1. Isomers
Molecules with same molecular formula but their respective atoms are arranged differently in space 1

2. Same molecular formula but different structural formulae
CHAIN ISOMERISM
STRUCTURAL ISOMERISM
POSITION ISOMERISM
Same molecular formula but different structural formulae
FUNCTIONAL GROUP ISOMERISM
GEOMETRICAL ISOMERISM
Occurs due to the restricted rotation of C=C double bonds... two forms… E and Z (CIS and TRANS)
STEREOISOMERISM
Same molecular formula but atoms occupy different positions in space.
OPTICAL ISOMERISM
Occurs when molecules have a chiral centre. Get two non- superimposable mirror images.

3. Structural Isomers
Structural isomers have the same molecular formulas but they differ in their structural formulas. 3

4. Example: octane
Find all possible structural isomers for octane

5. Stereoisomers
Stereo isomers have the same structural formulas but they differ in their spatial arrangements.
There are two types of stereoisomerism
Geometrical isomerism
Optical isomerism 5

6. Geometrical isomers
Geometrical isomers occur in organic molecules where rotation around a bond is restricted
This occurs most often around C=C
The most common cases are around asymmetric non-cyclic alkenes 6

7. Geometric Isomers in alkenes
A cis isomer is one in which the substituents are on the same side of the C=C
A trans isomer is one in which the substituents are on the opposite sides of the C=C 7

8. Properties of Geometrical Isomers
The chemical properties of geometrical isomers tend to be similar but their physical properties are different 8

9. Properties of Geometrical Isomers
The trans isomer has a much higher melting point. Unlike the cis isomer there is little intra-molecular hydrogen bonding 9

10. Optical Isomers
Optical isomerism is present in all compounds that contain at least one asymmetric (chiral) carbon atom
An asymmetric carbon atom has four different atoms or groups attached
In this case there are two different ways to arrange the four groups around the chiral carbon atom (shown in red) 10

11. Optical Isomers
While these structures may look identical, in three dimensions they are mirror images of each other. Such molecules are called enantiomers. 11

12. Distinguishing Enantiomers
Optical isomers can be distinguished by the way they interact with plane polarized light

13. Polarized light
Polarized light is light that has been passed through a polarizing prism or filter. As a result the light vibrates in a single plane.

14. Distinguishing Enantiomers
Optical isomers can be distinguished by the way they interact with plane polarized light. One enantiomer will rotate polarized light to the right, the other to the left.

15. Properties of Optical Isomers
Apart from their optical activity enantiomers generally have similar physical and chemical properties.
The chemical properties may be significantly different when the enantiomers interact with other optically active compounds.
Thalidomide has two optical isomers.
One is a tranquilizer, the other is a
Powerful teratogen.