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We've seen that bulky substituents prefer
to be in the equatorial position versus the axial position
in our chair conformation.
Sometimes you need to actually quantify that energy difference
with numbers, especially for complicated
poly-substituted cyclohexanes.
And so here's a chart that shows you
an estimation of the difference in energy
between axial and equatorial.
So a chloro group prefers to be in the equatorial position
by two kilojoules per mole.
And OH group 4.2, methyl group, 7.6, and ethyl group,
8, and isopropyl group, 9.2, and the bulkiest group,
a tertbutal alkyl group, 22.8.
So the tertbutal group much prefers
to be in the equatorial position by 22.8 kilojoules per mole,
since it is the bulkiest set of all the ones here.
Let's look at an example of a tri-substituted cyclohexane
And let's see if we can figure out
which chair conformation is the most stable.
So let's start by drawing our cyclohexane ring.
And this molecule is called neomenthyl chloride.
So neomenthyl chloride has an isopropyl group coming out
at us, a carbon 1, a chloro group coming out at us
carbon 2.
And a methyl group going away from us at carbon 4.
So this is carbon 1, 2, 3, 4, 5, and 6.
So that is
If I wanted to draw a chair conformation
for neomenthyl chloride it's I go ahead and very carefully
draw my chair conformation.
Then I go up.
Down, up, down, up, down.
And do my equatorial, so down, up, down, up, down, and up.
So let's go ahead and draw the other chair.
So very slowly you draw your or other chair.
So it looks something like that.
So this is now carbon 1.
So we go down, up, down, up, down, and up.
And then the equatorial one, so up, down, up, down, up,
and down.
Let's go ahead and put our groups in.
So at carbon 1 for neomenthyl chloride,
we have an isopropyl group up.
So here's carbon 1 on my first chair conformation.
So an isopropyl group up, like that.
And at carbon 2, we have chlorine up.
So here's carbon 2.
So we're going to put a chlorine up like that.
And at carbon 4, we have a methyl group.
So over here is carbon 4.
The methyl group needs to be down relative
to the plane of the ring.
So there's my methyl group going down.
When this chair conformation undergoes a ring inversion,
and when it undergoes a ring flipping, the isopropyl group
at carbon 1-- this is now carbon 1
on my second chair conformation.
The isopropyl group was up an axial.
So it's going to end up up an equatorial like that.
At carbon 2, this is carbon 2, we
had a chlorine that was up an equatorial.
So the chlorine is going to end up
an axial after the ring flipping.
See the previous few videos for how to do ring flipping.
And at carbon 4, we had a methyl group which was down an axial.
So the methyl group ends up down an equatorial.
So now we have two chair conformations.
And it's not immediately obvious which chair conformation
is the most stable.
To figure that out, we need to check out our table here.
So if we start with the isopropyl group on the chair
conformation on the left, it's axial on the chair conformation
on the right, it's equatorial.
And we know there's an energy difference associated
with that change in position of the isopropyl group.
That energy difference is 9.2 kilojoules per mole.
So the conformation on the left has
an additional 9.2 kilojoules per mole of strain
by having that isopropyl group axial.
Let's look next at chlorine so here
I have chlorine equatorial on the left.
And then here I have chlorine axial on the right.
So we know that the chlorine prefers to be equatorial.
Let's see how much strain it would give us to put it axial.
So what is the energy difference?
It's 2.0.
So for the molecule-- for the conformation,
I should say on the right, having that chlorine
axial introduces another two kilojoules per mole of strain.
Finally, the methyl group.
Here I have the methyl group axial on the left.
On the right, it's equatorial.
So, again, it prefers to be equatorial.
So putting it axial is going to introduce some extra strain.
How much strain by putting that methyl group axial?
7.6 kilojoules per mole.
So the conformation on the left has
9.2 from the isopropyl group and 7.6 from the methyl group.
So obviously, the conformation on the left
is much higher in energy than the conformation on the right.
So the conformation on the right,
is the most stable conformation.
The molecule will spend most of its time
in the conformation on the right.
So the chlorine is axial.
So for neomenthyl chloride most of the time
the molecule is spent in the conformation
on the right with the chlorine axial.
And that can be important for chemical reactions, which
we'll talk about in a few minutes.
Let's compare neomenthyl chloride to menthyl chloride.
So they're very similar in terms of dot structure.
So let's go ahead and draw the cyclohexane ring
for menthyl chlorides.
Menthyl chloride also has an isopropyl group
coming out at you at carbon 1.
At carbon 2, the chlorine is now going away from us.
And at carbon 4, the methyl group is going away from us.
So the only difference between menthyl chloride and neomenthyl
chloride is at carbon 2.
So at carbon 2, one has the chlorine going out at you.
And the neomenthyl chloride and for this one,
for neomenthyl chloride the chlorine is going away from us.
So everything else is the same.
So let's go ahead and draw our chair conformations
for menthyl chloride.
So here's my chair conformation.
So here we go up, and down, up, down, up, down.
Equatorial also down, up, down, up, down, and up.
Let's go ahead and do the other one now.
So we very carefully get our cyclohexane ring.
And then this is now carbon 1.
So down, up, down, up, down, up, and then up, and down,
up, and down, up, and down.
So now we have our two chairs.
Let's focus in on carbon 1 where my isopropyl group
is coming out at me.
It's going up relative to the plane.
So my isopropyl group is going up at carbon 1 right here.
At carbon 2, my chlorine is going down.
So here's carbon 2.
My carbon is going down.
That must mean the chlorine is axial, like that.
And at carbon 3, my methyl group is going down-- sorry,
carbon 4-- my methyl group is going down.
So at carbon 4, right here, my methyl group
must be going down.
When this conformation undergoes ring flipping,
the isopropyl group at carbon 1 will now go here.
It was up an axial, so now it's going
to go up an equatorial at carbon 1.
My chlorine was down an axial at carbon 2.
So when it undergoes ring flipping,
it's going to be down an equatorial.
And my methyl group at carbon 4--
so here is carbon 4, my second chair conformation.
My methyl group was down an axial,
so now it's going to be down an equatorial.
So I'll go ahead and fit my methyl group in there.
Which one of these two conformations
is the most stable?
Well, once again, we start with our isopropyl group.
On the left, it's axial, on the right, it's equatorial.
Having the isopropyl group axial introduces--
we've already seen that's going to introduce
9.2 kilojoules per mole of extra strain.
So we have 9.2 kilojoules additional energy on the left.
The chlorine on the left, it's axial.
On the right, it's equatorial.
So, once again, the one on the left
is going to get an additional strain
of 2 kilojoules per mole.
So we're going to go ahead and add 2 kilojoules.
And finally, we have a methyl group axial and a methyl group
equatorial.
So, once again, the axial methyl group
is going to introduce 7.6 of additional strain.
So obviously, the molecule, the conformation on the left
is much higher in energy than the conformation on the right.
So the conformation on the right is going to be the most stable.
All of your groups are equatorial.
So menthyl chloride is going to spend most of its time
in this conformation, most of its time
with the chlorine equatorial.
And in a reaction that we will cover in future videos,
the chlorine atom has to be axial in order for the reaction
to occur.
So because menthyl chloride spends
most of its time in this conformation,
it's not going to react as fast via that mechanism
because most of the time the chlorine is spent equatorial.
Only a small portion is spent with the chlorine axial.
That's in contrast to the previous example
of neomenthyl chloride where neomenthyl chloride spends
most of its time with the chlorine in the axial position.
So for this reaction, since neomenthyl chloride
spends most of its time with the chlorine in the axial position,
it's actually easiest for this mechanism to proceed.
And so the mechanism will proceed
and the reaction will proceed much faster
for neomenthyl chloride than for menthyl chloride
by a factor of about 200.
So this is why we have to understand conformations
in order to understand how some reactions occur.