Main content
Course: Organic chemistry > Unit 3
Lesson 4: Conformations of cycloalkanesChair and boat shapes for cyclohexane
Chair and Boat Shapes for Cyclohexane. Created by Sal Khan.
Want to join the conversation?
The chair configuration is only for cyclohexane or it can happen in others molecules?Really thanks for who answer me.(15 votes)
I've seen chair conformation defined in that way before. That is, the specific structure for cyclohexane. However, other cycloalkanes like cyclodecane, for example, also has a chair conformation. The "chair" reference is just referring to the repeating zig-zag shape across the longitudinal axis of the molecule.
Remember, configuration is a reference to the molecular make-up (i.e., constituent groups like an -OH group or a -Cl group, for example); whereas, conformation refers to the arrangement of the molecular structure in space or 3D structure. Hope this helps.(34 votes)
Are the axial hydrogens in parallel to one another?
Thanks(6 votes)- HOw do you tell if an axial or equatorial is wedged in or wedged out?(1 vote)
Will the equatorial hydrogens be perfectly parallel to those bonds..??
I mean, wont the repulsions make them tetrahedrals a little irregular .?
Or will the angles still remain 109.5??(4 votes)
In a chair conformation, the bond angles for each carbon are about 109 degrees, so the tetrahedrals are pretty close. Actually, these bonds are a bit wobbly too, enough for the carbons to wobble between boat and chair conformations on occasion. Accounting for wobble, and a tiny bit of repulsion between different molecules, the hydrogens won't be perfectly parallel but they tend to be pretty close, and it really helps to think of them as parallel when drawing these and considering the implications of cyclohexanes with things other than hydrogen attached.
It's really good to check the next video from around 3 :30 to see why they can be considered parallel.(11 votes)
What is the name of the original method used to portray the Cyclohexane molecule?00:20(4 votes)
if you mean before he draws it as tetrahedral then that's called its skeletal structure.(6 votes)
Please help me visualize this equilibrium correctly.
He says cyclohexane is in equilibrium between the two chair-shapes. Before he put the hydrogens on there, I would have said the two chair shapes were equivalent. You could get from one to the other simply by flipping the first chair in 3 dimensions. So that confused me.
Then he adds the hydrogens and says the difference between the two sides of the equation is that the axial hydrogens have become equatorial and vice versa. But if the ring itself hasn't undergone any change in shape from one side of the equilibrium equation to the other, then all that's really happening is the hydrogens are spinning around on their respective carbons.
That doesn't seem like the picture he was trying to get across, though, so I'd appreciate some direction from whoever will share it.(4 votes)- Hello Matterhacker,
You're right, in the case of an unsubstituted cyclohexane, you do simply get the same chair if you flip from one chair conformation to the other.
However, the conformation changes are very important when there are groups besides hydrogen on the carbons. If we have one methyl group (methylcyclohexane), for example, it will actually have a very large impact. Because the methyl group is much larger than a single hydrogen, it will favor being in the equatorial position to avoid large steric interactions with the other axial hydrogens when it is in the axial position (you can kind of think of steric interactions as the different atoms bumping into each other).
Because of this preference for the methyl group to be equatorial in methylcyclohexane, its equilibrium will favor the equatorial conformation; whereas the equilibrium of cyclohexane won't favor either conformation.(6 votes)
Is there a possible arrangement where the carbon atoms are in one plane and if not, why?(2 votes)
Possible maybe, but extremely unstable and thus unlikely to exist for any length of time.
To see why ask yourself these questions:
1) What is the preferred bond angle for sp³ hybridized carbon atoms?
2) What would the bond angles be in a perfectly flat hexagon?
This phenomenon is known as "angle strain" and raises the energy of (i.e. destabilizes) the flat conformation.
Does that help you answer your questions?(5 votes)
Are the two different chair shapes (11:50) stereoisomers of each other?(2 votes)
Yes, conformational isomerism is a type of steroisomerism.(3 votes)
- is chair configuration a correct term ? I think it is better to say chair conformation .
because when two compounds can convert together by rotation on single bonds , we call them conformation.
thank you for your great videos.(2 votes)- Yeah Sal made a mistake calling them configurations when in chemistry we call them conformations. They made an annotation about it at2:05.(3 votes)
what is the difference between spiro and bicyclo?(2 votes)
Bicyclic molecules have 2 rings that are joined together.
Spiro is a special case of a bicyclic molecule where 1 carbon atom joins both rings together, other bicyclic molecules have 2 carbon atoms joining the rings together.
It’s sort of hard to explain without drawing, but if you look up “spiro molecules” and “bicyclic molecules” you should see what I mean.(2 votes)
- So if the chair and boat conformations are trying to show the 3-dimensionality of the cyclohexane, then are the carbons viewed as in the same plane while the hydrogens are going up and to the side at angles to best show the tetrahedral shape of the molecule? Or is that wrong? Secondly, how do you decide if an axial hydrogen is pointing down or up? Is it just that you pick the one that will best show the geometry?(2 votes)
- So each carbon in cyclohexane has tetrahedral geometry and the chair conformation is cyclohexane trying to adopt those ideal 109.5° bond angles while still being a hexagonal ring. If we imagine a plane roughly through the center of the molecule, we have some carbons below that plane and some carbons above it. So the carbons do not exist in the same plane, some are below and others are above each other.
Each carbon will have a hydrogen above and below a horizontal axis relative to the carbon. And these hydrogens can be further divided into axial and equatorial ones; either being directly above or below, or to the side of the carbon respectively. Starting at the head rest carbon (which here in Sal's drawing we can think of as the highest carbon) the upper hydrogen will be axial and the lower hydrogen will be equatorial. So we would describe this as axial up and equatorial down.
Moving to the second carbon (either one) the combinations switch, now the upper carbon is equatorial and the lower carbon is axial. Now we would describe this as axial down and equatorial up. And we continue in this alternating order for the rest of the carbons.
When we move into a new conformation like the boat conformation then the hydrogens on the 'flap' which is moving change their axial/equatorial status. So the foot rest carbon (the lowest drawn carbon) with and axial down and equatorial up hydrogen now has its axial down turn into an equatorial down and its equatorial down turn into an axial up.
Hope that helps.(2 votes)
00:20Video transcript
For all the cyclic molecules
we've dealt with so far, we've just drawn them as rings. For example, for cyclohexane
we've literally just drawn it as a hexagon. So we've drawn cyclohexane
like that. Now, we know from the last
several videos that all the bonds for carbon don't sit
in the same plane. If we take the example
of methane, that's the simplest example. You have your carbon sitting
in the middle. You'll have kind of a hydrogen
popping out like that, another hydrogen that's in the plane
of the screen, another one that's behind the screen,
and another one that is straight up. So you kind of have this
tetrahedral structure, and in the case of methane you have
that 109.5 degree bond angles. Carbon likes to form bonds
of this shape. It won't always be
109.5 degrees. It'll be something close to
it, depending on what the different atoms or molecules
are that it is bonded to. So given that, what would a
cyclohexane molecule actually look like if we try to
visualize it in three dimensions? So to think about that, let's
think about these two bonds first. I'll try my best to
draw it in one of its three-dimensional shapes. So those bonds right there,
I will draw like that. And then this down here, in
orange, I will draw like this. And then this up here,
in magenta, I will draw like that. And then, let me see, in, in
purple, I'll do these two right over here, and I'll
draw them like this. So you have that
and like that. This hopefully makes clear that
over there is that end over there, this end over here
is this end over here, and this way that I've drawn the
cyclohexane is called a chair configuration. Chair shape. And it might be obvious. It looks like a chair. That's the back of the chair,
this is where you would sit down on the chair, and I guess
the back of your calves would go against here. Your knees would sit on it
someplace like that. That's called the chair
configuration. Now another configuration that
it could be in is called the boat configuration. And so if I were to put this
exact one in the boat configuration, if I take it
from a slightly different perspective, if I'm looking at
it, kind of, head on, it would look something like this in
the boat configuration. It would look like this. Now I want to use the purple. It would look like that. Now, the first thing you're
probably saying is, Sal, you said that the reason why it
looks like this is because carbon likes to form these kind
of tetrahedral, or this tripod shaped bonds. I don't see the tripod shaped
bonds either here or here. Let me draw that boat a little
bit, at least this end of the boat a little bit better. There you go. And you say, well I don't see
that tripod shape over there. And to see the tripod
shape, you just have to draw the hydrogens. So let me draw some
hydrogens here. So let me draw a hydrogen
here that will go straight down like this. A hydrogen that goes straight
down over here, a hydrogen that goes straight up over here,
straight up over here, straight down over here,
straight up over there. I've now drawn one hydrogen
on every carbon. And now let me draw
some hydrogens. Let me draw a hydrogen here that
goes straight up, not up really-- to the side
over here. So a hydrogen there, let me draw
a hydrogen over here that does the same thing. So those guys have
their hydrogens. A hydrogen right over here. And then, let's see, this guy
needs his hydrogens still, so he'll have a hydrogen that goes
down like that, and a hydrogen and it goes
like that. And this guy will have a
hydrogen that goes like that. And when you see it like this,
if you look at any one carbon on this molecule, if you look at
any one carbon, you can see that's forming the same
tetrahedral shape that has a tripod at every one. Over here, you have that close
to, roughly 109 degree, 110 degree angle between each of
the constituents that are bonding to the carbon. Now, I've drawn the different
hydrogens that are coming off of these carbons in different
colors, and I've done it for a purpose. The ones that are going straight
up or straight down, we call those axial hydrogens. And the ones I drew in orange
that are kind of going to the side in some level, we call
these equatorial. These are equatorial
hydrogens. And the reason why it's useful
to know that name is when we talk about the different
configurations, the different chair and boats, whether
something is equatorial or axial can change if this were to
flip up, or vice versa, and things like that. And we'll talk more about
that in the next video. And the reason why they're
called equatorial is if you think about it, and it's
sometimes hard to visualize, this bond right here is parallel
to this bond right over there. And this bond right over here
is parallel to that. It's parallel, the equatorial
bonds are parallel, to some part of the ring. So that one is parallel to
that right over there. Actually I should even, I could
even color-code that. This, well, I don't want
to use that same color. This is parallel to this and
this is parallel to that. And we could do it for all
the equatorial bonds. So for example, I don't
want to-- I'm running out of colors here. So this right here is parallel
to this, and this, and that over there. So we could keep doing
it for all of them. I could do it for the other
set right here. This guy right here
is parallel to that guy over there. I didn't quite draw it like
that, but hopefully it makes the idea clear. And I'll do one more of
these just to show what's parallel to what. This bond is parallel to that. So the ones that are parallel to
some part of the ring we're calling equatorial. And the ones that kind of jump
out of the ring, that aren't parallel to any other part
of the ring, we're calling those axial. And the way I've drawn it here,
the axials are the ones that point up and point
straight up and point straight down. We can do the same thing on
a boat configuration. Now, one question you might ask
is, well, there's these two configurations. Both of these would result in
tetrahedral type shapes at each of the carbons. In fact, let me draw
it for you. So this axial hydrogen is
pointing straight down, this one is pointing straight down. Here, this hydrogen is actually
going to point straight down because
we flipped it up. And then over here you would
have a hydrogen point straight up, and then one that's
kind of pointing down. This gives a tripod there. To have the tripod over here,
you'll have to have a hydrogen that points a little bit like
that, one that's pointing a little bit like that, along,
well, you can kind of view it along the same plane as this
guy would be parallel. It's hard to see it in
this, but he would actually parallel to that. This guy would be out like this,
and then this guy would have an axial hydrogen, and
then he would have one equatorial one just like that. So you could draw the tripod
shapes in either the chair or boat configuration. But one question is, well,
what's more stable? That's actually one of the main
points of being able to visually think about the three
dimensional structure of any of these hydrocarbons, or in
this case cyclohexane. So in this situation, we know
from past videos, that all of these carbons with their
hydrogens around them, these bonds, these have electron
clouds around them. The electron clouds are
negative, and so they want to get as far away from each
other as possible. In this chair configuration, you
have this carbon up here, the ch2 we could consider it,
has two hydrogens and is connected to the rest
of the ring. It's as far as possible from
this ch2 as possible. So in that situation, we have
a lower potential energy, or it is a more stable shape. Or more stable configuration. In the boat configuration,
this ch2 up here is much closer to this ch2, I mean,
that's really the main difference between the two. And they want to get away
from each other. They want to repel ech other. So this one will have higher
potential energy, or it will be less stable. So this is just a starting
point of how to visualize cyclic hydrocarbons and we'll
use this information in the next video to think a little
bit more about, maybe, the different chair configurations
that a molecule could have, and what could be more stable. In this situation, in the case
of just cyclohexane, the two chair configurations
are equally stable. And let me just touch
on that a second. So you have, well,
I don't have to-- actually, let me see. I won't copy and paste. I'll just redraw
the other chair configuration for this guy. Actually let me just do it
separately over here because I've made the colors
here so confusing. Let me draw two, the same
cyclohexane, but in two different chair configurations
that it could be equilibrium in. So you could have this one, you
could have this one, so this could be one chair
configuration, and I'll draw it like this. And then the same hydrocarbon
could be in-- or the same cyclohexane could be in
equilibrium with the this other chair configuration
that looks like this. Let me have a little
more space here. So it looks like this. Let me do the pink. It goes up like that,
like that. Let me make sure I'm-- no,
I want to do it actually. This pink guy goes like this. And then the blue guy is going
to be just like this. So notice, in this situation
this carbon appears kind of at the top of the chair, and this
carbon is at the bottom, and then they've flipped. But these are equally stable
configurations. But one way to think about is
all of the axial guys on this carbon here turned into
equatorial on this carbon and vice versa on the two. Let me show it to you. Let me just draw the hydrogens
on this carbon. This carbon's hydrogens has an
axial hydrogen, and has an equatorial hydrogen, whose bond
would be parallel to that just like that. And this guy would have an
equatorial hydrogen whose bond is parallel to actually
both of these guys. And an axial hydrogen. But when it flips, and I'm
just drawing those guys' hydrogens, but when this
structure flips like that, what happens? Well, this hydrogen over here
goes into this position, and this yellow hydrogen over here
goes into this position. So over here, it was
equatorial, and now it becomes axial. The same argument can
be made over here. This equatorial hydrogen, when
it flips-- when this whole blue part flips down--
now becomes axial. And this axial hydrogen, when
you flip it down, becomes equatorial. And you can actually do that
for all of the hydrogens. Over here you have an
axial hydrogen. Once you flip it, you have an
axial hydrogen, and then you have an equatorial hydrogen. When you flip it, these
two equatorial hydrogens become axial. So they become axial and then
both of these guys become equatorial. So let me do that in yellow. Both this guy and this guy
become equatorial So this and that become equatorial. They become parallel
to the other end. And you could do it for these
two hydrogens, as well. So that's another interesting
to think about. And this is really just practice
on visualizing what's going on when we when
we visualize--