hey guys, it's professor Dave, let's talk about cyclohexane chair conformations
so it's going to be very important to be able to
describe the conformations of cyclic compounds especially
cyclohexane because it is so prevalent organic chemistry
so what we want to do is take a look at what we have here
and understand that their are many possible conformations for cyclohexane
but they all have different energies and the one that is at the lowest
energy is the chair conformation. the reason that the chair conformation is at
the lowest energy
is because it allows for flawless tetrahedral geometry, 109.5 degree bond
angles at every center
and if we look closely it's all staggered
interactions. everything that we see here involves
staggered interactions between neighboring group so this is the lowest
energy
and cyclohexane will spend the most time in this conformation
so what we want to do is be able to draw chair conformations
let's take a look at how to do that
so when we draw cyclic molecules in line notation we're used to drawing them
in this top-down view like this
but just as with Newman projections
we had to rotate our perspective to be able to assess
conformations we've got to do the same thing with these molecules so what we
want to be able to do
is draw them edge-on
so what we need to understand is that there are two carbon molecules
this guy and this guy that are in the plane of the board
but that there are two that are closer to us than the rest that would be these
two in the front and there are two
at the back that are further away from us than the rest
and that would be these two and then the hydrogen atoms are going to be
oriented in a way that obeys tetrahedral geometry so at first
a lot of these lines and angles may seem very arbitrary
but what we want to be able to do is see the pyramid
at each and each spot here
so
each of these carbons is sp3 hybridized
tetrahedral geometry and that's what we can see here with this model
these are all
this is meant to reflect
the geometry at each of these carbons here now they can be tricky at first
to draw chairs
so just point out a few things, the reason it's called the chair
is cuz it kinda looks like a chair, you could almost imagine somebody
lying there and and sunning themselves and having a lovely day
now a couple a common mistakes,we've got bow-tie syndrome
this is no good we do not want to draw bow ties nor do we want to draw
lightning bolts
so let's cross these out here that's no good we don't want to bow ties
we don't want a lightning bolt in fact we don't want any the lines on the chair
to even approach verticality, if you have vertical lines on your chair
it's not gonna work out because we need to be able to look at a chair
and be able to tell which is the leftmost and which is the rightmost
carbon
so here that's not going to work right over here this works very clearly that's
left most carbon that's the rightmost carbon and so
that's going to be the best way for us to
able to assess certain things about these conformations
so let's take a look
at something called a chair flip and what happens when that occurs
so i've redrawn a cyclohexane chair here
and you're gonna want your chairs to look exactly like this so quick tip:
the chair is comprised of three sets of parallel lines
this line and this line should be essentially parallel
these lines should be parallel and these lines should be parallel
and all the things I mentioned before apply, remember we want to be able to
clearly see which is the leftmost
and which is the rightmost carbon because that's going to enable us to
keep track of things
when we flip the ring so let's take a look also at the substituents
on the ring. we have all these hydrogens we have 12 hydrogens
right now each carbon on the ring is going to have one group
projecting up from the ring and one group projecting down from the ring, in
addition
each carbon is going to have one axial substituent and one
equatorial substituent. now for axial you can think of this as
lying on a vertical axis that might help you remember
and so the red hydrogens on the left are the axial
hydrogens, those are projecting directly up or down
vertically from the ring. the green ones here
are the equatorial, you can think of them almost as lying more
nearly to the equator let's say of the molecule
so notice how those are angled, now this seems relatively arbitrary but once
again as I said before
these angles are very specific because they represent the tetrahedral geometry
that is occurring at every sp3 hybridized carbon so we must
illustrate the 109.5 degree bond angles so these groups
all have to be projecting exactly this way, if you had a hydrogen atom for
example going vertically upwards at that position
that would be wrong because that would be defying the tetrahedral geometry
of the chair so I know at first it seems a little bit arbitrary
but you really must have them look exactly like this
so notice as well that the axial position changes
upwards or downwards on alternating carbons that may help on
on carbon one it's up on carbon two it's down and so forth
this is exactly how you want to draw the chair
now let's talk a bit about what is happening happening when the
chair flips to the other chair conformation
now unfortunately it's very hard to illustrate with these models so
you can perhaps find information online that will show you what is happening
but this is a very common misunderstanding, what we need to understand
is that
these bonds are constantly rotating and because of that
these carbons sort of shift upwards and downwards with respect to
each other
generating other confirmations and at the end
you get the other chair conformation so when we draw the other chair
conformation. it's very tempting to think that what has happened
is simply a rotation or
something like that but that in fact is not what it is
what is happening it is that each carbon is being pushed
upwards or downwards so in order for the chair to get to the other chair
conformation
this carbon is being pushed down these are being pushed
up these are being pushed down and this one is being pushed up and so the
carbons are all in the same relative positions
but now carbon one is downwards carbon four is upwards et cetera
so I've numbered the carbons to make it perfectly clear
which carbons are which, over here is carbon 1
so that's gonna stay the leftmost carbon over here is carbon four that's going to
stay the rightmost carbon
carbons 2 & 3 are away from us these are the carbons
that are further from us than the board
so those will remain in those positions and carbons 5 & 6
are the ones that are closer to us than the rest in the molecule
and so those will remain closer so
that is what's happening with the chair flip and again I suggest you look at an
animation to better
visualize what is going on but it is not simply a rotation
it is actually a shifting of every atom with respect to one another.
so what is happening to the substituents
when we perform the ring flip
in fact what happens is that every group that was axial
will now be equatorial and every group that was equatorial
will now be axial so let's take a look at carbon 1
we've got the hydrogen that is axial
here now as this carbon gets shifted down
what's gonna end up happening is that goes down too but now will be assigned
to the equatorial position whereas the hydrogen that was equatorial
at carbon one gets pushed down will now assume
the axial position so we can see that is what is happening on each one of these
carbons
and it is certainly a rule that any group that is
axial in one chair conformation
will be equatorial in the other and vice versa
so this should help you be able to draw both chair conformations
for any given cyclohexane molecule.
now that we know each chair has axial and equatorial substituents
we need to understand a little bit about the energetic discrepancy between these
positions
so we take a look here let's say we're looking at these three
groups that are in the axial positions on their respective carbons
we need to understand that there's an element of steric hindrance
contained in these positions by these
diaxial interactions so what we need to understand is that
groups in the axial positions have an opportunity to create more steric
hindrance
than the groups in the equatorial positions because the groups in the
equatorial positions
are anti with respect to the rest of the ring being the bulkier group
so the equatorial position is the lower energy position
the axial position is the higher energy position
and what that means is that when we look at a substituted cyclohexane molecule
one of the chairs may be at a lower energy than the other one
and therefore the molecule will spend more time in that lower energy chair
which is going to be important for us to know to be able to predict reaction kinetics
and things like that so let's take a look
at just methyl cyclohexane. now let's say we put a methyl group here on this
carbon
in this particular chair the carbon
is in the equatorial position projecting upwards
from the rightmost carbon now when we flipped the ring
remember that this carbon gets pushed down these get pushed up these get
pushed down
and this one gets pushed up and so this carbon will still be the rightmost
carbon
and the methyl group will still be projecting
up from the rain however equatorial becomes axial
axial becomes equatorial when you flip the ring so this
equatorial methyl group will now
be axial. now given what we said about the energetic discrepancy
between equatorial and axial positions it should be easy for us to figure out
that this chair is going to be the more energetically favorable
because the singular substituent is in the
equatorial position rather then the axial
so whereas this would be generating diaxial interactions and therefore
more steric hindrance in this case it is not
so this chair will be at
a lower energy. now let's look at a disubstituted cyclohexane
so here on the left we have a methyl group projecting
upwards in the axial position and then we have a tert-butyl group
projecting upwards in the equatorial position
now when we flip the ring remember what we said about
each group staying on the same carbon so
this would be let's say we number this like this
so we've got
a methyl on three so that will remain
on three and as it was projecting upwards from the ring
it will still be up from the ring however axial when that carbon
moves down, became equatorial and then this tert-butyl group
as it was projecting up will remain up but where it was equatorial
it will now be axial now so in
each case we have one substituent axial and one substituent
equatorial however they do not contribute the same amount of steric
hindrance
because the tert-butyl group is a much larger bulkier group
so we will place a higher premium on
that group being equatorial over the smaller methyl group so given these two
options here
this is going to be the lower energy chair conformation
because it places the bulkier tert-butyl group
in the equatorial position so even though this methyl is
axial it's okay because in this chair the tert-butyl group is actually on that
is going to generate a lot more steric hindrance
so this is the worse of the two so now we should be able to
assess the different energies of the different chair conformations
and I also want to point out there is often a lot of confusion
and a desire to try to assign up
or down or cis or trans with axial and equatorial
and that's something that we shouldn't do, we should not associate that
terminology with one another so for example
here this methyl and this tert-butyl group are cis to one another
but that in no way implies that the two groups would both be axial or equatorial
or that one would be axial or equatorial
in fact if I move this methyl group over one
the up position on this carbon would be equatorial
so simply by moving one substituent to another carbon
they would still be cis with respect to one another
however they would then both be equatorial so there's absolutely no
correlation
between cis and trans or up and down from the ring
and whether those substituents are axial or equatorial, in fact
every single substituent on a cyclohexane chair
will be axial in one of the two chairs and will be
equatorial in the other necessarily.
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and as always feel free to email me with questions