1. Isomerism

2. Isomers are compounds that have the same molecular formula but have different arrangements of atoms in space. Isomers have different physical and chemical properties (depending on the type of isomerism). E.g. Butane and 2-methyl propane

3. Starter: make these molecules in the order given (5 minutes) alkanes alkenes H CH 3 | | H – C – C – H | | CH 3 H CH 3 CH 3 | | H – C – C – H | | H H H CH 3 | | H – C = C – H | | CH 3 H CH 3 CH 3 | | H – C = C – H | | H H Do you notice whether it is easier to make 2 after 1 compared with 4 after 3? How is it different? Why is it different? Challenge name these molecules

4. Same molecular formula but different arrangement of their atoms. CHAIN ISOMERISM POSITIONAL ISOMERISM FUNCTIONAL GROUP ISOMERISM GEOMETRIC ISOMERISM OPTICAL ISOMERISM Atoms are bonded in a different order. Atoms are bonded in the same order but arranged differently in space. Rotation prevented by double bond. Non- superimposable mirror images.

5. Stereoisomerism (Spatial isomerism) SAME molecular formula, SAME functional groups, SAME bonds, but DIFFERENT arrangement in space. (a)This often occurs with a double bond Geometric Isomerism! (b) Or.. if a central atom is attached to 4 different groups if a central atom is attached to 4 different groups Optical Isomerism!

6. Geometric isomerism. o This type of isomerism results from restricted rotation somewhere in the molecule (usually C=C). o HOWEVER.......

7. Pi bonds are formed by a sideways overlap between p- orbitals, and if they are twisted then this overlap is disrupted and the Pi bond is broken. Therefore use this as a clue!

8. trans-1,2-dichloroethene cis-1,2-dichloroethene “on this side” “across” Cis and trans isomers may have different properties e.g. In the way they react with biological molecules. Cis- MP -80ºC, BP 60ºC Trans – MP -50ºC, BP 48ºC

9. So what do we call this?

10. E-Z isomerism If 2 groups with the highest priority are on the same side they are a Z isomer. Z –Zusammen = together E –Entgegen = opposite

11. Rules 1. Separate the two sides of the double bond and treat them separately. 2. The atom with the highest atomic number is given the highest priority.

12. Examples

13. Comparing stereoisomerism and structural isomerism Bob being a stereoisomer Bob being a structural isomer Normal Bob

14. TASK: Illustrate the structures of the three different isomers of C 5 H 12.

15. The Same or Not the Same Slides 2-17

16. E.g. Optical isomers Specific to compounds with four different groups on the same carbon atom called Chiral carbons Four groups allows two different arrangements in space Non-superimposable mirror images or Enantiomers

17. Optical isomers Chiral cmpds have the same properties as each other. Except for Optical activity Interaction with biological systems; i.e. enzyme action Bulb Monochromatic light source Polarising filter Second Parallel Polarising filter Plain Sample in solution detector Plane Polarised Light

18. Plain Sample in solution Optical isomers Chiral cmpds have the same properties as each other. Except for Optical activity Interaction with biological systems; i.e. enzyme action Bulb Monochromatic light source Polarising filter Second Non-parallel Polarising filter detector

19. Sample in solution Optical isomers The polarising filters are perpendicular so no light should get through. However, With the optical isomer in place light is transmitted. Bulb Monochromatic light source Polarising filter Second Non-parallel Polarising filter detector

20. Optical isomers One enantiomer or optical isomer rotates the beam of plane polarised light clockwise. i.e. it is dextrorotatory e.g. d-glucose e.g. (+) glucose The second polariser must be rotated clockwise to stop light being transmitted again. Bulb Monochromatic light source Polarising filter Second Polarising filter Sample in solution detector

21. Optical isomers One enantiomer or optical isomer rotates the beam of plane polarised light anticlockwise. i.e. it is laevorotatory e.g. l-glucose e.g. (-) glucose The second polariser must be rotated anticlockwise to stop light being transmitted again. Bulb Monochromatic light source Polarising filter Second Polarising filter Sample in solution detector

22. Optically active substances often occur as a 50/50 mixture of the two enantiomers. This is known as a racemic mixture It has no effect on plane polarised light.

24. Optical isomers Implications for pharmaceutical compounds. Preparation Dosage Side effects e.g lactic acid e.g. thalidomide.

25. Some compounds exhibiting chirality have enantiomers with very different properties. Eg. thalidomide One enantiomer relieved morning sickness for mothers, the other hinder the development of babies and they were born with their limbs not formed properly.