Isomerism refers to compounds with the same molecular formula but different structural arrangements. There are several types of isomerism: 1. Structural isomers have the same molecular formula but different structural formulas. This includes chain isomers, positional isomers, and functional isomers. 2. Stereoisomers have the same structure and bond order but different spatial arrangements. This includes geometric isomers, enantiomers, and diastereomers. 3. Geometric isomers involve double bonds that do not allow free rotation, giving cis and trans orientations. Enantiomers are non-superimposable mirror images, while diastereomers contain at least two chiral carbons and are not

1. by: G. Semblante

3. OH
OH
O H H
H
H HO O
H H H
H H
OH
HO OH
HO OH H
OH OH
H
H
H O OH H
O
H H H HO
OH
H
HO H H OH
OH OH H
HO
OH
H
O OH H
H H
H O
H OH HO
OH
HO H
H OH
OH
OH H H
HO
OH

5. Isomerism is the phenomenon
whereby certain compounds, with
the same molecular formula, exist in
different forms owing to their
different organizations of atoms.
The concept of isomerism illustrates
the fundamental importance of
molecular structure and shape in
organic chemistry.

6. e.g. The formula C4H10 represents two possible structural formulae, butane and methylpropane:
STRUCTURAL ISOMERS
The structural isomerism or constitutional
isomerism arises when atoms within a molecule
are arranged in different orders. The structural
isomers have the same molecular formula but
different structural formulae.
The structural isomers usually show different
physical and chemical properties.
The structural isomerism is further divided into:
•Chain isomerism
•Positional isomerism
•Functional isomerism

7. CHAIN ISOMERISM
arises due to different arrangements of carbon
atoms leading to linear and branched chains.
The chain isomers have same molecular
formula but different types of chains i.e.,
linear and branched.
The chain isomers have almost similar
chemical properties but different physical
properties.
For example, the branched chain isomers have
lower boiling points than that of their linear
counterparts. It is because, the linear ones
have more surface area of contact and hence
the intermolecular forces of attraction are
maximum.

8. E.g. Two chain isomers possible with the molecular formula, C4H12.
i.e., n-butane: a linear chain isomer.
isobutane (or 2-methylpropane): a branched isomer

9. The positional isomerism arises due to
different positions of side chains, substituents,
functional groups, double bonds, triple bonds
etc., on the parent chain.
E.g.
1) Propyl chloride and isopropyl chloride are
the positional isomers with the molecular
formula, C3H7Cl. These isomers arise due to
difference in the position of the chloro group
on the main chain.

10. Ex 2) In the following positional isomers i.e., but-1-ene and
but-2-ene the position of double is different but they
have same molecular formula, C4H8.
Ex 3) The functional group, -OH is at different positions on the
main chain of the following alcohols. They have the
same molecular formula, C3H7OH.

11. Ex. 4) In the following positional isomers i.e., ortho xylene,
meta xylene and para xylene, the relative positions of
methyl groups on the benzene ring are different

12. The functional isomers have the same molecular
formula but possess different functional groups.
The functional isomers show different physical as
well as chemical properties.
*different Functional groups
e.g. the molecular formula C3H60 could either a
propanal(aldehyde) or propanone(ketone)

13. For C3H6O2 can form either be a carbolylic acid or
an ester

14. STEREOISOMERS
Stereoisomers have the same structure and
bond order but their atoms and groups of
atoms are arranged differently in space.
 They have different spatial
arrangements and their molecules are
not superimposable.
Types of Stereisomers:
1] Geometric Isomerism (GI)
Involves a double bond, usually C=C, that does
not allow free rotation about the double bond
(unlike a C-C single bond).

15. Geometric Isomers
They are not superimposable.
e.g. cis-but-2-ene and trans-but-2-ene
 Each the 2 doubly bonded C atom should have 2 different
substituents

16. GI orientations:
cis (z) = bulky groups are in the same side
trans(E) = bulky groups are in the opposite side
GI calculation:
GI = 2n
where n=no. of double bonds

17. Ex. How many GI in 1,4-dichlorobuta-1,3-diene
Cl-CH=CH-CH=CH-Cl
GI= 22 = 4 (cis-cis;cis-trans;trans-cis;trans-trans)

18. 2. ENANTIOMERS
Enantiomers are stereoisomers which are mirror image of each
other and non-superimposable.
 There should be at least one chiral carbon(sp3C where it
contains 4 different substituents)
 A chiral compound is an optically active, which can rotate a
plane of polarized light.
 Achiral compound is superimposable on its mirror image
 Ex: glyceraldehyde & lactic acid

20. 3] DIASTEREOMERS
 isomers that are non-mirror compounds
but contains at least 2 chiral carbons within
the molecule.
 No.of Stereoisomers = 2n
where n is the no, of chiral carbon
 Ex: 2,3-dichloropentane
CH3–CH–CH –CH2 – CH3
│ │
Cl Cl
No. of stereoisomers= 22 = 4 isomers

21.  2,3-dichloropentane with 4 isomers:
CH3–CH–CH –CH2–CH3
│ │
Cl Cl
Diastereomers: A&B ; A&D ; B&C ; C&D
Enantiomers: A&C ; B&D