1. Structure of polymer
chains:Isomerism
By:-Anshul Gautampurkar
Class:-S.Y. PPE
Roll no:-T2704

2. ISOMERS
• Compounds having same molecular
formula but different structural formula
• Differ from each other in physical and
chemical properties are known as
“Isomers” and this phenomenon is called
“Isomerism”.
• Isomerism is due to the difference in the
arrangement of atoms in molecules

3. CATEGORIZATION
• Types of isomerism
Isomerism can broadly be divided into two
main classes.
 STRUCTURAL ISOMERISM.
 STEREO ISOMERISM

4. Classification in tabular form 

5. • Structural isomerism
When isomerism is due to the
difference in the arrangement of atoms
within the molecule, without
any reference to space, the
phenomenon is referred to as
“Structural Isomerism”.
• Stereo isomerism
Stereo isomers are isomeric molecules
that have the same molecular formula
and sequence of bonded atoms
(constitution), but that differ only in
the three-dimensional orientations of
their atoms in space

6. 4-octene
4-octene
The above mentioned example of 4-octene is a stereo isomer in which
the chemical formulae remains same but the orientation in space
changes.

7. Chain Isomerism
• This type of isomerism comes under structural
isomerism.
• Chain isomers have same molecular formula but differ
in the order in which C-atoms are bonded to each other.
• Also known as skeletal isomerism.
• Components of the (usually carbon) skeleton are
distinctly re-ordered to create different structures.
• some of the examples are:-
n-pentane isopentane neopentane

8. Configurational stereo isomers
• Configurational isomers are
stereoisomers that cannot be
converted into one another by
rotation around a single bond.
• These can be separated easily at room
temperature from each other.
• These can only get interconverted by
breaking σ and π bonds or by changing
the configurations of stereocenters.
• There are two types of
configurational stereo isomers:-

9. Configurational stereo isomerism in
alkenes
• The carbon-carbon double bond is formed between two sp2
hybridized carbons, and consists of two occupied molecular
orbitals , a sigma orbital and a pi orbital.
• Rotation of the end groups of a double bond relative to
each other destroys the p-orbital overlap that creates the pi
orbital or bond.
• Because the pi bond has a bond energy of roughly 60
kcal/mole, this resistance to rotation stabilizes the planar
configuration of this functional group.
• As a result, certain disubstituted alkenes may exist as a pair
of configurational stereoisomers, often designated cis and
trans.

10. • The essential requirement for this stereoisomerism is
that each carbon of the double bond must have two
different substituent groups (one may be hydrogen).
• This is illustrated by the following general formulas.
• In the first example, the left-hand double bond carbon
has two identical substituents (A) so stereoisomerism
about the double bond is not possible (reversing
substituents on the right-hand carbon gives the same
configuration).
• In the next two examples, each double bond carbon
atom has two different substituent groups and
stereoisomerism exists, regardless of whether the two
substituents on one carbon are the same as those on
the other.

11. Conformational stereo isomers
• Conformational isomerism is a form of stereoisomerism in which the
isomers can be interconverted exclusively by rotations about formally
single bonds.
• Such isomers are generally referred to as conformational isomers or
conformers and specifically as rotamers
• when the rotation leading to different conformations is restricted
(hindered) rotation, in the sense that there exists a rotational energy
barrier that needs to be overcome to convert one conformer to another.
• The rotational barrier, or barrier to rotation, is the activation energy
required to interconvert rotamers.
• Most conformational interconversions in simple molecules occur rapidly at
room temperature. Consequently, isolation of pure conformers is usually
not possible.
• Specific conformers require special nomenclature terms such as
staggered, eclipsed, gauche and anti when they are designated.

12. Types of conformational isomers
Butane has three rotamers : two gauche conformers, which are
enantiomeric and an anti conformer, where the four carbon centres are
coplanar. The three eclipsed conformations with dihedral angles of 0°,120°
and 240° are not considered to be rotamers , but are instead transition
states.

13. Techniques for study of
conformational isomerism
• Most information on conformational isomerism comes from single
crystal X-ray diffraction studies.
• IR spectroscopy is ordinarily used to measure conformer ratios. For
the axial and equatorial conformer of bromocyclohexane, νCBr differs
by almost 50 cm−1.
• The dynamics of conformational (and other kinds of) isomerism can be
monitored by NMR spectroscopy at varying temperatures. The
technique applies to barriers of 8-14 kcal/mol, and species exhibiting
such dynamics are often called "fluxional".

14. • We call these different spatial orientations of the atoms
of a molecule that result from rotations or twisting about
single bonds conformations
• In the case of hexane, we have an unbranched chain of six
carbons which is often written as a linear formula:
CH3CH2CH2CH2CH2CH3. We know this is not strictly
true, since the carbon atoms all have a tetrahedral
configuration. The actual shape of the extended chain is
therefore zig-zag in nature. However, there is facile
rotation about the carbon-carbon bonds, and the six-
carbon chain easily coils up to assume a rather different
shape. Many conformations of hexane are possible and two
are illustrated below.
Extended Coiled
chain  chain 

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