Leah here from leah4sci.com and in this video we're going to look at the chair conformations
for cyclohexane.
When you hear the word cyclohexane, you think of hexagon.
‘Cyclo’ is a ring, ‘hex’ is six and ‘ane’ tells us there's only single bonds.
And here we have one, two, three, four, five, six.
The problem with this structure is that all the carbon atoms are sp3 hybridized.
This means that carbon prefers to be tetrahedral with an ideal bond angle of approximately
109.5 degrees.
But if you look at this structure, a planar hexagon has angles that are much, much bigger
than 109.5, so what does that do for the structure?
Let's take a look at the model kit.
You can follow along for getting your own kit at leah4sci.com/kit.
Here we have cyclohexane where each of these represents a carbon atom with four bonds.
So we have two going to the ring and two hydrogen atoms.
If you look at the top down view, it kinda looks like hexagon but, take a look at what
happens when I try to flatten it out.
You see how that looks like the kit is about to break?
This molecule is very, very unstable and if I leave it as a flat hexagon, it will break.
Carbon atoms will compensate for this but trying to readopt that 109.5 configuration
as follows.
We'll take the nearby carbons and bend them: alternating up and down.
Now looks like a nice and comfortable conformation.
Take a look.
Every other carbon: one is up, one is down.
Up, down.
But all the carbon atoms appear to have that comfortable 109.5 configuration making it
much more stable.
In fact, cyclohexane is the most stable cycloalkane because it can adopt this configuration.
I have seen chair conformations represented in some ridiculous ways that tell us nothing
about what's going on, where these substituents are located, what atom is up or down.
That doesn't work.
Instead, let's see how to take our model kit and put it onto two dimensional paper using
the drawing chair conformations tutorial link below.
We'll start with a simple parallel lines for the chair, add the carbon that is facing up,
the carbon that is facing down, and then we'll go ahead and add our substituents.
I’ve added color substituents that help you follow along.
Notice that I have red in the up position and blue in the down and this is just to differentiate
between the hydrogen atoms.
Why is this called a “chair conformation?”
Well if I line it up like this, I want you to imagine a beach chair.
This is where you put your head, this is where you put your body, this is where you relax
your legs, and you just lay down and relax on this chair conformation.
Studying Organic Chemistry, of course!
Now let's see this in two dimensions on the screen.
I've matched up the colors to help you see clearly what is going up, what is going down,
and where everything faces.
Now that we have it on the kit and in the drawing, let's try to understand exactly what
it is that we're looking at.
There are four things you need to recognize when looking at a chair conformation, to help
you understand everything that's about to follow from stability, interactions, and even
what's going to happen in your reaction mechanisms.
The first two are simple, if I take this cyclohexane and I look at it from top down or look at
it from the side, I want to clearly distinguish between the substituents that are facing up
and out of the page compared to the substituents that are facing down or going into the page.
We've shown this on the kit with red hydrogens facing up and blue hydrogens facing down.
And we'll show this on the hexagon where the red hydrogens are on a wedge coming out on
the page and the blue hydrogens are on dashes going into the page.
That was the easy part.
A little more difficult to understand is the nature of the substituents; particularly those
that are axial versus equatorial.
We'll represent this on the kit, green is axial and white equatorial, but I'll draw
it in grey because white won't show up on the screen.
Here we have another chair conformation with green and white substituents.
But you'll notice that here I alternated green, white, green, white compared to the previous
one which had all the red going up and all the blue going down.
And that's because we're looking at the specific orientation of each substituent.
If you'll look at this from the chair conformation perspective, you'll notice that all of the
green substituents are pointing straight up and down.
In fact, if we plot this on a graph, they're approximately on the y-axis.
Given that they're y-axis substituents, we call this axial, and the axial substituents,
if drawn correctly, should be pointing straight up or straight down.
If you look at the from the top view, the green substituents are pointing straight up
and straight down at you.
The white substituents don't really have a specific direction, they appeared to be slightly
angled up and slightly angled down.
But if you imagine this chair as a globe, the chair component would be the middle of
the circle and all of the white substituents are pointing outward.
So do you see how they perfectly surround the perimeter or the equator?
If we look at it from the side and KNOW that it's going towards the outside, this is what
we call the equatorial substituents because they're pointing towards the equator.
They're not straight up, they're not straight down, you should draw them with a slightly
angled line, and that represents equatorial.
One point of confusion comes up with which direction to put your equatorial substituents.
If you're drawing the chair like this, this one is pointing down and it's towards the
left.
This one here is pointing up but it's towards the right.
What do you do with the substituents that are pointing straight at you?
Notice this one right here is directly at you?
Well you can't draw a straight line outwards, so what you do is you tilt it slightly to
the right so you draw it to the right or you can tilt it slightly to the left and draw
it to the left.
I've seen drawings where people say, wait that's incorrect, it should be right, it should
be left.
And the answer is, not necessarily.
It's pointing straight at you, but because we can't show that on paper, we tilt it slightly
to one direction or slightly to the other direction and both are correct.
Students who follow my drawing tutorial, do not lose points for careless mistakes because
it's very easy to see on their drawings what is up, what is down, what is axial, and what
is equatorial.
And on your exams, that is exactly what your professor is looking for.
Let's see what this looks like on paper, starting with the simple chair skeleton.
We'll add the axial substituents and I want you to recognize the pattern.
Remember on the kit, I showed you that the green and white were alternating because the
axial went up, down, up, down.
Well we have the same pattern here, up, down, up, down, up, and down.
The white substituents we'll show in gray, and we'll start with this one going down but
it's equatorial so it's not an axis, a straight up and down, but it's slightly down, and this
one here is slightly up.
This is the one that comes straight out at you, so if we look at it slightly from one
side or slightly from the other, you just slightly offset it just a little bit so it
doesn't look like it's straight down and you're good to go.
This one is to the left, and again to the left, one is up, one is down.
And here, just like the one coming forward was straight at you, this one’s going straight
back.
But since we can't show that, we offset it slightly to one side.
Some professors will draw it on that side, same thing.
You're simply representing an equatorial substituent behind the ring pointing far away into the
equator.
Why is it so important to understand the relationship between substituents?
In chemistry, every reaction is all about the stability.
If a molecule is happy, it'll be stable and unreactive.
If it is unhappy, it'll be unstable and very reactive.
So we need to understand the interactions between the cyclohexane substituents to determine
what is considered more stable and what is considered less stable.
Let's look on paper first.
Substituents that are in the axial position are relatively close to each other.
They're pointing in the same direction.
They can get in each other's Van der Waals’ space or personal space and that makes them
very unhappy.
In Organic Chemistry, if you're trying to understand the concept, relate it to a human
characteristic to help it make sense.
I want you to imagine that you're speaking to someone, but they're very, very close.
Unless they're someone you know and you're comfortable enough getting very close giving
them a hug, you don't want strangers up in your personal space.
And if someone's talking to you that closely, you'll either take a step back or push them
away.
And if you're too shy to do that you'll gonna feel very very tense and uncomfortable the
entire time.
Axial substituents, if they're really big, will get in each other's personal space making
them very, very uncomfortable.
Let's take a look the model kit.
Looking at the axial substituents, they don't appear too unfriendly.
This hydrogen and this hydrogen are relatively small and they're almost in each other's personal
space.
But, atoms have to be next to each other, so they'd rather have something small.
You'd feel much more comfortable if a little kid stands next to you than if a big, fat,
adult starts talking in your face, right?
What happens if I swap that hydrogen for a methyl group?
This carbon here has one, two, three, hydrogens and now, the atoms are much closer to each
other and they're starting to encroach on each other's personal space.
What if I add a second methyl group: once again carbon with three hydrogens.
Now, look how close they are.
They're definitely getting into each other's personal space.
They're not comfortable enough to tell the other one to back up so the entire molecule
will bend, giving it more space.
They're too close?
It'll slightly bend out of the way.
This is causing strain on the molecule and this is what adds to the instability.
Now I've added two more CH3 groups to our carbon, making it an isopropyl substituent:
three carbons bound at the second carbon to the parent chain.
And now look what happens.
It looks pretty innocent and unassuming but carbon bonds can rotate.
And when this thing rotates, look at that!
Definitely in each other's personal space.
Makes it very, very uncomfortable and very, very unstable.
Axial substituents will have these diaxial interactions and this is what you want to
avoid at all costs.
I’ve taken this same molecule but moved the substituents to the equatorial positions.
Notice here we have the methyl group and here we have the isopropyl group.
They're still a little bit close to each other, but the interactions are not as bad as they
were before.
And that's because they're in the equatorial position.
The key to recognize here is when a substituent is equatorial it points out to the middle
of nowhere.
It's not pointing into the face of another axial substituent but instead, it points out
to nowhere.
There's much less going on there.
If you think back to Newman projections tutorial series linked below, you'll see that we do
have some gauge interactions going on here but it's much, much less unfavorable compared
to the diaxial interactions that we saw earlier.
What does this mean?
Axial substituents are highly unfavored, equatorial substituents are highly favored.
And to draw a stable molecule, your goal is to put as many of the big and bulky substituents
in the equatorial position, where the bulkier it is, the more likely it wants to be equatorial
and the rest can be axial.
Because they'll be less unfavorable.
Here we have the two molecules on the screen and they're not the same structure.
I did not convert one into the other.
But on the kit, I actually broke off bonds and moved things around.
Notice that if this is carbon one, then the methyl is on carbon three.
Here the methyl is on carbon two.
What I want to show you here, is how in the first structure, where the isopropyl and the
methyl are both axial.
We have that diaxial interaction, that invasion of personal space, that makes it very unstable.
In the second structure, it almost looks like the two groups are closer to each other, so
it looks like it should be less stable.
Think back to what we saw on the model kit and if you're not comfortable rewind the video
and look again.
In the diaxial interactions, they were right on top of each other, they were too close.
In the equatorial, even though there are neighboring carbons, notice I brought the groups closer
together but they were facing out in different directions on the equator.
This one right here is pointing down and towards the back.
This one right here is pointing up coming a little forward, and that means they're not
that close to each other, they're not in each other's personal space and that makes it much,
much, much, more stable.
The fact that one is cis and one is trans has nothing to do here, because we're simply
focusing on the diaxial versus the equatorial interactions.
But how do you take a molecule that has diaxial interactions and make it more stable to put
the substituents equatorial without breaking bonds like I did?
That happens through the inter-conversion or ring flip of the chair conformation and
that's exactly what we'll look at in the next video.
You can find the entire chair conformation tutorial series along with the chair conformation
practice quiz on my website Leah4sci.com/chairs.