Chapter 4

1. Chapter 4 • Stereochemistry. • Stereochemistry of open chain alkane. • Stereochemistry of ring alkane. Chapter 4 - stereochemistry

2. Stereochemistry Spatial arrangement of atoms in a molecule or ion. Function and reactivity of a molecule is determined by its 3-dimensional geometry. Chapter 4 - stereochemistry

3. Chapter 4 • Stereochemistry. • Stereochemistry of open chain alkane. • Stereochemistry of ring alkane. Chapter 4 - stereochemistry

4. Stereochemistry of open chain alkane. • Conformational Isomers vs. Constitutional Isomers. • Modes of display - Sawhorse, Wedges & Dashes representations, and Newman projection. • Staggered, eclipsed, torsional or dihedral angle, torsional strain, anti and gauche conformation, steric strain. Chapter 4 - stereochemistry

5. ConformationalIsomers vs. Constitutional Isomers Conformers Constitutional isomers Chapter 4 - stereochemistry

6. H H Br H Back carbon H Front carbon Br Sawhorse representation Newman projection Wedges & Dashes representation Modes of display Chapter 4 - stereochemistry

7. Ethane  = 0o Dihedral angle =  = 60o Eclipsed Staggered At room temperature there is sufficient thermal energy for molecules to rotate among the different conformations. Chapter 4 - stereochemistry

8. Ethane 2.4Å 2.4Å Staggered Chapter 4 - stereochemistry

9. Ethane 2.2Å Eclipsed Chapter 4 - stereochemistry

10. Ethane Chapter 4 - stereochemistry

11. H H H H H H H H H H H H Ethane 3(H-H) = 12 kJ/mol 12 kJ/mol E Each(H-H) = (12 kJ/mol)/3 = 4 kJ/mol Chapter 4 - stereochemistry

12. Propane Staggered Chapter 4 - stereochemistry

13. Propane Chapter 4 - stereochemistry

14. H H H H CH3 H Propane H H H 2(H-H) = 2x4 = 8 kJ/mol CH3 H H 14 kJ/mol Eclipse (CH3-H) = 14-8 = 6 kJ/mol E (CH3-H) = 3.8kJ/mol Chapter 4 - stereochemistry

15.  = 0o Steric hinderance  = 60o CH3 CH3 CH3 H CH3 H CH3 H  = 180o H H H H H CH3 H H H H gauche anti eclipsed Butane Chapter 4 - stereochemistry

16. Butane Chapter 4 - stereochemistry

17. Higher straight Chain Alkanes Zigzag with some gauche conformations.       Chapter 4 - stereochemistry

18. Problems • Draw Newman projections of the most stable and least stable conformations of 3-Methylhexane. • Using wedges and dashes to represent the bonds of the most stable conformation of 3-Methylhexane. • Construct a qualitative potential energy diagram for rotation about the C-C bond of 1,2-Dibromoethane. Include the Newman projections of the conformations at the maximum and minimum points. Also give the names of the various conformations. Chapter 4 - stereochemistry

19. Chapter 4 • Stereochemistry. • Stereochemistry of open chain alkane. • Stereochemistry of ring alkane. Chapter 4 - stereochemistry

20. Strains • Torsional - due to eclipsing of bonds on neighboring atoms. • Steric - due to repulsive interactions when atoms approach each other too closely. • Angle - due to expansion or compression of normal hybridized bond angles - sp, 180o; sp2, 120o; sp3, 109o. Chapter 4 - stereochemistry

21. Straight Chain: Torsional Steric Ring: Torsional Steric Angle Strains in Alkanes Chapter 4 - stereochemistry

22. Cyclopropane angle torsional Chapter 4 - stereochemistry

23. Cyclobutane Non-planar More torsional strain due to more atoms. Less angle strain Chapter 4 - stereochemistry

24. Cyclopentane Non-planar More torsional strain. No angle strain Chapter 4 - stereochemistry

25. Cyclohexane • Drawing. • Ring flip. • Boat and chair form. • Axial vs. equitorial. • Use of Alchemy. Chapter 4 - stereochemistry

26. Monosubstituted Cyclohexane • 1,3 diaxial interaction. • Axial & equitorial equilibrium calculation. • Use of Alchemy Chapter 4 - stereochemistry

27. Disubstituted Cyclohexane • 1,2-Disubstituted cyclohexane. • 1,4- Disubstituted cyclohexane. • Use of Alchemy Chapter 4 - stereochemistry

28. Policyclic rings • Fused rings stereochemistry. • Use of Alchemy Chapter 4 - stereochemistry