basic data of streochemistry

1. Presented by
Dr. Gopalkrushna
H. Murhekar

2.  Introduction
 History
Chiral and Achiral molecules
Optical Activity
Racemic Mixtures
Baeyer Strain Theory
 Conformations
References

3. Stereochemistry, a subdiscipline of chemistry,
involves the study of the relative spatial
arrangement of atoms that form the structure of
molecules and their manipulation. An important
branch of stereochemistry is the study of
chiral molecules. Stereochemistry is also known
as 3D chemistry because the prefix "stereo-"
means "three-dimensionality".
The study of stereochemistry focuses on
streoisomer and spans the
entire spectrum of organic, inorganic, physical,
boilogical and especially supramolecular chemistry.

4. History
Louis Pasteur could rightly be described
as the first stereochemist, having observed in
1849 that salts of tartaric acid collected
from wine production vessels could rotate plane
polarized light , but that salts from other sources
did not. This property, the only physical property
in which the two types of tartrate salts differed, is
due to optical isomerism.
In 1874, Jacobus Henricus van 't Hoff and Joseph
LeBel explained optical activity in terms of the
tetrahedral arrangement of the atoms bound to
carbon.

5. Stereoisomers – compounds with the same connectivity,
different arrangement in space.
Enantiomers – stereoisomers that are non-superimposible
mirror images; only properties that differ are
direction (+ or -) of optical rotation.
Diastereomers – stereoisomers that are not mirror images;
different compounds with different physical
properties.

6. • Although everything has a mirror image, mirror images may or may not be
superimposable.
• Some molecules are like hands. Left and right hands are mirror images, but they are not
identical, or superimposable.
Chiral and Achiral Molecules

7. 7
• Other molecules are like socks. Two socks from
a pair are mirror images that are
superimposable. A sock and its mirror image
are identical.
• A molecule or object that is superimposable on
its mirror image is said to be achiral.
• A molecule or object that is not
superimposable on its mirror image is said to
be chiral.

8. Optical Activity
• Enantiomers rotate the plane of polarized light in opposite directions,
but same number of degrees.

9. Polarimeter

10. Racemic Mixtures
If optically active reagents combine to form a chiral molecule, a racemic
mixture is formed. It is optically inactive.

11. Johann Friedrich Wilhelm
Adolf von Baeyer
• First graduate student of Kekule’
• Developed theory that cycloalkanes possess different amounts
of strain or stability, depending on the size of the ring.
• Awarded Nobel Prize in 1905 for work with organic dyes.
"I have never set up an experiment to see whether
I was right, but to see how the materials behave".
Baeyer Strain Theory

12. The bond angel is of 109˚ 28' (or 109.5˚) for carbon atom in tetrahedral
geometry (methane molecule).
Baeyer observed different bond angles for different cycloalkanes and also
observed some different properties and stability.
On this basis, he proposed angle strain theory. The theory explains reactivity
and stability of cycloalkanes.
Baeyer proposed that the optimum overlap of atomic orbitals is achieved for
bond angel of 109.5o .
In short, it is ideal bond angle for alkane compounds. Effective and optimum
overlap of atomic orbitals produces maximum bond strength and stable
molecule.
If bond angles deviate from the ideal then ring produce strain. Higher the
strain higher the instability.
Higher strain produce increased reactivity and increases heat of combustion.
Baeyer proposed “any deviation of bond angle from ideal bond angle value
(109.5o ) will produce a strain in molecule. Higher the deviation lesser the
instability”

13. 4-13

14. Conformations
different spatial arrangements of atoms that result from rotations about
single (σ) bonds
Conformer: a specific conformation of a molecule
Conformational Analysis of Ethane
Sawhorse
C C
H
H
H
H
H
H
HH
H
H H
H

15. Staggered EclipsedNewman projection
There are two conformations of ethane:
Dihedral (torsion) angle: angle between an atom (group) on the front atom of a Newman
Projection and an atom (group) on the back atom
H
H
H
H
H
H
Front
carbon
Back
carbon

16. 16
Energy vs. dihedral angle for ethane

17. Conformation of Cyclohexane

19. Energy Diagram of Cyclohexane

20. References