule. When we look straight on, the
front carbon, they're obviously blocking each other. But this way you can see the
front carbon's hydrogens are staggered relative
to the back ones. So this is right here, once
again, this is staggered. Now let's draw the eclipsed
conformation as a Newman Projection. So as a Newman Projection
the front's going to look the same. You have a hydrogen there, you
have this hydrogen, you have that hydrogen, and then you
have that blue, or I guess that purple hydrogen,
down here. So that's the front of it. But the back, they're
right behind it's. So let me draw the
back carbon. The front carbon is kind of
represented by just that dot. The back carbon we'll
represent like that. And the staggered conformation,
if this guy's really behind that, you're going
to have to draw it like, right there, like
right behind it. But since that's a little bit
messy, normally when people draw a staggered projection-- an
eclipsed conformation, as a Newman Projection. Instead of directly eclipsing
that last, that back hydrogen, they'll put it to the right a
little, or they'll push it off a little bit. So that's that hydrogen. That magenta hydrogen
is right there. It's really right behind the
front one, but this is just so you can actually see
that it's there. And then finally you have this
blue one down there. So that blue one is going
to be right over here. So this is the eclipsed
conformation as a Newman Projection. And as you can see,
it's eclipsed. The back hydrogens are eclipsed
by the front ones. If I were to draw it perfectly,
they would be right behind it. Now there's one other piece of,
I guess one more idea, I want to introduce you to, and
that's the notion of the angle between the different
hydrogens. So if you wanted to say, well,
what is this angle? What is this angle between the
blue hydrogen and this pink hydrogen right here? Now, when you actually think
of it in three dimensions, it's like, wow, you can't really
say the angle over here between blue and the pink. But on a Newman Projection, when
you're just saying how much are they rotated away from
each other, this angle right here is called
a dihedral angle. Sometimes it's just said, you
know, this hydrogen relative to that hydrogen has a DA of,
in this case, 60 degrees. In this case, this hydrogen
relative to that hydrogen has a dihedral angle of
zero degrees. And it's a way of saying,
how staggered, or how eclipsed, you are. Now, one last thing I touched on
the idea, so again, why do we even care? Well, all of these hydrogens
have electron clouds around them, and all of these
bonds have electron clouds around them. And electron clouds, they're all
negative, so they want to get as far away from each
other as possible. And they're all stable now,
because they've bonded in ways that they have nice
stable structures. Everyone feels like they
have full valence shells, full orbitals. And so the electron
clouds want to get away from each other. Now in this situation, in the
eclipsed conformation, this hydrogen and this hydrogen--
let me do it in-- this hydrogen and this hydrogen are
closer to each other than when you go to the staggered
conformation. The staggered conformation, the
closest hydrogen to this guy is going to be either that
hydrogen or that hydrogen. But they're both further away
than this hydrogen was in the eclipsed conformation. So in general, the staggered
conformation is going to be more stable. It's going to have a lower
potential energy. You can imagine that if you
start with an eclipsed conformation, these guys are all
going to want to get away from each other. So it's kind of like this is
the wound conformation. It has higher potential
energy. So it'll want to unwind and
it'll want to unwind to a staggered conformation, because
in this conformation, all of the hydrogens
have gotten as far away from each other.