Conformational analysis.pptx

Isomers

1. Conformational analysis CHEM 320 Chemistry @ PMB

2. Contents Conformation vs configuration Rotation around single bond Ring strain Conformation of Cyclohexane Monosubstituted cyclohexanes Disubstituted cyclohexanes Fused cyclohexanes Elimination reactions in cyclic compounds Text books: Bruice ‘Organic Chemistry’ Clayden , Greevers , Warren, Wothers ‘Organic Chemistry’ Please get a set of molecular models to use in the module

3. Conformation r efers to the three-dimensional shape of a molecule Do not confuse conformation with configuration Changing the configuration of a molecule means that bonds are broken

4. Conformation / Configuration A different configuration is a different molecule Changing the conformation of a compound means rotating about a bond, but not breaking it Conformations of a compound are readily interconvertible and are all the same molecule

5. Rotation around carbon-carbon single bond:

6. Cyclic compounds If flat, what would bond angles be? sp 3 carbon atom - bond angle 109.5 o In rings not always possible If bond angle differs from ideal 109.5 o then there will be strain in the molecule ( angle strain ) Torsion strain is caused by repulsion between bonding electrons of one substituent and bonding electrons of neigbouring substituent Steric strain is caused by atoms or groups of atoms that are approaching each other closely Cyclic compounds twist and bend to minimize the different kind of strains

7. Cyclopropane

8. Envelope conformation

9. Explain trend in blue graph

10. Conformation of cyclohexane

11. Chair conformation & boat conformation

12. Chair conformation of cyclohexane Newman projection

13. Boat conformation of cyclohexane

14. Draw two chair conformations for each of the following compounds and predict which one will be the more stable one: cis -1,2-dibromocyclohexane trans -1,2-dibromocyclohexane cis -1,3-dibromocyclohexane trans -1,3-dibromocyclohexane cis -1,4-dibromocyclohexane trans -1,4-dibromocyclohexane

15. Δ G o = -RT ln K K = e - Δ G°/RT

16. Decalins

17. Nucleophilic substitution reactions