Leah here, from leah4sci.com and in this video we'll look at the ring flips or interconversion
between cyclohexane chair conformations as well as the boat intermediate.
In the last video we learned about the basics of the chair conformation as well as the concept
of stability when it comes to axial versus equatorial substituents.
If i'm given a molecule as a flat hexagon, I have an option of drawing two different
chair conformations, if you're not comfortable with this drawing style follow along with
the drawing tutorial link below.
But the key here is to not even pay attention to the molecule when you're drawing your chairs,
we'll start by drawing two different versions of the chair and then place the methyl group
in the up position.
I know that it's up because methyl is on a wedge and a wedge is coming up and out of
the page directly at you, but the question is "Do I prefer to put it as an axial up or
equatorial up?".
Both are technically correct so which one am I going to see for methylcyclohexane?
What's the difference between these two?
In the first one, the methyl substituent is axial, the second one has the methyl equatorial
and since axial is less stable than equatorial, equatorial would be the more favored structure.
Does that mean that the molecule will exist as an equatorial substituent at all times?
Is this the chair conformation we're going to see in nature?
When would we see this kind of conformation, and the answer is that we're going to see
both.
The chair structure can occur one way or the other at random and they'll actually exists
in some sort of equilibrium because the chair will flip from one version to the other.
Ring flips are tricky if you simply memorize what's going on, so let's switch over to the
model kit and understand the nature of the interconversion, understand what is happening
with that ring flip and then I'll show you quickly do it on paper.
Here we have cyclohexane and a random chair conformation and as a quick reminder, the
red substituents are what we see going up and blue are what we see going down.
I want you to pay special attention to how these move when we do the ring flip.
In order to do a ring flip, you start at any point in the molecule, you can start with
any of the carbons that are up, you could start with carbons that are down, it doesn't
matter.
Pick a convention and stick with it so that it make sense.
Take one carbon and I'll start with the one that's pointing up and flip it down, then
take the opposite carbons so if this is carbon one, that would be two, three, and four, and
flip that one up.
This was initially an axial carbon that became equatorial and this was initially an equatorial
carbon that is now axial, now this one is down, this one went up, this one went down,
this one went up.
Everything changed it's position from this ring flip so let's do that one more time.
Now that this one is up, we're going to bend it so it goes down, carbon on the opposite
end, bend it and it goes up.
Now let's take a look at what happened, notice that the red substituents started out up and
no matter how I flip this ring, the red substituents will always stay in the up position.
Up will always stay up.
Look at the blue substituents, no matter how many times I flip it, they're all in the down
position.
Substituents that start at going down are going to remain going down, so what is it
that changes in a chair conformation ring flip?
Let's take a look at another chair and you'll remember this one from the last video, because
it has the green substituents are all axial and the white are all equatorial they are
outside to the equator, watch what happens when I do a ring flip here.
We'll start with this carbon that's up and drag it down, we'll start with this carbon
that's down and drag it up, look what happened, the green substituents are no longer axial,
every single one became equatorial, the white substituents that were pointing to the equator
are now all axial pointing up and down and the green ones are equatorial so even though
up stays up and down stays down, substituents that start out axial, will go equatorial after
a ring flip.
Substituents that start out equitorial, will go axial after a ring flip.
Now that you what changes and what stays the same, let's take a look at the steps within
that ring flip transition.
The first thing we do is take a substituent that is up or down and bring it to the opposite
direction, but as we bend it upward, watch what happens.
This thing now is flat, it's not up, it's not down, it's not a chair anymore, this part
here, this is still a chair.
This part is not a chair so we call this the "Half-Chair", it has to go through the half
chair on it's way up, but on it's way up it tends to bend to the side, because this confirmation
here, has substituents getting in each other's way so it slightly bends to the side.
This is called a "Twist Boat" but then it has to untwist and get into this conformation
which kind of looks like a canoe, this is called the "Boat" conformation.
With the boat conformation you can start twisting the other direction so once again we get a
twist boat as this starts to straighten out into a half-chair and goes down for the other
chair conformation.
Again, we have the chair, half-chair, twist boat, boat, twist boat again, half-chair,
and a chair conformation.
Let's see what this looks like on paper.
We start out with a stable chair conformation and relatively low energy, remember cyclohexane
is most stable in a chair conformation, but in order to do a ring flip, we drag up this
carbon.
That means the left half of the molecule is still going to look like a chair.
This gives us the half chair because the left side is flat, it's not up, it's not down,
it's half a chair, half a flat unstable thing.
It's unstable because these carbons have very undesirable bond angles and that makes the
molecules unhappy, it wants to get out of that position as quickly as it can.
In transitioning to a boat conformation we have a twist boat, which is so much tricky
to draw but you basically do an X and then add the two terminal carbons.
So these two lines, that means these 4 carbons are represented here on the x and then these
two carbons would be these two.
The twist boat is more stable than the boat because the two hydrogens that were facing
each other on the boat conformation are not as close as the boat itself but it's still
relatively unstable.
So we'll draw it lower than a half chair but definitely higher than the chair which is
the most stable structure.
In order to transition into the other version of the chair we have to undergo the unstable
boat conformation which starts to look like a chair again except that both sides have
their substituents facing up.
If we redraw it like this, you can start to envision a canoe or a boat.
Why a boat conformations so much less stable than chairs?
well look at what we have here.
Most of the carbons seems pretty happy, the bond angles look pretty good.
But, at the very top of the boat, these two carbons here, we have hydrogen atoms that
are very close to each other, these are called the flagpole hydrogens and they’re kind
of getting in each other's space.
When atoms get to close to each other, when they invade personal space it’s considered
very, very unstable.
This is why you get the twist boat because the flagpole hydrogens want to go away from
each other.
It's not as visible here but take a look from the top, they're very close to each other,
very unstable, twist the boat and now they have some space.
Twist the boat, they have some space.
This is what makes them more stable but ultimately the molecules waiting for this.
When you go into a full chair conformation, everything is relatively far away and so much
more stable.
Now that we have a boat we can start transitioning into the other chair conformation.
Once again we do the twist boat, this time we do the X in the opposite direction.
I know it's not fully visible here but if you had substituents you could start showing
axial going equatorial and equatorial going axial.
How do you know what to draw for the final chair?
well watch the progress.
This carbon here was dragged up so that it's in the up position, that's right here.
That means we have to start dragging this carbon, this carbon down.
So, we re-draw the chair as we saw initially, but now the carbon on the right is in the
up position, the carbon on the left that is on its way down is now planar and this gives
us another half chair which allows us to continue dragging that carbon all the way down for
that final and very stable opposite chair conformation.
Why did I draw them this way on the screen?
If you had to show this as an energy diagram, you start out with a very stable chair and
raise the energy all the way to the half chair.
It goes down slightly for the twist boat, higher in energy for the boat and click down
for the twist boat so it can go all the way up to the half chair so it can go all the
way down to the stable chair conformation.
Let's apply this to the initial molecule that we saw.
It's natural to look at the molecule, draw a chair, analyze a chair, and attempt to flip
it.
But now that you understand what's going on, let's switch over the model kit and apply
the ring flip to our initial molecule.
We were given a methyl cyclo hexane which can be drawn as hexagon with the methyl or
written as either of two chair conformations.
How do we do the ring flip?
I'd like to put my substituents on the up corner, you can put it wherever you want.
Remember this is a ring so you can go round and round and round put it wherever you want
as long as you stay consistent.
We have the methyl group here in the chair conformation with the up position.
So we simply take that methyl, drag it down, take the other end of the chair and drag it
up.
All the intermediates were there, so we went from chair, half chair, twist boat, boat,
twist boat again, other half chair and chair.
But in short, like that one up, one down.
Try this, it's fun.
Once you understand what is going to happen, in other words, the methyl group starts out
in the up position.
the methyl group ends up in the up position.
The only thing that changed is having the axial, first is equatorial.
Once you get that you don't have to worry about it when you're drawing making the entire
ring flip process so much easier.
How so?
If we're trying to figure out what to draw, we don't even need our initial molecule to
figure out what boat chair conformations will look like.
What you do instead is just draw two chair conformations skeletons so that the parallel
lines are opposite in nature.
For the first structure, I have left over right.
For the second structure I’ll have right over left.
Again this is taught in the drawing chairs tutorial linked below.
Here's the best part.
Take your cyclohexane and number it so that you know where the substituents are located.
We only have a methyl so it's pretty easy put a number 1.
Anywhere on this chair conformation, put a number 1.
So we put the 1 here.
Now ask yourself, in order to do a ring flip, what happened?
Well, I chose to take that carbon number 1 and drag it down or you can start by taking
carbon number 4 and dragging it up.
It doesn't matter as long as you're consistent.
If carbon 1 came down, this is that same carbon number 1.
Now that I know what's what, I simply ask myself, what substituent is where?
Carbon number 1 has a methyl in the up position.
Up position on the structure has to be axial, up position on this structure has to be equatorial
because the other substituent is down in this case equatorial, here it's axial.
If we had a second substituent, you would do the same thing.
For example, if we have another methyl here also in the up position.
I would continue numbering two and three clockwise.
This is important, I don't care where you start, I don't care where you continue.
But if your hexagon is clockwise, your chairs have to be clockwise.
If the hexagon is counter, the chairs have to be counter.
We're going clockwise so we’ll go 2,3, so we'll draw 2, 3 and stop.
We don't need to number all the way through 6.
1,2,3, in a clockwise direction.
Where is methyl on carbon 3?
The up position, once again we have a wedge is axial on the left structure and equatorial
on the right structure.
And since we have a gray line, let's put a hydrogen so you don't get confused with the
methyl groups and there we have it a quick and simple ring flip that starts out by simply
drawing both chairs figuring out where your substituents are located and drawing them
onto the molecule.
Once again let's figure out which one is more stable.
On the left we have two substituents that are axial.
On the right we have 2 substituents that are equatorial.
And since we know the axial substituents will have unfavorable diaxial interactions, we
know the left structure is less stable and the right structure is more stable.
One last thing I wanna share with chair conformations.
Structures will typically exist in equilibrium with each other.
If we go back to the previous example, when there is one methyl group, the equilibrium
will tend to favor equatorial, slightly more than axial.
When there are two methyl groups, equilibrium will favor this one much more because the
more the difference between stability, the more time the molecule will spend in the more
stable position.
Two equatorial is so much more stable so the molecule will tend to favor this structure
but it'll still go back and forth, you will still see that ring happening again and again
. But if you have a molecule that has a tert-butyl substituent which is a very very big bulky
group, that's a C (CH3)3.
It's like carrying a giant umbrella.
This molecule is going to spend 99% of it’s time where the tert-butyl group is in the
equatorial substituent.
You can think of this as a tert-butyl group locks the ring equatorial and that means anything
you have is going to remain locked in that position.
Keep this in mind when you get to reactions especially,especially when you're doing E2
reactions on cyclohexane.
Ready to try some practice problems?
You can find the entire chair conformation tutorial series along with the practice quiz
on my website leah4sci.com/chairs.