Conformers and Cyclic Alkanes
I. Conformational Isomerism
We'll continue to visit the alkane family in this unit. The C atoms in alkanes are bonded
through single bonds known as sigma (!) bonds.
ROTATION CAN OCCUR ABOUT A SINGLE BOND. Molecules are not STATIC structures.
They "wiggle" constantly via rotations about single bonds.
Compounds that differ from each other only by rotations about one or more single
bonds are called CONFORMATIONAL ISOMERS.
The easiest way to visualize these isomers (on paper) is using a drawing called the
NEWMAN PROJECTION.
A Newman projection gives you a perspective on a molecule by viewing down
a bond to determine how the groups bonded to the atoms on each each end of
the bond line up or are arranged in space.
look down this bond...
C
C
Z
C
X
Y
B
A
Y
C
B
A
Z
That's a Newman
projection of our fictitious
molecule.
X
...this is what you
see. The circle in the
middle just indicates
that two atoms in a
bond are lined up
along an axis going
into or out of the paper.
Here's what is good about the Newman projection: Suppose you were interested
in the changes in the molecule when rotations occur about that C-C single bond.
The Newman projection allows you to see the spacial effect easily.
Let's look at some conformational isomers of n-butane.
H
1
CH3
H
3
CH3
H
H
dihedral angle or
torsional angle, 60o
2
H
H3C 4
H
H
CH3
H
Let's look down the C2-C3 bond. We'll put C2 in front.
CH3
H
H
H
60o
H
rotation
H
H
bond
CH
H 3
CH3
The atoms on C2 and
C3 are staggered. When
the methyl groups are as
far apart as possible, the
conformer is ANTI.
H
H
60o bond
rotation
H
H
CH
H 3
H
H
rotation
H
CH3
CH3
This rotation
staggers the
atoms again. This
time the methyl
groups are
positioned
GAUCHE to each
other.
H
H
H
60o bond
H3C
H
rotation
60o bond
rotation
CH3
This rotation puts
the methyl groups
on top of each other.
This is the
ECLIPSED position.
H
H
60o bond
This rotation puts the
atoms on C2 and C3
"on top" of each other.
This is one of the
ECLIPSED positions.
CH
H3C 3
H
H3C
H
H
CH3
This rotation
staggers the
atoms again. This
puts the methyl
groups in the
GAUCHE conformer
position.
CH3
60o bond
H
H
rotation
H
H
This rotation gives
an ECLIPSED conformer.
CH3
Back to start.
What is the SIGNIFICANCE of these bond rotations? Is one conformer preferred
over another?
Let's study the potential energy diagram below to answer these questions.
Notice we mapped the conformers we just drew on an E vs degree of rotation
plot. Some of the conformers sit on energy peaks, some sit in energy troughs.
Again, what is the significance of this?
You can see from the diagram that there is an energy difference between the
conformers. The lowest energy conformer has the methyl groups anti while
the highest energy (and least stable) conformer has the methyl groups eclipsed.
Groups can rotate about a single bond but not without climbing an energy barrier
to do so. Free rotation is hindered by interactions of adjacent groups within the molecule.
Rotation about
a sigma bond
is allowed and occurs
constantly. But energy
barriers to rotation
occur based on the
resistance to overlap
of the electron clouds
repulsion
The most stable conformations are the ones that have minimized electron cloud repulsions.
STAGGERED confomrations are lower energy for this reason. ECLIPSED conformations cause
electron cloud repulsion. TORSIONAL STRAIN ACCOMPANIES "CROWDED
CONFORMATIONS. ECLIPSED CONFORMERS HAVE HIGH TORSIONAL STRAIN.
The van der Waals (VDW) radius of the atom (the distance from the center of an electron cloud
surrounding an atom or group to the outer edge of the cloud) determines how crowded they will be
during bond rotation. ELECTRON CLOUD REPULSION HINDERS ROTATION.
A methyl group will have a much larger van der Waals radius than a single H atom.
VDW radius of H atom
VDW
radius
of CH3
methyl anti-conformer of n-butane. The dots indicate the VDW surface.
methyl eclipsed conformer of n-butane. Look at the VDW
radii of the methyl groups. Can you see why this
is a high energy conformer compared to the anti conformer?
By the way, we have the software to do this kind of 3-D modeling in class so
we will.
Going back to the energy diagram, we can appreciate now why, in order for the conformers to
convert, they have to overcome energy barriers, the height of which depends upon the energy
difference between the conformers. To go from gauche (E) to gauche (B) would require climbing
a 3.6 kcal energy barrier during rotation because the methyl eclipsed conformer (A) must be
passed through. Going from gauche (B) to methyl anti (D) requires climbing an energy barrier
of only 2.9 kcal. The rotation of (B) to (D) does not require going through the methyl eclipsed
conformer (A).
The preferred conformation, from a thermodynamic standpoint, it is the one that
minimizes VDW repulsion. THIS WILL BE THE ANTI CONFORMER THAT
SITS THE LARGEST GROUPS AT A DIHEDRAL ANGLE OF 180o
II. Heat of combustion of cyclic alkanes-ring strain
It was originally thought that cyclic alkanes were flat or planar. For this reason,
von Baeyer proposed that planar cyclopentane, with bond angles of 108o (like
a regular pentagon) would be the most stable ring structure. Cyclohexane,
with bond angles of 120o, would be less stable.
flat cyclopentane
flat cyclohexane
The smaller rings would be even less stable, with cylcopropane and cyclobutane
having bond angles of 60o and 90o respectively. von Baeyer called the stability
differences ANGLE STRAIN or RING STRAIN. It arose from deviation from a
109.5o tetrahedral angle.
Models made of the cycloalkanes strongly suggested that, if one allowed the rings
to be non-planar, this angle strain could be relieved (at least partially) for rings with
5 or more atoms. In fact, the models showed, and HARD DATA confirmed, that
CYCLOHEXANE in fact had NO angle strain and its sigma bond angles were
109.5o.
Combustion data (burning the sample and measuring the heat) allows us to
calculate angle strain data for the cycloalkanes.
Adding CH2 groups raises the heat of combustion of alkanes. BUT, if we
calculate the value on a per CH2 basis, any differences mean that the structures
hold more (or less) energy
heat of combustion increases...but on a per CH2 basis...?
The heat of combustion per CH2
for cyclohexane is close to the
value found for acyclic (open chain)
alkanes
Per CH2, the heat of combustion DECREASES from cyclopropane to cyclohexane and
then increases modestly again to cyclodecane.
Cyclohexane is at a minimum and is the benchmark for a ring structure WITH NO
ANGLE STRAIN. The difference between the per CH2 value for a cylcoalkane and
cyclohexane is a MEASURE OF ANGLE STRAIN OR RING STRAIN.
#C
3
4
5
6
7
8
9
10
Ring strain KJ/mol
44
28
5
0
4
5
6
6
The source of the ring strain lies in the fact that the bonding AOs can't overlap
efficiently and this results in "bent" bonds.
poorly
overlapped
AOs in cyclopropane
good overlap
strong bond
poor overlap
weak bond
cyclobutane and larger rings can and do rotate their ring C atoms out-of-plae
to relieve angle strain and increase stability.
A stable conformation of cyclopentane is the folded or "envelope" form as shown below:
all H atoms
eclipsed
one C atom
rotates out of the
plane of the other
C atoms
relieves torsional strain
caused by eclipsed H atoms.
Just like with butane, the rings change conformation through SINGLE BOND ROTATIONS
The rotations relieve ANGLE STRAIN, due to the restrictions of a ring structure, and
TORSIONAL STRAIN, which happens with VDW repulsion (electron clouds of
atoms or groups forced close together)
III. Conformers of cyclohexane
The six membered ring is common in nature and widespread in naturally occuring compounds.
It is worthwhile to study cyclohexane in detail because what we learn will extend to 6 membered
rings that are heterocycles--rings where one or more of the C atoms are replaced with another
atom (O and N being most common)
How must cyclohexane fold to acheive bond angles of 109.5o ? The most stable
conformation is known as the "chair" conformation because it looks somewhat
like a reclining chair.
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
120o
flat,
bond angles,
not consistent with
reality.
H
H
H
H
H
H
two chair conformers of cyclohexane.
The H atoms are perfectly staggered.
equatorial positions
on cyclohexane
axial positions on
cyclohexane
axial
equitorial
FOR CYCLOHEXANE, THERE ARE 6 AXIAL POSITIONS AND
6 EQUITORIAL POSITIONS.
Pay attention to this: When chair cyclohexane "flips", or converts from one chair
form to another, the black hydrogens go where the white hydrogens were and vice
versa. The positions on the C atoms of a ring structure are known as AXIAL and
EQUATORIAL. The axial positions are above and below the ring carbons. The
equitorial positions are sort of in the same plane as the ring C atoms.
axial
C1
C4
C4
C1
equitorial
The equitorial positions become axial and the axial positions become equitorial.
Let's take a look at a couple of other conformers of cyclohexane and highlight
some of their features.
Remember this. ALL of these conformers (butane included) lie at minimum or maximum
PE levels. There are a myriad of conformations in between. We are looking at extremes
because the extremes teach us why a particular conformer is FAVORED or DISFAVORED.
What we are doing here is TRYING to UNDERSTAND the dynamics of molecules and the
interactions within their structures (intramolecular push and pull).
flagpole positions-causes
torsional strain due to VDW
repulsion
BOAT
flagpole positionstorsional strain relieved
by "twist" (C-C bond rotation)
TWIST BOAT
This conformation lies at an energy maximum
As chair cyclohexane interconverts, it must
adopt this conformation, followed by conversion
to the twist-boat form.
HALF-CHAIR
COMPARING NEWMAN PROJECTIONS OF THE CONFORMERS
This time we want to "line up" two pair of C atoms in the ring...
H
H
view
flagpole
H atoms
axial and equatorial
positions staggered.
axial and equatorial positions
eclipsed.
view
INCREASING POTENTIAL
ENERGY
IV. Replacing H atom(s) with methyl (or something else) on cyclohexane
THE EQUATORIAL POSITION IS MOST SPACIOUS. A GROUP BIGGER THAN
H WILL PREFER THE EQUATORIAL POSITION.
H
H
H
H
H
H
H
H
H
K
H
H
H
H
H
CH3
H
H
H
H
H
CH3
H
H
H
axial methyl
equatorial methyl
K=
[equatorial]
= 21
[axial]
95%
5%
IT'S ALL ABOUT SPACE (OR LACK OF CROWDING): TWO METHYL GROUPS...
cis-1,4-dimethylcyclohexane
H
CH3 H
H
H
H
H
H
H3C
CH3
H
H
H
H
H
H
H
H
trans-1,4-dimethylcyclohexane
H
H
most
stable
H
CH3
H
H
H
H
H
H
H
H
H
for 1,4 this
means 1
axial methyl,
1 equatorial
methyl
trans means
"across from"
H
H
H
H
CH3
CH3 H
H
H3C
H
H
H
H
cis means
"same side"
H
H
CH3
for 1,4 this
H means 2
axial methyls
or 2 equatorial
methyls
cis-1,3-dimethylcyclohexane
H
H
H
H
H
H
H
H
H
H3C
H
CH3
in this case,
cis will put
both methyls
axial or
equatorial
H
H
H
H
H
H
H
H
H
H
most
stable
CH3
CH3
H
H
trans-1,3-dimethylcyclohexane
H
H
H
H
H
H
H
H
CH3
H
H
H
H
H
CH3
H
H
H3C
int this case,
trans will put
1 methyl axial
and 1 methyl
equatorial
H
H
H
CH3
IN 1,4 SUBSTITUTION, THE TRANS ISOMER IS MOST STABLE
IN 1,3 SUBSTITUTION, THE CIS ISOMER IS MOST STABLE.
LOOK AT THE STRUCTURES TO SEE WHY. FOR BOTH ISOMERS, THE METHYLS
ARE IN EQUATORIAL POSITIONS.
CIS AND TRANS ARE JUST POSITIONAL NAMES...CIS MEAN SAME SIDE (OF RING)
TRANS MEANS ACROSS FROM (OPPOSITE SIDES OF RING)
V. Bridged Bicyclic compounds (nomenclature)
There's one other type of cyclic structure to mention. We'll not tear apart the
structure, we'll just learn a bit about how to name it.
Some rings can share (bonded together) 2 carbons. These are called bridged bicyclic
compounds. After the notes above, you should realize these are not flat molecules.
bridgehead
C atoms
1
2
3
5
4
What these molecules have in common is that the rings share 2 carbons, called the
bridgehead carbons.
Let's name them...
1C
0C
Structure 1:
4 carbons total, butane
3 bridges
1 carbon bridge
1 carbon bridge
0 carbon bridge
bicyclo[1.1.0]butane
1C
Structure 2:
5 C atoms total, pentane
3 bridges
1 C bridge
1 C bridge
1 C bridge
bicyclo[1.1.1]pentane
Structure 3:
7 C atoms total, heptane
3 bridges
2 C bridge
2 C bridge
1 C bridge
bicyclo[2.2.1]heptane
Structure 4:
8 C atoms total, octane
3 bridges
2 C bridge
2 C bridge
2 C bridge
bicyclo[2.2.2]octane
Strcture 5:
8 C atoms total, octane
3 bridges
3 C atom bridge
3 C atom bridge
0 C atom bridge
bicyclo[3.3.0]octane