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Stereochemistry
of organic
compounds-i
Stereochemistry
• 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 "threedimensionality".
• The study of stereochemistry focuses on
stereoisomers and spans the entire spectrum of
organic, inorganic, biological, physical and especially
supramolecular chemistry.
Isomerism
The concept of isomerism is an important feature of
the study of organic compounds. It relates to the
existence of different compounds which have the
same molecular formula but different properties.
Such compounds are called isomers. The difference in
properties of isomers is due to the difference in the
relative arrangements of atoms in their molecules.
There are two types of isomerism:
• Structural isomerism
• Space or stereo isomerism
Structural isomerism
Structural isomerism, or constitutional isomerism (per IUPAC), is a form
of isomerism in which molecules with the same molecular formula have
bonded together in different orders, as opposed to stereoisomerism.
There are multiple synonyms for constitutional isomers.
There are various types of structural isomerism:
 Chain isomerism: In chain isomerism, or skeletal isomerism, components of the
(usually carbon) skeleton are distinctly re-ordered to create different
structures. Pentane exists as three isomers: n-pentane (often called simply
"pentane"), isopentane (2-methylbutane) and neopentane (dimethylpropane).
 position isomerism (regioisomerism): In position isomerism a functional group
or other substituent changes position on a parent structure.
Functional isomers are structural isomers that have the same molecular
formula (that is, the same number of atoms of the same elements), but
the atoms are connected in different ways so that the groupings are
dissimilar. These groups of atoms are called functional groups,
functionalities. Another way to say this is that two compounds with the
same molecular formula, but different functional groups, are functional
isomers.
Metamerism: This type of is due to the difference in the distribution of
carbon atoms on either side of functional group in compounds having the
same molecular formula.
Tautomerism: This is a special type of functional isomerism where the isomers
exist simultaneously in dynamic equilibrium with each other. The isomers
involved in equilibrium are called tautomers.
Stereoisomerism
The isomeric compounds which differ from each other only in the relative
arrangement of atoms in space but resemble one another with respect
to which atoms are linked to which other atoms are called
stereoisomers and the phenomenon is known as stereoisomerism.
There are three types of stereoisomerism:
•
Optical isomerism
•
Geometrical isomerism
 Optical isomerism: In this the isomeric compounds resemble
one another in their chemical reactions and most of the
physical properties but differ in their effect on polarized light.
Molecular chirality
•
A molecule is considered chiral if there exists another molecule that is
of identical composition, but which is arranged in a non-superposable
mirror image. The presence of an asymmetric carbon atom is often
the feature that causes chirality in molecules.
Types of optical isomerism
1. Enantiomerism
2. Diastereomerism
Enantiomerism : An enantiomer is one of the two stereoisomers that
are mirror images of each other that are non-super imposable
and which rotate the plane polarized light equally but in
opposite directions. The phenomenon of mirror image isomerism
is called enantiomerism.
Fundamental condition of enantiomerism
Enantiomeric molecules are always non-super imposable mirror images of
each other. The non-super imposability of mirror images invariably arises
due to chiral nature of the molecules. A molecule is termed as chiral if
it has no plane or centre of symmetry and is therefore, non-super
imposable on its mirror image.
Therefore, chirality is the fundamental and only condition of
enantiomerism.
Characteristics of enantiomers
1. They have identical physical properties such as m.p, b.p, densities,
solubility, refractive index.
2. They have identical chemical properties except in reactions with
other optically active compounds.
3. They have different biological properties.
4. When equal quantities of enantiomers are mixed together it results
in the formation of an optically inactive form called racemate.
Representation of stereoisomers
The flying wedge formulae
Diatereoisomerism
•
Diastereomers (sometimes called diastereoisomers) are a type of a
stereoisomer. Diastereomerism occurs when two or more
stereoisomers of a compound have different configurations at one
or more (but not all) of the equivalent (related) stereocenters
and are not mirror images of each other. When two
diastereoisomers differ from each other at only one stereocenter
they are epimers. Each stereocenter gives rise to two different
configurations and thus increases the number of stereoisomers by
a factor of two.
• Diastereomers differ from enantiomers in that the latter are
pairs of stereoisomers that differ in all stereocenters and are
therefore mirror images of one another. Enantiomers of a
compound with more than one stereocenter are also
Diastereomers of the other stereoisomers of that compound
that are not their mirror image. Diastereomers have different
physical properties (unlike enantiomers) and different chemical
reactivity.
Characteristics of Diastereomers:
1. They show similar but not identical chemical properties.
2. They have different physical properties such as m.p, b.p,
density, solubility, refractive indices.
3. On account of different physical properties, they can be
rather easily separated through fractional crystallization,
fractional distillation, chromatography.
Erythro and Threo isomers
•
•
If similar groups are on same side in the fischer projection formula,
the diastereomer is known as erythro isomer.
If the two similar groups are on the opposite sides in the projection
formula, the diastereomer is known as threo isomer.
Meso compound
A Meso compound or Meso isomer is a non-optically active
member of a set of stereoisomers, at least two of which are
optically active. This means that despite containing two or
more chiral centers it is not chiral. A Meso compound is
"super posable" on its mirror image.
Racemization
• In chemistry, Racemization refers to the conversion of an
enantiomerically pure mixture (one where only one enantiomer
is present) into a mixture where more than one of the
enantiomers are present. If the racemization results in a
mixture where the enantiomers are present in equal quantities,
the resulting sample is described as racemic or a racemate.
• Racemization can be brought about by one of the following:
 By action of heat
 By action of chemical reagents
 Autoracemisation
Comparison of internal and external compensation
External compensation
Internal compensation
 This is due to mirror image
relationship between structures
of the molecules of two
enantiomers mixed together.
 In this the molecule of one
enantiomer cancels the rotation
of the other enantiomer.
 The racemate forms is a
mixture of two isomeric
compounds.
 It is a reversible process and
the racemate mixture can be
resolved into enantiomers.
 This is due to mirror image
relationship between the
structures of two halves of the
same molecule.
 One half of the molecule
counter balances the rotation
of the other half.
 Meso or internally compensated
compound represent a single
pure compound.
 It is a permanent effect and it
is not possible to resolve into
optically active forms.
Rectus and sinister configuration
• Identify each of the chiral centers (most commonly an sp3 C
with 4 different groups attached)
• Then at each chiral center.... Assign the priority (high = 1 to
low = 4) to each group attached to the chirality center based
on atomic number.
• Reposition the molecule so that the lowest priority group is
away from you as if you were looking along the C-(4) σ bond.
If you are using a model, grasp the lowest priority group in
your fist.
• Determine the relative direction of the priority order of the
three higher priority groups (1 to 2 to 3)
• If this is clockwise then it is the R-stereoisomer (Latin; rectus
= right handed)
• If this is counter-clockwise then it is the S-stereoisomer
(Latin; sinister = left handed)
Assignment
 Explain the following terms: 1. polarised light 2.optical
activity 3.specific rotation 4.structural isomerism
5.enantiomers 6.diastereomers 7.meso compounds
 What is resolution? Give two methods for this purpose with
examples.
 What is meant by internal and external compensation? How do
they differ from each other?
 With the help of suitable examples, explain the terms threo
and erythro isomers.
 Explain what is Walden inversion? Explain necessary and
sufficient condition of optical activity?
 Cis-alkene has higher boiling point than trans- alkene. Why?
 Write short note on isomerism exhibited by maleic and fumaric
acid?