Geometry of Organic Compounds
H
O
C
C
H
H
H
H
H
H
C
C
sp3
sp2
sp
Tetrahedral
Planar
Linear
Tetrahedron
H
How to Draw a Tetrahedral Structure
T bonds
Two
b d on the
th plane
l
off the
th paper, one towards
t
d you andd one away from
f
you.
In such structures, the bonds on the plane should be adjacent to each other. The
following two structures are not so right.
right
Drawing a bond in between the bonds on the plane may give a wrong configuration
The structures can also be drawn with one or no bonds on the plane of the paper
Drawing Ethane - Projections
Replacing one H-atom in methane with a CH3 group gives ethane.
W d structures
Wedge
t t
Sawhorse Projection
Newman Projection
Viewed from
the front
carbon to the
back through
the C-C bond
The front carbon is represented as a dot
and bonds are drawn from the dot.
dot
The back carbon is drawn as a circle and
bonds start from the circle
circle.
Dihedral Angle
The angle defined by X-C-C and C-C-Y plane is termed as dihedral angle.
Dihedral Angle
C
X
C
Y
Torsional angle is an alternative term used
for dihedral angle.
angle While dihedral angle is
measured from 0-360º, torsional angle is negative
measured from 0-180º, as a positive value
in the clockwise direction and a negative
value in the anti-clockwise direction.
0º
positive
180º
Conformations
Different spatial arrangements a molecule can adopt through rotation about sigma bonds
bonds.
Based on the dihedral angle between two groups of interest, a molecular conformation
can be defined using the following.
-30º
30º
syn
clinal
anti
-150º
periplanar
anti-periplanar
anti
periplanar
150º
anti-clinal
anti
clinal
syn-periplanar
syn-clinal
Conformations
Staggered - A low energy conformation where the bonds on adjacent atoms bisect each
other (60º dihedral angle), maximizing the separation.
Eclipsed - A high energy conformation where the bonds on adjacent atoms are aligned
a b
with each other (0º dihedral angle).
b
a
b
a
Gauche - Description given to two substitutents attached to adjacent atoms when their
bonds are at 60º with respect to each other.
a
60º
b
Strain in Organic Molecules
Steric
St
i (non-bonded
(
b d d iinteractions)
t
ti ) – Destabilization
D t bili ti ddue tto the
th repulsion
l i between
b t
the
th
electron clouds of atoms or groups. Groups compete to occupy common space. Strain
due to close contact of atoms separated by four or more bonds
Torsional – Strain resulting from eclipsing interactions between bonds on adjacent
carbon atoms. Actual reasons are highly debated.
Angular – Destabilization due to distortion of a bond angle from its optimal value
Possible Reasons for Torsional Strain
Eclipsed l
d
Staggered
d
Conformations of Ethane
2.9 kcal/mol
0
The staggered conformation is more stable than the eclipsed conformation by 12 kJ/mol
or 2.9 kcal/mol. There are infinite number of conformations in between both these
conformers
Conformations of Propane
Conformations of Butane
Butane has steric and torsional strain in its eclipsed conformation
Conformational Analysis of Butane
Energy Corresponding to Interactions in Alkane Conformations
T t l destabilization
Total
d t bili ti = 19 kJ/mol
kJ/ l
Total destabilization = 16 kJ/mol
Gauche Effect
The Gauche effect denotes the existence of a gauche conformer, which is more stable
than the anti conformer. This effect is present in 1,2-difluoroethane (CH2FCH2F) for
which the gauche conformation is more stable by 3.4 kJ/mol in the gas phase. Another
example
l is
i 1,2-dimethoxyethane.
1 2 di th
th
The increased stability of the gauche conformer is explained based on the possibility of
hyperconjugation between C-F and C-H bonds. This is initiated by the very high
electronegativity of fluorine.
fluorine
Effect of Intramolecular H-bonding
IIntramolecular
t
l l H-bonding
H b di between
b t
t
two
vicinal
i i l groups confers
f
an appreciable
i bl amountt
of stability (8-20 KJ/mol) to a conformer.
For effective Intramolecular H-bonding to occur, the donor and the acceptor groups
must be close to each other,
other which is possible only in the eclipsed or gauche
conformation.
The gauche conformations with a torsion angle of 60-70° between the interacting
ggroups
p are ideallyy suited for intramolecular H-bonding.
g
In general, 2-substituted ethanols of the type, X-CH2-CH2-OH, where X = OH, NH2, F,
Cl Br, OCH
Cl,
OC 3, NHCH
C 3, N(CH
(C 3)2 etc. have
h
the
h preferred
f
d gauche
h conformations
f
i
with
i h OH
O
and X forming intramolecular H-bonds. Usually the dihedral angle is slightly greater
than 60°.