Stereochemistry deals with the three-dimensional arrangements of atoms in molecules. There are different types of isomers including stereoisomers which have the same molecular formula but different spatial arrangements. Configurational isomers have fixed atomic arrangements while conformational isomers can rotate around bonds. Geometric isomers have different arrangements in space and optical isomers are mirror images that rotate plane-polarized light in opposite directions. The stability and reactivity of cyclic compounds is influenced by their conformations, with the chair form typically being most stable for cyclohexane. Chirality is important in biology as enantiomers can have different biological effects.

1. STEREO-
CHEMISTRY
PRESENTED BY:-
Dr. Om Prakash

2. ⚫ STEREOCHEMISTRY
Stereochemistry is the “chemistry of space” i.e. it
deals with the spatial arrangements of atoms and
groups in a molecule.

3. ELEMENTS OF SYMMETRY
A symmetry element is a geometrical entity such as point a line a plane about
which a symmetry operation is performed.
Axis of symmetry
An axis of symmetry is a line about which a figure is symmetrical. If a figure can be folded such
tha
t the two parts exactly match, the fold line is an axis of symmetry.

4. Plane of Symmetry:

5. Centre of symmetry:
•Alternating axis of symmetry:
CH₃
H
H
COOH
COOH
H
H
CH₃
COS

6. Isomerism
The compounds having same molecular formula but possesses
different properties (either physical or chemical or both) are called as
isomers and the phenomenon is called isomerism.

7. Molecules with the same molecular
formula
Whether superimposable?
No
Yes
Homomers
(Identical)
ISOMERS
(Structural Isomers)
Whether same connectivity/
Constitution?
Yes No
STEREOISOMERS CONSTITUTIONAL
ISOMERS
Configurational
Isomers
Conformational
Isomers
Geometrical Isomers Optical Isomers
Chain Isomers
Functional
Isomers
Positional
Isomers
Metamerism
Tautomerism

8. TYPES OF STEREOISOMERS
CONFIGURATIONAL ISOMERISM
•It contain fixed configuration i.e. rotation of groups about a bond is
restricted.
CONFORMATIONAL ISOMERISM
•Rotation of groups or atom towards C-C is free by different angles to
give infinite number of conformers.

9. TYPES OF CONFIGURATIONAL ISOMERISM
GEOMETRICAL ISOMERISM
•Compounds having same molecular formula but different geometrical
arrangement of groups or atoms in three dimensional space are known
as geometrical isomerism. They exhibit different properties.
OPTICAL ISOMERISM
•Compounds having same molecular formula, same physical and
chemical properties but differ in their optical properties towards plane
of polarized light are called optical isomers and this phenomenon is
known as optical isomerism.

10. Designation of configuration in Alkenes in Geometrical Isomerism
⚫ 1) Cis and trans configuration-

11. 2) E and Z configuration-
A new system of nomenclature was developed by
Cahn , Ingold and Prelog.
This nomenclature is based on priority rule.
E- entgegen (opposite) : Have same substituents on the opposite side of
the double bond, with more complex molecules having high priority
groups on opposite sides.
Z- zusammen (together): Have same substituents on the same side of the
double bond with more complex molecules high priority groups on the
same sides.

12. 1) Atom or group having higher atomic number directly attached
to stereogenic centre occupy higher priority.
2) If priority is not decided by first atom, then follow the atomic
number of second atom and so on.
3) Priority of isotopes is based on isotopic mass, greater the
isotopic mass, higher will be the priority.
4) Some groups considered as-
C C
C=C C C=C C C
C C
Priority Rule given by Cahn, Ingold and Prelog

13. examples :-
C=C
H₃C
H₅C₂
H
C=C
C=C-H
CH
CH₃
CH₃
C₂H₅
2
1
2
1
1
2 (3 Z, 5 E) 1,3,5 heptatriene
3 [sec-propyl] - (3 Z, 5 Z ) 1 yne 3,5
octadiene

14. Calculation of number of Geometrical Isomerism in Polyenes
When both ends of polyenes are different then –
No. of G.I.= 2n
where n= number of C=C bond
When both ends of polyenes are same then-
a) If number of C=C are even, i.e. n=2,4,6,8, etc.
No. of G.I.= 2n-1
+ 2(n/2)-1
b) If number of C=C are odd, i.e. n= 3,5,7,etc.
No. of G.I.= 2n-1
+ 2(n+1/2)-1

15. OPTICAL ACTIVITY: It is ability of chiral molecule to rotate the plane of
plane polarized light, measured using a polarimeter.
PLANE POLARIZED LIGHT: Light that has been passed through a nicol
prism or other polarizing medium so that all of the vibrations are in
the same plane.

16. POLARIMETER: An instrument used to measure optical activity.
A simple polarimeter consist of a light source, polarizing lens, sample
tube and analyzing lens.
Counterclockwise
Levorotatory (-)
Clockwise Dextrorotatory
(+)

17. Specific Rotation
Observed rotation depends on the length of the cell and
concentration, as well as the strength of optical activity,
temperature, and wavelength of light.
Here,
a = observed rotation in degrees
l = pathlength (length of tube in decimeter)
c = concentration in g/ml
[α] = observed rotation
λ = wavelength of emitted light

20. OPTICAL ACTIVITY IN THE ABSENCE OF A CHIRAL CARBON
⚫ Allenes-
• Spirans-
• Biphenyls-

21. ENANTIOMERS
They are configurational isomers, optically active compound and
they are the non superimposable mirror images of each other.
They possess same physical and chemical properties but differ in
their optical activity towards plane of polarized light.

22. MESO COMPOUNDS
It is optically inactive compound like d-l mixture but from meso
compound (+)d & (-)l compound cannot be separated.
Meso compounds are optically inactive due to internal
compensation present in the molecule.

23. DIASTEREOMERS
Those stereoisomers which are not the mirror images of each
other are called diastereomers.
Diastereomers are different compounds therefore they possesses
different physical and chemical properties.

24. CONFIGURATION
The arrangement of atom that characterize a particular stereoisomers
is called its configuration.
The following methods are used to assign configuration :
A. Relative configuration ( D & L system)
B. Absolute configuration (R & S system )

26. R & S notation

28. CONFORMATIONAL ANALYSIS OF CYCLOBUTANE

29. CONFORMATION OF CYCLOALKANES
Stability of cycloalkanes (Baeyer’s Strain Theory)-
According to Baeyer’s-
Cyclopropane < Cyclobutane < Cyclopentane > Cyclohexane

30. This theory is applicable only to cyclopropane, cyclobutane and cyclopentane. He
assumed that all cyclic compounds are planar. But cyclopropane is planar and
other cycloalkanes are not planar.
Other types of strains are:-
1. Angle Strain (Baeyer Strain)- This results when the bond angle is different
from the desired tetrahedral bond angle of 109.5 °.
2. Torsional Strain (Pitzer Strain)- This is caused by repulsion of the bonding
electron of one substituent with the bonding electrons of another substituents
on the adjacent atom.
3. Steric Strain (Vander Waals Strain)- This is caused by atoms or groups
approaching each other too closely.
d= ½[109.5°-α]
where α = (2n-2)/n 90°
d= angle deviation
n= number of C-C bonds in cycloalkane

31. CONFORMATIONAL ANALYSIS OF CYCLOHEXANES
Sachse (1890) proposed that cyclohexane and larger rings are not
planer but they are puckered in which all angles are tetrahedral and
thus the rings are stainless.
Cyclohexane exists in two forms which are called the Chair and
Boat conformation.
Mohr proposed these forms are readily undergo interconversion
by rotation about single bond.
Both chair and boat forms are free from angle strain.
Chair conformation is most stable than the boat
conformation.
The chair conformation is rigid because when is changed to the
boat conformation , some angular deformation is necessary
The energy barrier for this transformation is 10.8 kcal/mole.

34. CONFORMATIONS OF FUSED RING: DECALINS
Trans decalin is more stable than the cis by 2.7 kcal/mole.

35. EFFECT OF CONFORMATION ON REACTIVITY
• E2 reaction in acyclic compounds-

36. • Esterification and hydrolysis in cyclic compounds-
• Elimination reaction in cyclic compounds-

37. BIOLOGICAL SIGNIFICANCE OF ENANTIOMERS
Limonene is a compounds extracted by some plants and it exists in two
enantiomers that are characterised by specific rotation, [α]D
, values of +123
and -123, respectively.
Found in orange Found in lemon

38. ROLE OF ENANTIOMERS IN DRUGS
S – thalidomide R - thalidomide
Sedative
(to calm
nervousness)
Teratogen
(causes birth
defects)

39. BIOLOGICAL SIGNIFICANCE OF CHIRALITY
(Enantiomers of epinephrine)
(-) - Epinephrine (+) - Epinephrine
Better fit, more active Poorer fit, less active