1. Isomerism
SCH4U Spring 2012

2. Isomerism
Isomerism occurs when there are two or more compounds (called isomers) with the same molecular formula but different arrangements of their atoms
Isomers have different physical and chemical properties
There are two main classes of isomerism:
a) structural isomerism
b) stereoisomerism

3. Structural isomerism
Structural isomers are molecules with the same molecular formula but with different structural arrangements of the atoms
There are three types of structural isomers:
a) Chain isomers
b) Positional isomers
c) Functional group isomers

4. Chain isomers Isomers have different arrangements of the carbon chain
E.g. C4H10

5. Chain isomers
As the number of carbon atoms increases , the number of possible isomers increases
How many isomers would C5H12 have?
What are they?
Use molecular kits, draw them

6. Positional Isomers
Isomers have the same carbon skeleton but the functional group is in different positions in the molecule
E.g. C6H4Cl2

7. Positional isomers
What would be a positional isomer of butan-1-ol?

8. Functional group isomers
Isomers have the same molecular formula but different functional groups
E.g. C3H8O
CH3CH2CH2OH, propan-1-ol H3COCH2CH3 , methoxyethane

9. Functional group isomers
What would be a functional group isomer of cyclohexane?

10. Stereoisomerism
Stereoisomers have identical molecular formula and the atoms are linked together in the same order but have different relative positions in space
There are two types of stereoisomers:
a) Geometric (or cis-trans ) isomers
b) Optical isomers

11. Geometric isomers
Geometric or cis-trans isomers exist because the (pie) π bond of the C=C bond prevents free rotation
Cis – same side
Trans – opposite side of the double bond
E.g. 1,2-dichloroethene
two different groups on the left-hand end of the bond and two different groups on the right-hand end.

12. Optical isomers
This type of isomerism occurs when a molecule has one or more asymmetric carbon atoms i.e. a carbon atom linked to four different atoms or groups
The optical isomers have a chiral centre and are mirror images of each other
E.g. butan-2-ol
One of the enantiomers is simply a non-superimposable mirror image of the other one.

13. Optical isomers
Identify all chiral centre(s), if any: