This document provides an overview of the topics that will be covered in the conformational analysis chemistry module, including: - The differences between conformation and configuration - Rotation around single carbon-carbon bonds - Sources of strain in cyclic compounds and how their structures minimize this strain - The chair and boat conformations of cyclohexane and factors that determine stability - The different conformations of monosubstituted and disubstituted cyclohexanes - Fused ring systems and decalins - Elimination reactions in cyclic compounds Students are instructed to obtain molecular models to aid in visualizing the 3D structures discussed.

1. Conformational analysis
CHEM 320

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 refers 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
Br Br

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:

9. Cyclic compounds
• If flat, what would bond angles be?
• sp3 carbon atom - bond angle 109.5o
• In rings not always possible
• If bond angle differs from ideal 109.5o 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

10. Cyclopropane

12. Envelope
conformation

13. Explain trend in blue graph

14. Conformation of cyclohexane

15. Chair conformation & boat conformation

17. H
H
H
H
H
H
H
H
H
H
H
H
Chair conformation of cyclohexane
Newman projection

18. Boat conformation of cyclohexane

20. 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

22. ΔGo = -RT ln K K = e-ΔG°/RT

23. Decalins

24. Nucleophilic substitution reactions