Organic Chemistry
By
Dr. Mehnaz Kamal
Assistant Professor,
Pharmaceutical Chemistry
Prince Sattam Bin Abdulaziz University
1- What is Conformational Isomers.
2-How they could be Classified?
3-. What is Eclipsed & Staggered
conformation
4-What are Newman " and sawhorse
projection.
5-Conformation of cycloalkane.
Conformations
Different internal arrangements of the atoms in a molecule that differ
by rotation(s) about one or more of the single bonds.
Conformations are NOT ISOMERS they are different arrangements
of the SAME MOLECULE.
The connection pattern of the atoms does not change.
Most Atoms Attached By A Single Bond Are Free To
Rotate Around The Bond So The Two Ends Of The
Bond Can Rotate Relative To Each Other
H
H
C
H
The Carbon And Three Hydrogen Atoms On
The Right End Of The Bond Can Spin Relative
To The Carbon And Other Three Hydrogens On
The Left End Of The Bond
fix
H
C
H
H
rotate
Conformations
 The conformation of a molecule containing two tetrahedral atoms linked
together can be represented as a "Newman " or as sawhorse projections.
John von Neumann
Newman projections
In the Newman projection the molecule is viewed along the
axis of a rotatable bond.
Conformations
*Sawhorse Projections
Sawhorse Projections are very similar to Newman Projections, but are used
more often because the carbon-carbon bond that is compressed in a Newman
Projection is fully drawn out in a Sawhorse Projection.
Carbon arrangement
Conformations of Lower Alkanes
 Interconversion of conformers proceed
by an infinite number of intermediate
conformers, however, only a few of
them have energy minima.
• Staggered conformation: a conformation
about a carbon-carbon single bond where the
atoms on one carbon are as far apart from
atoms on the adjacent carbon
• Eclipsed conformation: a conformation
about a carbon-carbon single bond where
atoms on one carbon are as close as possible
to the atoms on the adjacent carbon
H
H
H
H
H
H
H
H
H
H
HH
•Torsional Strain
•The force that opposes the rotation of one part of a molecule about
a bond while the other part of the molecule is held fixed
Electron Pair Repulsions Determine The Energy Difference
ECLIPSED
STAGGERED
Hd H
:
:
C C
H
:
C
a
D
C
.
Shorter Distance
Larger Repulsions
For Eclipsed
H
.
Longer Distance
Repulsions Are Less
For Staggered (D > D)
The torsional strain between eclipsed and staggered ethane is approximately 2.9
kcal/mol
H H
H
HH
H
+2.9 kcal/mol
H
H
H
HH
Conformations of Higher Alkanes
Each eclipsed conformation now consists of two eclipsed
C-H bonds and one C-H eclipsed with a CH3 bond. We
can infer that the latter eclipsing interaction has an
energetic "cost" of 3.3 kcal/mol.
Bear in mind that these energies do not measure electronic
repulsion in any absolute sense, but merely provide figures
relative to the more stable staggered state.
In the most stable conformation, the two methyl groups lie as far
apart from each other as possible with a dihedral angle of 180
degrees. This particular staggered conformation is called anti.
The other staggered conformation has a Me-Me dihedral
angle of 60 degrees and is called gauche
The gauche form is less stable than the anti form by 0.9
kcal/mol due to steric hindrance between the two methyl
groups.
(C2-C3 bond)
Conformations Of Butane (Newman)
CH3
SYN (0o)
H
H
H
GAUCHE
ANTI (180o)
CH3
(60o)
H
H
H
H
H
CH3
ECLIPSED (240o)
CH3
H
CH3
H
H
CH3
H
H
ECLIPSED (120o)
CH3
CH3
H
H
H
CH3
GAUCHE (300o)
CH3
H
H
H
CH3
H
CH3
H
H
H
H
Conformations Of Butane Sawhorse Projections
SYN
CH3CH3
H
H
H
H
CH3H CH
3
GAUCHE
H
ECLIPSED
CH3H
ANTI
H
H H
CH3
H
H
CH3
CH3H
H
CH3
H
CH3H
GAUCHE
H
H
H
ECLIPSED
H
CH3
H
CH3
H H
H
E
x
a
m
p
l
e
2-Stereoisomerism…cont.
I-Geometric isomerism…cont
2- Geometric isomerism in cyclic compounds
The most illustrative example is cyclohexane
H
H
CH3
OH
Rotation of group ( atoms ) around sigma bonds,
result in different conformations ( conformer )
Planar Cyclohexane
(doesn’t exist)
If cyclohexane were planar all of the hydrogens
would be eclipsed, resulting in torsional strain.
There would also be angle strain - a hexagon has
120o internal angles.
Cyclohexane is not planar ...
Ball-and-stick model
In chemistry, the ball-and-stick model is a molecular
model of a chemical substance which is to display
both the three-dimensional position of the atoms and
the bonds between them.
Chair
Boat
Stereochemistry
2- Geometric isomerism in cyclic compounds
Axial hydrogen ( Ha ) : H parallel to axis (in vertical plane)
Equatorial hydrogen ( He ) : H in plane (in horizontal plane)
upon flipping and reflipping between conformers, axial becomes
equatorial and equatorial becomes axial
Mono-substituted cyclic compounds, do not exhibit
geometrical ……… cis or trans
CH3
6
5
1
4
CH3
CH3
2
6
3
5
1
4
CH3
2
6
3
5
1
4
CH3
6
2
3
5
CH3
1
4
1,2
1,3
3
CH3
1,4
mono-substituted
2
2- Geometric isomerism in cyclic compounds
1,2-disubstituted : same as 1,4-disubstituted
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
Cis
both down = e,a
both up = a,e
Trans
up and down = a,a
down and up = e,e
1,4-disubistuted cyclohexane as 1,2
In summary : a,e
a,a
e,a
e,e
Stereochemistry
2- Geometric isomerism in cyclic compounds
1,3-disubstituted :
CH3
CH3
CH3
CH3
CH3
both down = e,e
both up = a,a
cis
CH3
a,a
both up
e,e
both down
CH3
CH3
CH3
CH3
CH3
CH3
a.e
up and down
e,a
down and up
trans
up and down = a,e
down and up = e,a
Label each of the following disubstituted rings as
cis or trans and as a,a ; e,e ; or a,e
Cl
H
H
Cl
Cl
Cl
H
H
Trans e,e
Cis a,e
H
H
Cl
H3C
Br
Cl
H
Trans e,e
H
Trans e,e
The Chair Conformation Has Lower Energy Than The
Boat
E
N
E
R
G
Y
29 kJ/mol
boat
0 kJ/mol
chair
•In a chair conformation, six H are equatorial and six are axial
Ha
He
Ha
Ha
He
He
He
Ha He
Ha
Ha
• For cyclohexane, there are two equivalent chair conformations
– all C-H bonds equatorial in one chair are axial in the other, and
vice versa
Ha
He
Ha
Ha
He
He
He
He
He Ha
He
Ha He
Ha
Ha
Ha
Ha
Ha He
He
He
Ha
Ha
Cis-trans isomerism
• trans-1,4-dimethylcyclohexane
– the diequatorial-methyl chair conformation is more
stable by approximately 2 x (-1.74) = -3.48 kcal/mol
CH3
H
H
H
CH3
H3 C
(less s table)
(more s table)
trans -1,4-Dimethylcyclohexane
CH3
H
•cis-1,4-dimethylcyclohexane
H
H
CH3
H3 C
H
H
CH3
cis -1,4-Dimethylcyclohexane
(conformations are of equal stability)
CH3