Stereochemistry, a subdiscipline of chemistry, that deals with the study of three dimensions structure, reactions, and properties of given molecules.

1. STEREOCHEMISTRY
Dr. Ashwani Dhingra
Associate Professor
GGSCOP, Yamunanagar
Introduction to Stereochemistry

2. Stereochemistry is the branch of science that deals with the study of three
dimensions structure, reactions and properties of given molecules.
The physical, chemical and biological properties of the molecules depends
upon the stereochemistry i.e. the position of atoms with in the
molecule.
Compounds having the same molecular formula but different structure are
known as Isomers and this phenomenon is known as Isomerism.
Introduction

3. Isomers
Chain Position Funtional Metamerism Tautomerism
Ring/Double
bond
Structural/
Constitutional
Stereo/Space
Configurational Conformational
Geometrical Optical
Cis-Trans EZ System Enantiomers
Diastereomers

4. Structural isomers possess same molecular formula but
different structural formula i.e. they have different IUPAC
names, the same or different functional groups, different physical
and chemical properties.
Stereo isomers possess same molecular and structural formula
but differ in Configuration (i.e. the particular three-dimensional
arrangement in which atoms are oriented in space. They have
identical IUPAC names (except for a prefix like cis or trans).
They always have the same functional group/s.
Isomers

5. Consitutional Vs Stereoisomers…

6. The compounds having same molecular formula but different carbon
skelton chain is known as Chain or Nuclear isomers: e.g. (Pentane,
C5H12)
Chain isomers

7. When the same substituent occupies different positions in organic
compounds having same molecular formula, the compounds exhibit
position isomers. In position isomerism, the basic carbon skeleton
remains unchanged.
CH2
H2C
H3C I CH3
C
H
H3C
n-propyl iodide or
1-iodopropane
Isopropyl iodide or
2-iodopropane.
I
Position isomers

8. Organic compounds having same molecular formula but different
functional groups exhibit functional isomers.
Functional isomers

9. C2H5 O C2H5
Diethyl ether
H3C O CH2 CH2 CH3
Methyl n-propyl ether
H3C O C
H
CH3
CH3
Methyl isopropyl ether
Metamerism
Some members of the same homologous series differ in the nature of
the alkyl groups attached to the polyvalent atom of the functional
group. Such compounds are said to exhibit metamerism.
Diethyl ether, methyl n-propyl ether and methyl isopropyl ether are
metamers. Since all of them have the same molecular formula i.e.
C4H10O but the alkyl groups are different.

10. It is a dynamic isomerism where one isomer is constantly changing
into the other and vice versa. This is a special case of functional
isomerism.
Alkyl cyanides (RCN) and alkyl isocyanides (RNC) are also
tautomers.
Tautomerism

11. Such type of isomers exist between ring/cyclic structure (like
cycloalkanes) and acyclic alkene since both are having same
molecular formula i.e. CnH2n.
H2C CH CH3
Propene
(C3H6)
CH2
H2
C
H2C
Cyclepropane
(C3H6)
Ring/Double bond Isomers

12. Conformational isomers are stereoisomerism that can be inter-
converted just by rotations about formally single bonds. The
different arrangements of atoms within a molecule due to
rotation about single bonds known as Conformers/Rotamers.
1,2-dichloroethane
Stereoisomerism

13. Configurational isomers are stereoisomers that can cannot be
converted into one another by rotation around a single bond. The
two main types of configurational isomers are geometric
isomers and optical isomers.
1,2-dichloroethene.
Stereoisomerism

14. Geometrical isomers contain double bonds, which cannot rotate,
however they can also arise from ring structures, wherein the rotation
of bonds is greatly restricted.
H
H3C
H
CH3
H
H3C
CH3
H
cis-1,2-dimethylcyclopropane trans-1,2-dimethylcyclopropane
H3C
H H
CH3 H3C
H CH3
H
cis-2-butene trans-2-butene
Geometrical isomerism

15. Cis-Trans & E-Z
Cis-Trans: The terms cis and trans are derived from Latin, in which cis
means "on the same side" and trans means "on the other side" or "across".
If the total number of different substituents on a double bond is 2, then
geometric isomers may be classified as cis- or trans-.

16. Cis-Trans & E-Z
E-Z: When the total number of substituents are 3 or 4, the isomers are
classified by assigning the order of priority (CIP rule) of subsituents at
each carbon of the double bond and giving an E designation to an alkene
with the high priority substituents having trans- configuration and Vice-
varsa.

17. 1-pentene 3-ethyl-3-hexene
However if one of the two carbon atoms of the double bond has two
identical substituents, then there are no cis-trans isomers for that
molecule
1,1-dichloroethene

18.  Trans isomers have higher melting points than the corresponding cis
isomers as the former is more symmetrical and fits well into the crystal
structure.
 Cis isomers have higher boiling point than corresponding trans isomers
because of the stronger intermolecular forces between the molecules of
the cis isomers than trans isomers.
Isomer melting point (°C) boiling point (°C)
Cis -80 60
Trans -50 48
Physical properties of Geometrical isomers

19. Dipole Moment: Generally Cis isomers have greatest dipole
moment than trans isomers with some exception.
Physical properties of Geometrical isomers

20. Identifying cis and trans isomers of given
alkenes

21. Optical activity: The ability of a molecule to rotate the path of plane
polarized light is known as optical activity and the compound which
have ability to change the path of plane polarized light are known as
optically active.
The necessarily and sufficient condition for a molecule to behave as
optically active is chirality or molecular dissimilarity.
Optical isomerism

22. 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.

23. Plane-Polarized Light through a Chiral
Compound

24. Polarimeter
An instrument used to measure optical activity. A simple polarimeter
consists of a light source, polarizing lens, sample tube and analyzing
lens.
589 nm -
D-line of
a sodium
lamp

25. The angle of rotation of plane polarized light by a 1.00 gram per cm-3
sample in a 1 dm tube.
[α ]D (D = sodium lamp, λ
= 589 mμ).
α
l × d
where α = observed rotation
l = length (dm)
d = concentration (g/cc)
(+)-alanine [ α ]D = +8.5
(-)-lactic acid [α ]D = -3.8
Specific Rotation depends on the nature of substance, wavelength of
light used, concentration of the solution, thickness of the layer, nature
of the solvent and temperature at which experiment is conducted.
Specific rotation
[ α ]D =

26. Dextrorotatory (+):
An optically active compound that rotates plane polarized light in a clockwise
direction. It is represented by (+) or (d).
Levorotatory (-):
An optically active compound that rotates plane polarized light in a counterclockwise
direction. It is represented by (-) or (l).
COOH COOH
H C OH
CH3
(+)/d - Lactic Acid
OH C H
CH3
(-)/l - Lactic Acid

27. Enantiomers:
Stereoisomers that are non-superimposible mirror images of each
other; only properties that differ are direction (+ or -) of optical
rotation.
Diastereomers:
Stereoisomers that are not mirror images; different compounds with
different physical and chemical properties.
An equimolar mixture of the two enantiomers is known as Racemic
mixture. Racemic mixtures are optically inactive.

28. Chirality
Chiral objects have Non-superimposable mirror images.
Achiral objects - objects that have superimposable mirror images
Non-superimposable mirror images - a mirror image that is not the same as the
image itself.

29. Assymetric Center
Chiral molecules - generally molecules containing an asymmetric center
Asymmetric (chiral) center - tetrahedral atom bonded to four different groups
indicated with an asterisk (*)

30. A point in a molecule where four different groups (or atoms) are
attached to carbon is called a Chiral center.

32. H Cl
H OH
H CH3
Label the stereogenic centers in each molecule
and decide if it is chiral….?
Br
Br

33. Thank You
Dr. Ashwani Dhingra
9996230055
ashwani1683@gnkgei.ac.in