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Course: Organic chemistry > Unit 3
Lesson 4: Conformations of cycloalkanesMonosubstituted cyclohexane
How to draw and analyze the two chair conformations for a monosubstituted cyclohexane.
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From6:50on, how come some percentage of the molecules will exist in the less stable conformation instead of all of them existing in the most stable conformation? Do these molecules have more energy for some reason? Or do they switch conformations freely, each molecule being in the less stable conformation, on average, 5% of the time?
Random side question: does it cost energy to change conformations?(6 votes)
You're right in that the molecules switch conformations freely. You can compare this to chemical equilibrium in a reaction; the only reason the concentrations of the reactants and products are stable is because they've reached an equilibrium, where the amount of reactants turning into products is proceeding at the same rate as products turning into reactants. The conformations are switching randomly, and when the molecule with the less stable conformation reaches about a 5% concentration, the rates of switching are equal.(4 votes)
Shouldn't the first conformation, stated to be equatorial up, be axial up? I thought that equatorial groups were parallel to some other part of the cycloalkane.(1 vote)
It is axial up. If you imagine the cyclohexane to be flat (although we know it is most stable in the chair formation), axial up/down refers to the position of the group or atom as being "above" or "below" the ring. Equatorial up/down still means that the atom or group is "above" or "below" relative to the ring, but it remains on the side of the ring, so the group or atom is equatorial (like an equator).(4 votes)
Based on the examples from these videos, I've noticed when larger groups are placed in the equatorial position, as opposed to the axial position, they tend to be more stable. Is this generally the "rule of thumb" or is it just a coincidence?(2 votes)- at the end Jay says the steric hindrance is 1,3 diaxial because of the interaction between the hydrogens of Carbon 1 and 3. There is also interaction occurring with the axial hydrogen on Carbon 5. Would this not be called 1,3,5 triaxial interaction?(2 votes)
- Why is it called 1,3 diaxial interaction and not 1,3,5 diaxial interaction? By writing 1,3, is it just implied that 5 is involved in the steric hindrance too?(1 vote)
It could potentially be called that, there's nothing wrong with that name. The idea with the given 1,3 name is that looking from the methyl group it will experience steric hindrance from those two neighboring axial hydrogens like you said. Numbering the carbons from either a clockwise or a counterclockwise direction will make both of those hydrogens attached to the number 3 carbons using one system or the other (especially if its monosubstituted). So to avoid redundancy chemists shortened it to 1,3 with the implication that the 5th axial group also experiences the same interaction.
Hope that helps.(2 votes)
Why is the cyclohexane monosubstituted?(1 vote)
Because only one hydrogen has been replaced by another group. If two hydrogens had been replaced it would be disubtituted. If three had it would be trisubstituted. Etc.(2 votes)
- What's the relation between temperature and the percentage of molecules found in the less stable chair conformation where the substituent is in the axial position? it's probably not a linear relation but what is it's overall characteristics?(1 vote)
What is steric hindrance?
Is it same as angle strain?
Or is it torsional strain?(1 vote)
Steric Hindrance - The Interaction between individual groups (e.g in this video between CH3 & H or C4H9 & H).
Angle strain - Is the "push" or "strain" that happens when the angle between the atoms is far from ideal (e.g the angle between carbon atoms in cyclopropane is 60°, far from the ideal 109.5°)
Torsional strain - Is the interaction of eclipsed groups or atoms in a molecule.(1 vote)
If the methyl or tert-butly groups are equatorial, do they not participate in any other interactions? Would there still be gauche interactions, etc?(1 vote)
Jay states that if the methyl substituent is axial it has steric interactions with the nearby axial hydrogens and that this makes the methyl group prefer to be equatorial.
In an earlier video "Double Newman diagram for methylcyclohexane", Sal stated that this preference was due to the interaction between the methyl group and the ring -CH2- (gauche conformation) when the methyl group is axial.
These are really just the same explanation, right??(1 vote)- The way I understand it is that Gauche conformation interactions and steric hindrance are closely related (if there is a substitute)(1 vote)
6:50Video transcript
- [Voiceover] Here we have
one chair conformation of methylcyclohexane,
and this is carbon one. You can see we have a methyl group that is axial up at carbon one. We also have a hydrogen, and
I've made the hydrogen green so we can tell it apart
from the other hydrogens. And this hydrogen is
equatorial down at carbon one. Now, this chair conformation
is in equilibrium with another chair conformation. And we can get to the
other chair conformation by what we call a ring flip. So if I lift this carbon up, and I pull this other carbon down here, and then we rotate it a little bit, we can see the other chair conformation of methylcyclohexane, so here it is. Now, when we look at carbon one, we can see we have a difference now. At carbon one, the methyl
group is now equatorial. Notice it's still up relative
to the plane of the ring, but it's an equatorial methyl group. And the hydrogen is now axial, so it's still down, but
it's an axial hydrogen. So that's what happens in a ring flip. We just saw the video of methylcyclohexane undergoing a ring flip,
and now we have to draw our two chair conformation. So, we saw how to do
this in an earlier video. If I'm trying to draw the
chair conformation on the left, I start with two parallel lines that are offset from each
other, so something like that. And then I draw a dotted line here, that intersects with the
top point on the top line, and a dotted line that intersects with the bottom point on the bottom line. And next, we draw a line
from the top dotted line down to the bottom, and then
from the bottom up to the top, and those are parallel. And then finally, we have
another set of parallel lines to give us a total of three
sets of parallel lines. We start at carbon one,
and we start axial up. So there's axial up, and then at carbon two
would be axial down. Carbon three, axial up. Carbon four, axial down. Five is up, and six is down. Next we put in equitorial, so at carbon one we start equatorial down, so I draw that down. At two would be equatorial up. At three would be down. At four would be up. At five would be down,
and six would be up. All right, this chair conformation is in equilibrium with
another chair conformation. So let's go ahead and
draw the other chair, and then I'll go back and
put in the different groups. So for the other chair, we
approach this the same way. We draw two parallel lines that
are offset from each other, so those are my two parallel lines. And then we draw our
dotted line that intersects pretty close to this top
point here on the top line. And then we draw another
dotted line, which is pretty close to this bottom
point on the bottom line. Next, we put in another
set of parallel bonds, so we put this one in, and then we draw-- Think about drawing a line
parallel to that, so like that, and then finally we can go
ahead and connect our dots. So we go like this, and these
lines should be parallel too. Now, this is carbon one, and we do the opposite of what we did before. We start axial down. So,
axial down at carbon one. This is carbon two, we go axial up. At carbon three is axial down. Carbon four, axial up. Carbon five, axial down. And finally, carbon six will be axial up. Go back to carbon one,
and we're putting in the equatorial bonds now. Since we started axial down,
then we would do equatorial up. So, this would be equatorial up, and then equatorial down at carbon two. At carbon three, would be equatorial up. At carbon four, equatorial down. At carbon five, equatorial up. And finally, at carbon
six, equatorial down. Now, let's finally put in our groups. So, we go back up here to the diagram, and this was carbon one. We had a methyl group axial up. So, that methyl group is axial up, and go ahead and put in a C-H three, right here at carbon one. And then we have this green hydrogen here, which was equatorial down, if
you look at how it's drawn. So, let me go ahead and
put in that hydrogen. So, this is our green hydrogen. Let me go ahead and circle
it and make it green so we can tell the difference here. And I could put in the other hydrogens, but I'm just not going to
on our chair conformations. Okay, it's really good practice
to draw all of these bonds, and you could put in the hydrogens, but I'm not worried about it. Our goal is to show the ring flip. So, at carbon one, let me
go ahead and highlight it, this carbon one became this carbon, right here, when we did the ring flip. And notice what happened
to our methyl group, just repeat what we saw in the video. We started axial up for our methyl group, and we still have this methyl group up, but notice it is now equatorial. So, we need to put in that
methyl group equatorial at carbon one, so we go ahead and do that. Remember, this is carbon one. And then, our hydrogen,
the green hydrogen, is still down relative
to the plane of the ring, but is now axial. So this hydrogen right
here is the green one. Let me go ahead and highlight it in green. It is now down, relative to our plane. So, the point of a ring flip is, any group that's axial becomes equatorial. Any group that's equatorial becomes axial. So that changes. But what doesn't change is, is it up or down relative
to the plane of the rings? Let me go ahead and highlight this here. The methyl group stays up. So here, it's axial up, and over here, it's still up relative to the plane, but it turns equatorial. And what was down, which
was our hydrogens-- Let me go ahead and highlight that here, this was equatorial down. The hydrogen stays down relative
to the plane of the ring, but it turns axial. So you need to practice
drawing all this stuff out. Now let's think about which
conformation is more stable. So we have two chair conformations, and one is more stable than the other. And let's go back up to this picture here, and let's look at the
interaction of this methyl group with these other axial hydrogens. So think about this hydrogen right here, and then this hydrogen right back here. This methyl group takes up a lot of space, and it's pretty close to
these hydrogens in space. If you rotate this, there's
some serious steric hindrance with putting the methyl
group in the axial position. However, for this
conformation on the right, with this methyl group out to the side in an equatorial position,
now this methyl group doesn't really interfere with
any of the axial hydrogens. So, down here, we have a hydrogen, right, which could interact a little
bit with these other hydrogens but nowhere near as much as putting that methyl group in this position. So the goal with a
relatively bulky substituent is to put it in the equatorial position, and that's going to be the
most stable conformation. It turns out, at equilibrium,
at room temperature, this conformation, this chair
conformation on the right is about 95% of all the chair
conformations of cyclohexane, and the one on the left is only about 5%. So, this is a substantial effect. The steric hindrance that
destabilizes this conformation, you can call this a one,
three diaxial interaction. So, if this is carbon one right here, so at carbon one we have
something that's axial, and then this would be carbon two, and this would be carbon three. So, carbon three, we have
something else that's axial. And these two can
interfere with each other. So that's why we call this a one, three, diaxial-- So, two things
that are axial, interaction. So, one, three diaxial interaction. It's the same idea for
this hydrogen back here. And this steric hindrance
destabilizes this conformation. The effect is even more pronounced when you have a larger
group that's interfering with these axial hydrogens. For example, if we changed our group from a methyl group to a tert-butyl group, let me go down here so we
can see a tert-butyl group. And I only drew part of the bonds here. Here you can see our
tert-butyl group at carbon one, and if you think about one,
three diaxial interactions, I could draw in some hydrogens here. Since you have all of
these methyl groups here, this is going to be even
more steric hindrance. And that's why at equilibrium, between these two conformations, 99.99% are going to be
in this conformation, with your really bulky tert-butyl group equatorial and out to the side. That decreases the steric hindrance.