Lecture 010 Transcript

Lecture 010 - cycloalkanes
Speaker: Jean-Claude Bradley

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Jean-Claude Bradley: So we're going to resume the Newman projection.
We left off at propane last time, so I'm going to do the Newman
projections for butane.

So when we look at butane, we've got four carbons. So we're looking
at the Newman projections for butane, and when we look at that,
there's actually two different Newman projections we can do with this
one because I could draw a box around the C-1 to C-2, or I could draw
a box between C-2 and C-3. So, with butane, we're going to look at
C-2 to C-3 because it gives you more different confirmations. But
generally, with molecules that have more than one possibility, I will
tell you which two carbons to do the Newman projection around. So,
if we're doing it from C-2 to C-3, then we draw the box.

Next step, we go and draw the circle, and it doesn't matter which one
you put in the front and which one you put in the back; as long as
they're consistent with all the projections, it'll work out. So, I'm
going to start first with a staggered conformation, they're a little
bit easier to draw. And I'm asking a question, if I decide to make
the left carbon the front, the question is what are the three things
that are connected to that carbon? A methyl, an H, and an H. So
that's what I'm going to put in the front of the Newman projection.
A methyl, an H, and an H.

Now, why did I put the methyl on the top instead of on the right? It
doesn't matter where you start, because when you generate all the
Newman projections, you're going to generate them all, regardless of
where you start. Okay, so the important point is just to put them
down there somewhere. For the right carbon, that'll be the one in
the back, and again we have a methyl, a hydrogen, and a hydrogen.
So, I'll put the methyl at the bottom arbitrarily, and then I'll put
the other two hydrogens.

So just like what we did for propane, I'm going to keep either the
front or the back constant, and I'm going to rotate the other part.
So in this case, let me make the back constant. So I have to put the
methyl on the bottom like that. And now I'm going to rotate, and I
generally rotate clockwise, so I'm going to rotate this clockwise
until the methyl group is on the top of the first hydrogen that it
comes across. You want to go to the center, as much as possible.
Okay. So, we have now the methyl on top of the hydrogen, and you
notice the other hydrogens on top of the other groups.

Now we want to continue rotating until we generate a duplicate
structure. So I'm going to keep rotating until the methyl group is
around the four o'clock position. Now it's really important that if
you decide you're rotating one side, then you don't change that.
After you draw the first projection, you have to maintain that
throughout all the Newman projections, otherwise you're going to get
confused and you're going to come up with the wrong counts for them.
So you'll notice here that the back here is going to remain unchanged.

So, now the two methyl groups are close to each other, so you can see
that the third Newman projection is different from the first one,
right? So now if I continue to rotate, I can draw one more. Okay,
again the back is constant. And now I'm going to show that the two
methyl groups are on top of each other. Okay. So, if I were to
continue to rotate clockwise, the methyl group would show up here.
Well, that actually is the same thing as the third structure, and as
I continue to rotate, I will basically create duplicate structures as
I go along, so these are the four conformations that are important
for butane, that's it.

Now, they have names, and the first one, because the methyl groups
are on the opposite sides, it's called Anti. So that's the Anti
conformation. Now, when the methyl is on top of a hydrogen, this is
called an Eclipse conformation, but we have to distinguish between
this eclipse and this eclipse, so we call this partially eclipsed.
Someone ran out of imagination when they went to the eclipse
structures, unfortunately. The staggered ones, though, are still
pretty interesting. When the two methyl groups are next to each
other, that's called Gauche. And the final one you might guess from
the partially eclipsed, we go to totally eclipsed when the two
methyls are on top of each other.

Okay, now we have to make the distinction between the name of the
conformation and the class of the conformation. When I refers to the
anti and the gauche, both of those conformations are staggered.
Okay? However, because we have more than one staggered conformation,
staggered is not the name of the conformation; it's simply the class
of the conformation. So whenever you have a conformation where the
front group is inbetween the back group, that's always going to be a
staggered conformation, but you'll have to be more specific, so
someone will know exactly which one you are referring to. So we use
staggered as a class, and of course we can also use eclipsed as a
class. Okay, so that's the nomenclature of the master here.

The other thing is that when we use the term gauche, anti and butane,
we can specify, Draw me the gauche conformation of n-butane, but I
can also say, that the two methyl groups are gauche to each other. I
can say that the two methyl groups are anti to each other in this
structure. So I can use that term also not just as a name, but to
describe the relationship in a Newman projection, and I'll be doing
that when it's relevant.

So what I want to do now is draw an energy diagram for this as we
rotate through. So the anti has the two methyl groups as far away
from each other as possible, okay? And so you remember that if you
have nothing that is attracting two groups, they'll naturally be
repulsed from each other, so the farther away I can put those groups,
the lower the energy. So we're going to predict that the anti will
be the lowest energy. The two eclipse structures, they're the groups
that are absolutely as close as they can be, and specifically for the
totally eclipsed, I have the two methyl groups in the closest
position, so that one we predict will have the highest energy in the
diagram. Okay, so I'm not going to give you specific number; I just
want you to be able to gauge the shape of this thing.

So if we do the same thing in an energy diagram that we did for
ethane, let's start with the lowest energy, being the anti, and as I
rotate I'm now going to go to an eclipse conformation. The eclipse
conformations will always be at the top of an energy hill, right?
Because as you reach the peak where they're on top of each other,
that's the most unstable that the rotation will take you, so I'm
going to go up here some more, like this. So again, maintaining the
back constant. All right, so that'll be the top, and as I continue
to rotate the methyl group, I'm going to go back to a staggered
conformation. Specifically the gauche.

So, if I want to try to guess where the energy could be, we know that
it's going to be at the bottom of an energy well, but it won't be as
low as the anti, because it actually is closer, the two methyl groups
are closer in the gauche than it is in the anti, so we're going to
predict somewhere in between. And then finally, when we have the two
methyl groups on top of each other, that'll be the highest energy.

If I continue the rotation, all I'm going to do is draw the mirror
image of this, so this would keep going on and on as I rotate. You
can actually calculate what the various energies are. Why is that
important? Because in this case, if you know what the energies are,
you can actually predict what percentage of the molecules are going
to have a certain conformation, which is important especially from
ecology, when drugs fit into receptors, you need to know what they
look like at the temperatures you're going to be using them, and
that's where this comes from, originally. I think that's all I
wanted to do with Newman projections. Do we have questions?

Let's talk a little bit about cycloalkanes. So, we're just talking
about molecules that have just carbon and hydrogen and that are in a
ring. The smallest ring you can make is three, so I can have
cyclopropane. Generally, three-membered rings are less stable than
larger rings, but it doesn't mean that they're impossible to make, so
cyclopropane is a gas, and you can buy it, and it's pretty stable,
but it's going to be less stable than cyclohexane, for example. But
when you're drawing this thing, you're drawing it flat, and each one
of these centers is sp3 hybridized, so whenever you draw a ring
that's an alkane, you're going to have one bond coming towards you,
and one bond going out. So if I want to draw the hydrogens on this
thing, I would do it like this.

Okay, and you'll notice that you can't rotate around those bonds. So
if I were to replace one of the H's with a methyl group, I find that
there are two possibilities. I can either have the methyl groups on
opposite sides of the ring, or they can be on the same side. So
those are two different compounds because I cannot rotate around this
sigma bond. All right, if I have an open-chain alkane, I can rotate
around any sigma bond I want, I have free rotation, and that's why
you have different conformations, but here we actually have two
different compounds, because they're locked in the same position,
just like the double bond locks a molecule in position. So we would
name this thing trans-dimethyl-cyclopropane. Or trans-1, 2.

So as I go from cyclopropane to cyclobutane to cyclopentane, if you
make molecular models of these things, you'll find that it's pretty
much going to stay flat. You're basically going to have a really
flat structure. With cyclopentane, you're going to see that it
buckles a little bit, but it's still really very close to being flat,
but when you hit cyclohexane, it's definitely not flat anymore.

So if we want to look at the conformations of cyclohexane, because
it's not flat, it's going to get bent in certain ways, and we want to
talk about the certain ways in which it's bent, because we absolutely
have to know what these molecules look like, depending on what we
want to do with them, so for cyclohexane, we're going to look at it
from the side. So this is side view of cyclohexane, and the reason
that it bends like that is because you have 109.5 degree angles, and
you can't make a flat hexagon that has those angles. So you find
that basically it can bend like this, or it can bend like this. And
those two conformations have the names of what objects they resemble;
this is called the chair conformation, and this is called the boat
conformation. And whenever you have a comparison between the chair
and the boat, the chair is more stable. Okay, so that means that if
I have a solution of cyclohexane, there are going to be more
molecules that are going to look like the chair than the boat, but
those two interconvert all the time. It's just that we want to know
what the most stable conformation is. If I just have cyclohexane,
there are only two conformations, but if I start to put some situates
on there, we're going to find that we sometimes have more than one
chair conformation.

Let me give you an example with methylcyclohexane. Okay, so here's a
skeletal formula of methylcyclohexane. That doesn't tell you at all
what the molecule looks like, because it's drawn flat here. What we
want to know is what does methylcyclohexane look like the most when
it's in 3D? So the first thing is we don't bother with the boat
conformations because the boat is going to be less stable, so what we
want to do is compare the chair conformations of methylcyclohexane
and see how we can analyze if one is more stable than the other.

Now to do this, you have to get used to drawing this little chair
side-view, so if you've managed to draw this, you'll find that you
have edges that are pointing either upwards or downwards, so let's
say this one here on the left, this is definitely pointing down, so
I'm going to draw a vertical line pointing down, like this. Exactly
vertical. This wedge here is pointing up, so I'm going to make a
vertical line pointing directly up. This wedge here is pointing
down, same thing. This is pointing up, I go up. Those are the easy
bonds to draw because they are exactly vertical.

The other bonds we have to draw are a little bit harder because all
the angles here are 109.5 degrees, right? So in order to do that I'm
going to draw an angle that's a little bit larger than 90, so I'm
going to do something like this. So not exactly horizontal, you want
to make that angle a little bit more than 90. I'm going to go around
the molecule and do that everywhere. Okay, so I strongly recommend
that you put the vertical bonds first because they're easier.

So I have to put the methyl group somewhere, and you'll notice
there're really only two kinds of bonds here, I have the bonds that
are going up and down, and I have the bonds that are kind of going
horizontally, so there's only two different places I can put the
methyl group. So on the left structure I'm going to put the methyl
group on the bonds that go up and down, and that of course means that
all the other things are hydrogen's. Okay, so the methylcyclohexane
is flipping around, and the other chair conformation I want to be
able to do for that. The easiest way to do that is learn to draw
this template once and for all, and then we're going to put the
groups on it in a different position, so once again, I'm flipping all
of the vertical bonds, and then the horizontal ones. So if I put the
methyl group on a vertical bond, now I want to put it on a horizontal
one. It doesn't really matter which horizontal one I put it on, but
I'm using this carbon because it's more accessible to me in terms of
my ability to draw a methyl group on it. It's easier than the bonds
that're in the back here of this projection, so that's the only
reason that I'm using this particular carbon. And the rest is loaded
up with hydrogen's.

Okay, so now I've drawn the two chair conformations of
methylcyclohexane. The difference between the two in terms of
stability is that when they methyl group is on one of these vertical
bonds, it's actually pretty close to these two hydrogen's. That's a
1, three interaction in cyclohexane. So the bigger the group, the
worse that that interaction is going to be, but still with methyl
it's significant enough that the molecule wants to put that methyl
group far away from the ring, so when it puts it on these horizontal
bonds, it's no longer close to these other hydrogen's.

Okay, so we know then that the structure with the methyl group on the
horizontal bond will be more stable. Okay, so these horizontal and
vertical bonds have names actually, the ones that're going straight
up and down are called axial, and the ones that are going near
horizontal are called equatorial. So equatorial should be easy to
remember, near the equator, horizontal. So in general what we want
to do is put the large group in the equatorial position. So if
you've seen the quiz for this material, you'll see that there's
questions like that, or a certain derivative of cyclohexane in its
most stable conformation, where's the methyl group? Well, in its most
stable conformation, they methyl group is equatorial, that's how you
answer those kinds of questions. Now when you only have one group,
it's pretty easy because there's only two choices, equatorial or
axial, but let's take a look at what happens when we have more than
one group on the cyclohexane ring.

So I'm going to again start with the skeletal formula so we know what
we're dealing with. So I have 1-methyl, 2-ethylcyclohexane here, and
notice that they're on the same side. Up and down will have nothing
to do with axial or equatorial, and you'll see when I draw the
picture. The up and down will have to do with whether it's coming
from the top of the ring or the bottom, and that's a little tricky
the first time you see that, so I'm drawing this skeletal formula so
we keep in mind what exactly we are trying to draw. I am looking for
the most stable conformation, so I ignore the boat. I draw my two
templates. So I can start ending where I want, it doesn't matter,
but I like to use these carbons in the front, because I have more
room to draw things with, but it really doesn't matter where you
start, you'll end up with the same answers. So I'll just arbitrarily
put the methyl group over here.

Now I have to put the ethyl group, where? I have to put it on the
next carbon, so the next carbon would be this one right here, for
example. Now, they're on the same side, so let me actually draw it
over here. In one of them, the methyl group will be equatorial; in
the other one the methyl group will be axial. The question is, where
is the ethyl group going to be? So what you're looking at is the side
view of the cyclohexane. You'll find that there's always one bond
that's higher than the other, so those two yellow dots that I've put,
those are both on the same side of the ring, they're both underneath
the ring, whereas these two dots are on top of the ring. So if I put
the methyl group on the yellow dot, I have to put the ethyl group on
the yellow also, and then I have hydrogen's everywhere else. When I
draw the other conformation, I flip both of those groups, so now the
ethyl group that was axial will now be equatorial at this position,
and the methyl group that was equatorial is now axial. Both of these
are the same compound, they're two different conformations of the
same molecule.

So now we want to determine which one is going to be more stable.
Well, you'll notice that, in each case you have one that's
equatorial, and one that's axial. We want to put all the groups, if
we can, equatorial. We can't do that, because we don't have one of
the conformations that does that, so we want to pick the conformation
that has the biggest group in the equatorial position. That would be
this one here. So even though we have the methyl group that's axial,
the ethyl will take over.

[question from student]

The question is, if there are cysts, are they both going to be axial
or equatorial? It depends on the position. For a 1, two
relationship, the cysts will have one axial, one equatorial.
However, if I have a 1, three relationship, then if they're cysts
they're both going to be axial, or both of them will be equatorial.
So in other words, don't try to memorize it or anything, just learn
how to draw the picture, then put them and see what it turns out to
be. It's the only way you really can do these problems.

Okay. Did I cover everything here? Do we have any other questions?

So these questions take time. I certainly cannot do them in my head;
I have to draw these out, so you definitely will have to. I'm going
to ask you questions about this, then I'm going to look at the
picture. So for example, this one, the methyl group is axial in the
most stable conformation. You couldn't possibly know that unless you
went through this process of drawing them out to find out exactly
what is the lowest conformation for the whole molecule. Okay, so
you've got some practice with the quiz, and some of the problems.