This document discusses stereochemistry and isomerism. It defines constitutional and stereoisomers, and describes different types of constitutional isomers like chain, position, functional, and tautomeric isomers. It also discusses configurational isomerism including optical isomers like enantiomers and diastereomers. Chirality and chiral centers are explained. Methods to represent 3D structures in 2D like Fischer projections are introduced. The document also covers topics like optical activity, polarimetry and racemic mixtures.

1. Stereochemistry
Dr Akhil Nagar
RCP-IPER
Shirpur

2. ISOMERISM
• The compounds having same molecular formula are called
isomers.
• two compounds show different properties even if they have
same atoms or groups present and have same covalent
bonding.
• Bond connectivity. It describes the sequence in which
different atoms or groups are bonded to each other in a
molecule.
• The isomers that differ in their bond connectivity are
known as constitutional isomers/ structural isomer.
• study of three-dimensional nature (spatial arrangement)
of molecules is known as stereochemistry.

4. Constitutional Isomers
• Constitutional isomers have same molecular formula but
different bond connectivity, that is, the sequence in which
atoms are bonded in a molecule, is different.
• Chain isomers
• Position isomers
• Functional isomers
• Metamers
• Tautomers.
• Chain isomers: These isomers differ in the way the carbon
atoms are bonded to each other in a carbon chain.
• Eg: Butane and 2-methylpropane (C4H10)

5. • Position isomers. These isomers differ in the position of
functional group in a carbon chain
Functional isomers. These isomers have same molecular
formula but they differ in the nature of functional groups
Eg: alcohols and ethers with same molecular formula

6. • Metamers. These isomers have same functional group but
they differ in the arrangement of alkyl groups around the
functional group.

7. Tautomers. These are the two forms of same compound, which
arise due to migration of a hydrogen atom in a compound. The
two forms are readily inter-convertible and exist in dynamic
equilibrium with each other. This phenomenon is known as
tautomerism.

8. • Examples of stereochemistry
• (–) Adrenaline has more hormonal activity than (+)
adrenaline.
• (–) Nicotine is more poisonous than (+) nicotine.
• One stereoisomer of Limonene has smell of lemon while the
other smells like oranges.

9. CONFIGURATIONAL ISOMERISM: The carbon is tetravalent
in nature and the non-planar tetrahedral geometry in carbon
compounds gave birth to the concept of stereochemistry. The
arrangement of different groups or atoms in space is termed as
configuration.
Stereoisomers :These differ from structural isomers, in that
structural isomers have different structural formulas, while
stereoisomers have the same general structural formula but
different orientations in space.

10. • There are different kind of stereoisomers:
• a. Optical isomers (e.g. enantiomers).
• b. Geometrical isomers (cis, trans isomers).
• c. Conformational isomers (conformers). (Diastereomers)
• Optical isomerism: Optical isomers are named like this
because of their effect on plane polarized light. Light is a
wave motion that contains oscillating electric and magnetic
fields. The electric field of ordinary light oscillates in all
planes. However, it is possible to obtain light with an electric
field that oscillates in only one plane. such light is called
plane polarized light or simply polarized light.

11. Enantiomers: They are optical isomers, with two
stereoisomers that are related to each other by reflection. They
are mirror image of each other that are non super imposable.
• Compounds that are enatiomers of each other have same
physical properties, except for the direction in which they
rotate polarized light.
Diastereomers: The stereoisomers of a compound, which are
not mirror images of each other are termed as diastereomers.
The diasteromers exist only when a compound has two or more
stereogenic centres.
• Conformational isomers (conformers): Its a form of
isomerism that describe the phenomenon of molecules with the
same structural formula but with different shapes due to
rotation about one or more bonds.

12. CHIRALITY
• The term ‘chiral’ is taken from Greek word cheir
meaning handedness.
• An object that is not superimposable on its mirror image
is called chiral and is unsymmetrical while the object that
is superimposable (exactly overlaps its image) on its
mirror image is called achiral and is symmetrical.

13. CHIRALITY: ENANTIOMERS AND DIASTEREOISOMERS
• A molecules in which carbon is attached to four different
groups or atoms is said to be Chiral in nature.
• The carbon is said as stereogenic center. Chiral molcule
dont have plane of symmetry. These molecules are optically
active.
• A molecule with one stereogenic centre always exists in
enantiomeric forms.

14. • Meso compounds. A compound having stereogenic centres
but optically inactive (achiral), due to presence of a plane of
symmetry, is termed as meso compound or a meso form.
• In meso compounds the plane divides the molecule into
halves that are mirror images of each other. This internal
compensation makes meso compounds optically inactive.
The achiral molecule CH2BrCl has a plane of symmetry, but
the chiral molecule CHBrClF does not.

15. Fischer Projection
Representation of 3-D Structures in 2-D
• The three dimensional structure of carbon compounds is
due to their tetrahedral geometry.
• A Fischer projection is represented by an additive cross
(+), where horizontal line represents the bonds above the
plane and vertical line represents the bonds below the
plane.

16. Writing the Fischer projection: Step 1. The structure of a
compound, whose Fischer projection is to be written, is first
written in a vertical manner. The carbon with the lowest rank, as
per IUPAC nomenclature, is written on the top.
Step 2. The chiral carbon in the structure is assumed to lie in the
plane of the paper while the groups attached vertically and
horizontaly to chiral carbon are assumed to lie below and above
the plane of paper, respectively.

17. For example, for Lactic acid CH3CH(OH)COOH, the Fischer projections
2-Hydroxypropanal

18. Fischer projection of 2-bromo-3-chlorobutane- compound with more
than one chiral centre.
Step 3. A Fischer projection can be rotated through 180° in the
plane of the paper since this does not change the configuration,
that is, spatial arrangement of groups around chiral carbon. A
rotation by 180° does not involve the interchange between
horizontal and vertical lines

19. Number of Stereoisomers of a Compound: The number of
stereoisomers possible for a compound is related to the number of
chiral centres (n) present in it as
Number of stereoisomers = 2n
Where n is the number of stereogenic or chiral centre(s) present in the
compound. Eg: Lactic acid CH3CH(OH)COOH with one chiral centre,
possible stereoisomers (21 = 2),

20. Eg: 2-bromo-3-chlorobutane which has two chiral centres. stereoisomers (22 = 4)
Lactic acid
2-bromo-3-chlorobutane

21. Threo and Erythro prefixes: Fischer projection of a stereoisomer
having two different chiral centres (may or may not be on the
adjacent carbon atoms), if two similar groups or atoms are on the
same side of carbon chain, the isomer is designated as erythro.
On the other hand, if two similar groups or atoms are on opposite
sides of the carbon chain, the isomer is designated as threo.

22. OPTICAL ACTIVITY: Ordinary light consists of light waves
vibrating in all planes. When passed through a ‘Polaroid lens’ or
‘Nicol prism’ light moves in one plane only and is called plane-
polarized light (PPL). An unsymmetrical or chiral compound,
placed in the path of plane-polarized light, will rotate the plane
of light. The compounds exhibiting such a behaviour are termed
optically active.

23. A compound rotating the plane of polarization towards right
(clockwise) is termed dextrorotatory (+), whereas a compound
rotating the plane of polarization towards left is termed
laevorotatory (–).
The optical activity is measured in the laboratory by polarimeter.
Polarimeter consists of a light source (usually D–line of sodium),
two polaroid lenses (one polarizer and one analyzer), and a
sample tube placed between polarizer and analyzer lenses.
If the sample tube is empty and polarizing axes of two lenses are
parallel, the intensity of light reaching the observer is
‘maximum’.

24. • Once the solution of sample is placed in the tube, the emerging
light is observed from the analyzing lens. If there is no change in
the intensity of light, the sample is said to be optically inactive.
• If the sample rotates the plane of polarization, the intensity of
the light observed through analyzer lens is found to decrease and
the sample is said to be optically active.
• The analyzer lens is then rotated to observe the maximum
intensity of light.
• If the rotation of the plane, towards right, the sample is
dextrorotatory, if rotation is towards left, laevorotatory.

26. • Specific rotation [α], is the rotation caused by a sample at a
concentration of 1.0 g/mL in a sample tube of length 1.0 dm
(10.0 cm). The value of specific rotation depends on the
concentration of sample, its structure, wavelength of light
source, temperature, length of the sample tube, and solvent.
• The temperature t (in oC) and wavelength λ of light source are
indicated as superscript and subscript respectively while writing
the specific rotation.

27. Racemic mixture: A mixture containing equal amounts of a pair of
enantiomers is termed as racemic mixture (also called racemic
modification). A racemic mixture is optically inactive. As the
enantiomers exhibit same specific rotation in opposite directions,
the specific rotation values of two isomers cancel each other,
resulting in zero optical activity. The racemic mixture is denoted by
(+). For example, (+) lactic acid.