CHEM 3013
ORGANIC CHEMISTRY I
LECTURE NOTES
CHAPTER 4
1.
Conformations of n-alkanes
Single bonds allow rotation around the sigma bond axis. The atoms attached to the bonds
will try to achieve a conformation (position of atoms in space due to bond rotation) where the local
interaction of all groups is minimized. By studying a few simple alkanes, we can derive some
"guidepost numbers" that represent some destabilizations which occur in common molecular
arrangements. These numbers can then be applied in more complex settings to evaluate the most
probable conformation of a molecule. This will be of great importance in understanding the
effect of structure on chemical reactivity.
a.
Ethane
A 3-D perspective drawing such as the saw horse is sometimes written as a side view
projection of convenience. The vertical lines in each side view projection are in the plane of the
paper. Hashed lines drawn to the left are behind the plane. Bold lines drawn to the right are out in
front of the plane.
A more useful representation to show rotation around a C-C bond is the Newman
projection. The view is down the C-C bond axis with one carbon eclipsing the other. Bonds
attached to the front carbon are shown by lines intersecting at the center of a circle. Bonds attached
to the rear carbon are shown by lines attached to the circumference of the circle.
H
H
H H
H
HH
H
H
H
H
H
H
H
H
H
HCCH
dihedral angle = 0
(rear H's behind
front H's)
Staggered
Conformation
H
H
H
HCCH
dihedral angle = 60
(rear H's bisect
front H,s)
H
H
H
E
HH
Newman
Projection
H
H
E
N
E
R
G
Y
H
H
H
H
Eclipsed
Conformation
H
Sideview
Projection
Sawhorse
Representation
H
HH
H
H
E
E
E
2.9 Kcal/Mole
S
0
60
S
120
180
Rotational Angle
Conformations of ethane
S
240
300
360
1
b.
2
Butane
Butane has a more complex conformational equilibrium than ethane. The fully eclipsed
form of butane 1 is 4.5 Kcal/mole more destabilized (higher in energy) than the anti form 4. Since
we know the value of two H,H eclipsing interactions from our analysis of ethane, i.e. 2 x 1
Kcal/mole = 2 Kcal/mole, we can now assign an energy to the CH3, CH 3 eclipsing interaction of
2.5 Kcal/mole. Similarly, the value of a CH 3, H eclipsing interaction can be determined as (3.81)/2 = 1.4 Kcal/mole. A final interaction that can be obtained from this study is the value of
gauche butane destabilization of 0.9Kcal/mole.
CH3
H 3C H
H
CH3
H
HH
H
H
CH3
H
H
CH3
H
H
H3CCH3
H
H
H
HH
Fully Eclipsed
1
H3CH
H
H
H
H
Gauche
2
H
CH3
H
CH
3
H
Partly Eclipsed
3
CH3
H
H
CH3
Anti
4
1
1
3
3
E
N
E
R
G
Y
H
H
H
CH3
CH3
CH3
H
H
H
CH3
4.5 Kcal/mole
3.8 Kcal/mole
2
2
0.9 Kcal/mole
0
60
4
120
180
Rotational Angle
240
300
360
Conformations of n-butane
2.
Conformations of Cyclohexanes
Von Baeyer predicted in 1885 that the stability of cycloalkanes would be related to their
angle strain and should be correlated to the internal bond angles of the planar structures. The
further away from the ideal tetrahedral angle of 109˚, the more strain the structure should have.
3
60
90
108
120
128.6
135
Bond Angles for Flat Cycloalkanes
Isomers can be compared by determining their Heats of Formation. Members of a
homologous series can be compared by evaluation of their heats of formation per -CH2- unit. By
doing this we can see that von Baeyer's prediction is in error, cyclopentane is not the most stable
cycloalkane, cyclohexane is. Moreover, this compound is strain free, it has the same heat of
formation per -CH2- unit as does n-hexane. Von Baeyer's mistaken assumption was that all these
molecules would be planar.
Heat of Formation for Acyclic (linear) -CH2Ring
Size
3
4
5
6
7
8
Heats of Formation
in Kcal/mole
+12.7
+ 6.8
- 18.4
- 29.5
- 28.2
- 29.7
Heats of Formation
in Kcal/mole per
CH2 unit
+ 4.2
+ 1.7
- 3.7
- 4.9
- 4.0
- 3.7
= -4.90 Kcal/mole
Total Strain
Energy in
Kcal/mole
+ 27
+ 26
+ 6
0
+ 6
+ 10
CH2 Strain Energy
in Kcal/mole
+ 9.1
+ 6.6
+ 1.2
0
+ 0.9
+ 1.2
Heats of Formation of Cycloalkanes
a.
Cyclopropane and Cyclobutane
Cyclopropane has no conformational mobility, and is thus forced to be planar. The six
pairs of eclipsing hydrogens in cyclopropane contribute 6.0 Kcal/mole of Torsional Energy . The
balance of the 27 Kcal/mole of Total Strain Energy is contributed by Angle Strain . Cyclobutane
has almost as much Total Strain as does cyclopropane because its greater number of ring
hydrogens (8 vs. 6) results in more Torsional Strain even though it has less Angle Strain. In
addition, evidence shows the cyclobutane is not quite flat. It has enough conformational mobility
that the ring can bend slightly, so that one carbon lies about 25˚ above the plane of the other
three.
4
HH
H
H
H
H
CH2
H
H
6 Pairs of Eclipsing H's
H
H
6 x 1 = 6.0 Kcal/mole Torsional Strain
Total Strain - Torsional Strain = Angle Strain
+ 27 Kcal/mole - 6.0Kcal/mole = 21 Kcal/mole Angle Strain
Strain in Cyclopropane
H
H
H
H
Not
Quite
Eclipsed
25deg.
H2C CH2
Bent Conformation of Cyclobutane
b.
Cyclopentane
Von Baeyer predicted cyclopentane to be strain free (CCC angles of 108˚),but experiments
indicate that it has 6 Kcal/mole of strain energy. Although cyclopentane has no Angle Strain it
does have considerable Torsional Strain due to the ten pairs of neighboring hydrogens.
Cyclopentane has enough conformational mobility to adopt a puckered shape in order to minimize
destabilization. In the envelope conformation, four of the ring carbons are in approximately the
same plane, with one carbon bent out of the plane.
Look down C1-C2 axis
H
1
H
CH2
2
2
H
1
H
CH2
CH2
Planar cyclopentane - Predicted Torsional Strain - 10 Kcal/mole
Actual Total Strain - 6 Kcal/mole
Cyclopentane deviates from planarity to relieve Torsional Strain
Envelope Conformation of Cyclopentane
3.
5
Conformations of Cyclohexane
Cyclohexane has no Strain Energy, that is it has no Angle Strain nor Torsional Strain.
Obviously, this precludes the possibility of cyclohexane existing in the planer structure (12 pairs of
eclipsing H's would result in at least 12 Kcal/mole of Torsional Energy). Cyclohexane has enough
flexibility that it can adopt a strain free structure known as the Chair conformation. In this
structure the bond angles are all close to tetrahedral and all pairs of hydrogens are completely
staggered with respect to one another. The latter point can be seen by looking down each carboncarbon bond in turn to produce the Newman projection shown below.
H
H
H
H
H
H
HH
H
H
H
H
H
H
H
H
H
H
Planar Conformation
H
H
H
H
H
Chair Conformation
CCC angles 120 deg.
At least 12 Kcal/mole
Torsional Strain
H
H
CCC angles 109 deg.
No Torsional Strain
H
H
H
H2
C
C
H2
H
H
H
H
Chair Conformation of Cyclohexane
a.
Axial - Equatorial Interconversion
The chair conformation has two distinct types of hydrogen atoms. These different
hydrogens are named the Axial and Equatorial hydrogens as shown below. The axial hydrogens
are perpendicular to the mean ring plane, whereas the equatorial hydrogens project out along the
ring circumference (i.e. equator). The molecular axis is a three-fold axis, rotation by 120˚ about
the axis leaves the molecular representation unchanged.
6
Six Equivalent Axial H's
Six Equivalent Equatorial
Hydrogens
Three-fold axis
of symmetry
Axial and Equatorial Hydrogens
The chair conformation of cyclohexane has two structurally different types of
hydrogen atoms (axial and equatorial), but attempts to discern any difference in type by chemical
reactions are unsuccessful. Usually, different "types" of hydrogens demonstrate (often subtle)
differences in chemical behavior. However, all hydrogens in cyclohexane behave identically.
Similarly, we might expect to find two isomeric forms of a monosubstituted cyclohexane
(substituent at the axial or equatorial position). This is not the case. Cyclohexane is a
dynamic structure. A concerted partial rotation about the carbon-carbon bonds changes one
chair conformation to another in which the axial and equatorial bonds have changed places. The
interconversion of chair conformations, usually referred to as a ring-flip, is shown below.
Ring
Flip
move this carbon
up
move this carbon
down
Interconversion of Chair Conformations
There are other, higher energy, cyclohexane conformations called the boat, twist-boat and
half-chair forms.
7
Flagpole
Boat - Four pairs of eclipsing hydrogens,
in addition to steric repulsions of
"flagpole" hydrogens
Twist Boat - The two "flagpole" hydrogens
are offset so that they do not
directly run into each other.
Half Chair- The four planar carbons result in
Eclipsing interactions
Conformations of Cyclohexane
half-chair 1
10.1 Kcal
E
N
E
R
G
Y
half-chair 2
boat
twist-boat
7.0 Kcal
5.5 Kcal
chair 1
chair 2
Energetics of Cyclohexane Forms
4
Monosubstituted Cyclohexanes
The interconversion of equatorial monosubstituted cyclohexane to its alternative chair form
generates a conformation which has the substituent axial. This form is no longer identical to the
equatorial form from which it arose. These two forms have differing energy content due to the
different environments of the substituent.
H
H
H
H
H
H
H
H
CH3
H
H
H
H
H
H
CH3
H
Axial-Equatorial Methylcyclohexane
H
H
H
H
H
H
H
8
1,3-Diaxial interaction, steric
repulsion, 1.8 Kcal/mole
Similar to Guache butane
worth 1.8 Kcal/mole
H
H
H
H
H
H
H
H
HH
2 3C
C
C
H2
H
CH3
H
H
H
H2 H
C
C
H2
H
H
CH3
CH3
H
No Destabilizing interactions
1,3-Diaxial Destabilization
The amount of energy required to convert from an equatorial to an axial form is called the
A-Value for a given substituent.
Typical A Values for Monosubstituted Cyclohexanes
(Kcal/mol)
F
Cl, Br
OH
OCH3
NH2
0.25
0.50
0.70
0.70
1.80
CH3
C2H5
n-C3H7
i-C3H7
t-C4H9
1.80
1.90
2.10
2.10
5.40
Typical A-Values for Monosubstituted Cyclohexanes
5.
Disubstituted Cyclohexanes
a.
Trans 1,2-Dimethylcyclohexane
This stereoisomer of dimethylcyclohexane has two substituents on opposite sides of the
cyclohexane ring in the chair conformation. This gives rise to two possible chair
conformers: one with both methyls equatorial and one with both axial. They have different
energies as a result of different destabilizing interactions present. The diequatorial conformer is
lower in energy than the diaxial conformer by 2.7 Kcal/mole. The diequatorial form will account
for 99% of the equilibrium mixture; the diaxial for only 1%.
CH3
CH3
CH3
CH3
Chair 2
Chair 1
H
H
H
H
H2 H
C
C
H2
H
CH3
CH3
H
The diequatorial conformation
has one butane gauche type
interaction...results in 0.9 Kcal/mol
destabilization energy
H
H
H
H2 CH3
C
H
C
H2
H
CH3
The diaxial conformation
has two 1,3-diaxial type
interactions...3.6 Kcal/mol
destabilization energy
The difference in energy between the two forms favors the
diequatorial conformer by 2.7 Kcal/mol.
Keq = e-∆G/RT
Keq = 0.01
Interconversion of trans-1,2dimethylcyclohexane
b.
cis-1,2-Dimethylcyclohexane
This stereoisomer of dimethylcyclohexane has two substituents on the same side of the
cyclohexane ring in the chair conformation. The two possible conformations each have one
substituent axial and one substituent equatorial. Thus they have the same extent of destabilizing
interactions.
9
10
CH3
CH3
CH3
CH3
Chair 2
Chair 1
H
H
H
H2 CH3
C
CH3
H
C
H2 H
H
H
H2 H
C
H
H
H
CH3
C
H2 CH
3
H
Same for this form.
The monoaxial-monoequatorial
form has one gauche type interaction
and one 1,3-diaxial type interaction for
2.7 Kcal/mole destabilization energy
There is no difference in energy between the two forms...
each will be present in equal amounts (50%:50%).
Interconversion of cis-1,2dimethylcyclohexane
c.
cis-1,3-dimethylcyclohexane
This compound has two substituents on the same side of the ring and has two possible
chair conformations. One conformation has a new type of 1,3-diaxial interaction...a 1,3-diaxial
methyl-methyl interaction. This interaction is worth 3.7 Kcal/mol of destabilizing energy.
CH3
CH3
Chair 2
Chair 1
H
H
H
H
H2 H
C
CH3
H
CH3
H
H
H
CH3H
NO destabilizing steric interactions
H2 H
C
H
H
H
CH3CH3
Two gauche butane type interactions
and one 1,3-diaxial methyl-methyl
interaction for a total destabilization
energy of 5.5 Kcal/mol.
Interactions in cis-1,3-dimethylcyclohexane
11