Isomerism

1. Isomerism Mr. Lane

2. Structural Isomerism • These are when the atoms are connected in different ways • Three types: • Chain Isomerism • Position Isomerism • Functional Group Isomerism

3. Structural Isomerism Chain Isomerism Different arrangements of the carbon chain. Hexane 2-Methylpentane

4. Structural Isomerism Position Isomerism Same carbon skeleton but the functional groups occupy different positions. Butan-1-ol Butan-2-ol

5. Structural Isomerism Functional Group Isomerism Isomers have the same molecular formulae but have different functional groups and so belong to different homologous series.

6. Questions • Draw and name all the chain isomers for C6H14 • Draw all the positional isomers for C6H13OH

7. Stereoisomerism • In stereoisomerism, all the atoms are connected to each other in the same way but they’re arranged differently in space • There are two types: • Optical isomerism (this will be covered in a future module) • Geometric isomerism Two different groups attached to each carbon, either end of the double bond.

8. Stereoisomerism: Geometric isomers With an alkane such as ethane, C2H6, there is free rotation about the carbon-carbon single bond. Viewed along the carbon – carbon bond, the three hydrogen atoms of each methyl group can rotate with respect to each other.

9. Stereoisomerism: Geometric isomers In an alkene such as ethene, C2H4, the double bond prevents this rotation. There is no rotation around the carbon-carbon double bond and the molecule is confined to a planar shape.

10. Stereoisomerism: Geometric isomers OR

11. Stereoisomerism: Geometric isomers OR The geometric isomers can be described in terms of cis and trans. Cis – Same side Trans – Opposite sides

12. Stereoisomerism: Geometric isomers Problem with cis and trans... Br H Br Cl C C C C F Cl F H Which is cis, which is trans? More sophisticated way of naming was introduced called the CIP rules (named after the chemists who developed the system, Cahn, Ingold and Prelog.)

13. CIP rules • Look at the two groups at the end of the double bond and rank the two groups in terms of the atomic number of the atoms concerned. • The atom with the higher atomic number takes precedence. • This is done for both ends of the double bond. • If the higher priority groups are on the same side of the double bond, then it is the Z isomer (from the German zusammen which is together). • If they are on opposite sides then it is the E isomer( from the Germanentgegen which is opposite).

14. CIP rules On left hand end of the double bond: C has a higher priority than H. On right hand end of the double bon: C has a higher priority than H. The carbons are on opposite sides of the double bond and so this is: (E) - but-2-ene On left hand end of the double bond: C has a higher priority than H. On right hand end of the double bon: C has a higher priority than H. The carbons are on the same side of the double bond and so this is: (Z) - but-2-ene

15. CIP rules (E) - 2-bromo-but-2-ene

16. CIP rules (z) - isomer (E) - isomer

17. CIP rules NB: Double bonds count as two bonds. (z) - isomer

18. CIP rules (z) - isomer