Hey guys, Professor Dave here, I wanna tell you about conformational analysis.
so let's take a look at a very simple molecule
a molecule of ethane, C2H6.
ethane is comprised entirely of single covalent bonds, or sigma bonds
and if we recall, sigma bonds are comprised of overlapping
molecular orbitals so they can rotate freely, which means any single covalent bond
is constantly rotating. if we rotate this carbon-hydrogen bond
not much really happens, but if we rotate this carbon-carbon bond
actually the molecule is going to assume different shapes called conformations.
and what that has to do with is the energy associated with these different conformations
with respect to how close or far away these neighboring groups are.
whether they're a little bit closer
or whether they're a little bit farther away. so we're gonna want to be able to
analyze these conformations
and and be able to draw them too, using things called
Newman projections.
the way we're used to seeing ethane in line rotation looks like this
and we want understand that these four atoms
those two hydrogens and those two carbons are in the plane of the board there
and then here's the wedge and dash down there on this carbon
and here's the wedge and the dash projecting upwards on this carbon. and so
we can see how this is what we're trying to represent on the board when we draw it this way
but if we want to examine conformational analysis
we're gonna have to look at this molecule in a slightly different way
we're going to want to look directly down a carbon-carbon bond
so basically it's as though we're placing ourselves in the board looking this way
and so looking at this model this is what we're doing, we're looking directly down
this bond. the reason we want to do that is because we want to understand something
about the orientation or the configuration
of these groups on this carbon
in relation to the groups that are projecting from the back carbon.
in order to draw this and analyze it we're gonna draw something called Newman projections
and that's this here so when we look at a Newman projection what we want to understand
is that we are looking directly at this carbon
this front carbon, and that is what is represented by the circle
the circle is the front carbon. because we're looking directly at this carbon
the carbon behind it and the bond connecting them
cannot be seen. we can imagine this is like a cylinder
that we're looking directly at the front of so the back carbon
and the bond between them cannot be seen. but what we can see
are the six groups that are projecting from these two carbons
so we take a look at the front carbon, all we need to do
is draw the three groups projecting at precisely the angle that we see them
so for looking head on we see a hydrogen atom directly down
and so that's why this bond originates from the center
because these groups are on the front carbon so we can see the front carbon
and the entirety of the bond projecting to the group in question
so we see straight down a hydrogen and then up
and both to the right and to the left we see two other hydrogen
atoms so these are the three hydrogen atoms projecting from the front carbon
that's why the bonds to those groups originate at the center of that circle
now we can't see the carbon behind the front carbon
but we can see the three groups projecting from them
now these bonds will start at the edge of the circle
to represent the fact that they are connected to the carbon that cannot be
seen behind the front carbon
so these lines will originate at the sides and we will see a hydrogen
directly upwards and we will see hydrogens directly
to the left and right downwards
here as well. so this Newman projection
represents precisely this conformation
of ethane and we would call this a staggered
conformation because of the dihedral angle here
between groups so this is a 60-degree
dihedral angle. this is the furthest away
that these groups can be as they rotate before starting to become closer to one another
so the other confirmation for ethane would be eclipsed
and so we can see here that these hydrogen atoms and actually all of them
are eclipsing one another in other words they are directly
overlapping. and there's a lot to discuss
in terms of energy here, so we have eclipsed
and then as it rotates to a 60-degree
dihedral angle we have a staggered conformation which is the furthest these
groups can be from one another
and then further we would go back to another eclipsed conformation.
so now let's go ahead and analyze the energies between these
different conformations.
so every atom is surrounded by an electron cloud
electrons repel one another because they're of like charge so that means that
every group projecting from this carbon is going to want to be as far away as it can be
from the group projecting from this carbon and the closer they will be to one another
the more steric hindrance there will be and the higher the molecule will be in
potential energy
think it like a compressed spring or magnets that have like
charge that are held closely to one another
the potential energy wants to be converted into the kinetic energy of motion
so a compressed spring will have a tendency to relax
and magnets of like charge held closely to one another
will have a tendency to push each other away. as it happens
it's the eclipsed conformation that has the groups more closely together. as you can see
this distance here is shorter than
if we go to the staggered conformation, there's the normal distance plus a
lateral distance
so then you're making a triangle and the hypotenuse is always going to be the longest
so that means that the staggered conformation holds the groups as far away
as they can possibly be
while still obeying the normal bond lengths
let's take a look here so let's take a look at this first staggered conformation
so here we can see the Newman projection hydrogens going
up and then down to the left and right on the front carbon and then staggered
on the back there, so that is that conformation, now let's rotate the back carbon
until we get to that first eclipsed conformation
that's up here so all the while from this conformation here
the potential energy was rising as these groups begin to
get closer to one another as the dihedral angle shrinks to zero
to get that eclipsed conformation and then as they continue
we will go back down to the next
staggered conformation which will be identical in energy to the other
because these are all identical hydrogen atoms but this is a measurable
energetic difference, and in fact we can measure that it's 12 kilojoules per mole
meaning if you had a a mole of ethane molecules
all in the eclipsed conformation they would exist at 12 kilojoules
more potential energy than entirely staggered so this can be measured and it's
important to understand this to be able to assess certain things about reaction mechanisms
so let's take a look at a slightly more complicated molecule
a molecule of butane, C4. so here's the four carbons, were gonna look directly down
the C2-C3 bond
the central bond so here's our butane, that's
how we would look at it in line notation, and now what we're going
to do is look directly down the C2-C3 bond
so here are the middle two carbons so we're looking directly
through this carbon and through that bond
so it's really the same as ethane, it's just that if we took
ethane and replaced one of the hydrogens on each carbon with a methyl group we'd get butane.
and so here we can see butane in a staggered conformation
so here's the front carbon and we can see that the three groups projecting
from the front carbon
are directly upwards we have this methyl group, CH3
and then down and to the left and right we have our two hydrogens
so that would be here CH3 and then are two hydrogens
from the back carbon we can see that
up and to the left and right are these two hydrogen atoms
and then directly downward we have this methyl group
so that's why these groups with the bonds
originating from the edge of the circle we have hydrogen hydrogen and then a
methyl here
now this is a staggered conformation and now that we've introduced one
different group it's not all the same group, the same hydrogens, there's methyl
and hydrogens
that introduces a discrepancy in the energies
of the different staggered conformations. this is necessarily
the best and lowest energy conformation
and it is called the anti conformation and that is because
this methyl group and this methyl group are in
an anti configuration with respect to one another meaning they are as far away
from each other as they can be
this is very important because the methyl group being much larger
than these individual hydrogen atoms generate more steric hindrance
so having these methyl groups closer together
puts the molecule at a higher energy than if hydrogen
atoms are closer to one another, so that means the
anti conformation is the lowest energy conformation that we can have.
another kind of staggered
interaction would be this
this is a gauche confirmation. it is still
a staggered conformation, we can see everything in the front is the same
but now we have the methyl group still
with a 60-degree dihedral angle but on this position up and to the right
which is closer to the other methyl group. now because
these are much larger groups that is gonna generate
a lot more steric hindrance
and so this is a gauche interaction we would say that these methyl groups are
gauche to one another
and that places this conformation at a higher energy
than this, however while anti is the lowest
gauche being the next lowest because also this would also be a gauche interaction
all of the eclipsed interactions are at higher energies
than the staggered interactions. however there's a discrepancy between
the eclipsed confirmations as well. here is one eclipsed interaction
we can see that this methyl group is directly over this hydrogen
we can see that these hydrogens are directly over one another
and we can see this hydrogen directly in front of this methyl group so there is
some steric hindrance associated with this conformation
now that is not as much steric hindrance however
as this fully eclipsed conformation where the methyl groups
are directly over one another. now that has the most steric hindrance because
that places the largest electron clouds
very close to one another so this will have the highest energy
of all the possible conformations. and there you have it
anti being the lowest energy
increasing in energy to the fully eclipsed and then back
to anti
so in your class you're probably going to be asked to draw
Newman projections, so I wanted to go over an example
to see if we can clarify how to do this. so I drew an arbitrary molecule
now in order to draw a newman projection you're going to have to agree
about which bond we are discussing and which direction we are looking at it from
so let's say I've drawn this so this means that I wanna look
directly down this bond and from this direction
so that means this is the front carbon, this is the back carbon
so what we want to do, here's our circle
this represents the front carbon which we've agreed is this
now what are the three groups that are projecting from the front carbon
we have a fluorine atom, we have a hydroxyl group, and then we have an
implied hydrogen, don't forget the implied hydrogen
that would be on a dash bond so
in which direction are these projecting considering the fact that we are
basically placing ourselves
in the plane of the board so what I notice is that the fluoro group is
directly down from me and also in the plane of the board so we're gonna draw
a straight line down
for that fluorine atom. now the wedge bond
means that this hydroxyl group is projecting out of the board
so that means from my position on the left
this wedge bond is, if I'm on this side of the molecule that means that the
hydroxyl group is projecting up
and to the left because it is out of the board this way
so that is going to be up here and then that implied hydrogen that would be on
the dash bond
which is going into the board, if I'm over here that's up and to my right
so here's that hydrogen so
that covers the front carbon, now here's the back carbon
that we cannot see but we can see the groups that are projecting from the back carbon
so what do we see, well from this side anything on a wedge bond will be going
to the left so this chlorine is going down and to the left and don't forget
if we are looking from the other side the molecule over here
anything on a wedge bond would be going to our right so try not to associate
dash and wedge with left and right
because it depends what side of the molecule you're on, but from this
perspective the chlorine atom on the wedge bond is going down and to my left
out of the board so let's draw that there. the bromine atom
on this dash bond is going down and to my right into the board
so i'm going to draw that there and then the rest of the molecule whatever it may be
in this case an ethyl group
you can either just draw two lines or
perhaps you're a professor wants you to draw CH2CH3
basically whatever else is going on, we're not interested in the conformation
around these bonds, we're strictly looking at this bond only
so CH2CH3 will suffice to describe
the rest the molecule there so this is an
accurate Newman projection for this
carbon-carbon bond in this direction
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