Alkanes

1. Alkanes Acyclic: CnH2n+2 Cyclic (one ring): CnH2n Bicyclic (two rings) : CnH2n-2 Only single bonds, sp3 hybridization, close to tetrahedral bond angles

2. Physical properties • Boiling points • Lower than other organic molecules of same size. • Lower attractive forces between molecules than in alcohols.

3. Strength Intermolecular Forces • Ionic Forces • Hydrogen Bonding • Dipole Dipole Forces • Dispersion Forces Dispersion Forces: due to fluctuating motion of the electrons in a molecule. Motion in one molecule is correlated with that in the other molecule.

4. Dispersion Forces and Molecular Structure Branching decreases surface area, reduces dispersion forces and, thus, boiling point.

5. 18.8 kJ Molecular Structure and Heat of Combustion Difference in heats of combustion indicates a greater stability of branched structures.

6. Isomerism and Naming • Hexane

7. 2-methylpentane

8. CycloAlkanes

9. Bicycloalkanes Parent name: name of alkane with same number of carbons. Number from bridgehead along largest bridge. If substituent choose bridgehead to assign low number to substituent. Size of bridges indicated by number of carbons in bridge.

10. Examples of numbering

11. 60 Eclipsed Conformation. Conformations • Rotations about single bonds produce different conformations. Staggered Conformation.

12. Newman Projections Staggered Conformation. Eclipsed Conformation. Less stable. More stable!

13. Rotational Profile of ethane

14. Torsional strain: Strain between groups on adjacent atoms. A-B-C-D. Worst when eclipsed; best when staggered. Bond angle strain: when a bond angle, A-B-C, diverges from the ideal (180, 120, 109) What are the forces in a molecular structure?

15. 120 deg. View from here yields view below. View from here yields view below. Rotation about C2 – C3 in butane Gauche conformation, Methyls closer, 60 deg, more repulsion, higher energy Anti conformation Methyls 180 deg, lower energy Anti!! Gauche!!

16. Energy Profile for Rotation in Butane Three hills (eclipsed) 120 apart. Three valleys (staggered forms) 120 apart;

17. Problem: Rotational profile of 2-methylbutane about C2-C3. First, staggered structures. 300 60 180 Rotate the front Me group. Relative energies….

18. Now, eclipsed…. 180 240 120 360 = 0 0 This was the high energy staggered structure,180 deg. Shown for reference only. Now relative energies…..

19. Now put on diagram… eclipsed staggered 120 60 360 300 180 240 0

20. Conformations of cycloalkanes: cyclopropane Planar ring (three points define a plane); sp3 hybrization: 109o. Hydrogens eclipsing. Torsional angle strain. Bond angle strain. Should be 109 but angle is 60o. Cyclopropane exhibits unusual reactivity for an alkane.

21. Folded, bent: less torsional strain but increased bond angle strain Conformation of cyclobutane Fold on diagonal Planar: eclipsing, torsional strain and bond angles of 90o

22. Cyclobutane molecular dynamics

23. Cyclopentane

24. Boat conformation Chair conformation Cyclohexane Ideal solution: Everything staggered and all angles tetrahedral.

25. Axial: Equatorial: Chair Conformation

26. Axial and Equatorial Axial Up/Equatorial Down: (A/E) Equatorial Up/Axial Down: (E/A) A/E E/A E/A A/E A/E E/A

27. Ring Flips Chair Boat or Twisted Boat A becomes E E becomes A Up stays Up Down stays Down Chair

28. Substituents: Axial vs Equatorial

29. Each repulsion is still about 3.6 kJ. Note that the gauche interaction in butane is about 3.8. Substituent Interactions Destabilizes axial substituent. Each repulsion is about 7.28/2 kJ = 3.6 kJ 1,3 diaxial repulsions Alternative description: gauche interactions

30. Newman Projection of methylcyclohexane Axial methyl group Equatorial methyl group gauche anti

31. 7.3 kJ (axial) 7.3 kJ (axial) Disubstituted cyclohexanes 1,2 dimethylcyclohexane 3.6 kJ (gauche) 3.6 kJ (gauche) interactions 0.0 kJ equatorial 0.0 kJ equatorial 7.3 + 3.6 = 10.9 kJ 7.3 + 3.6 = 10.9 kJ

32. 7.3 kJ (axial) 0.0 kJ equatorial 3.6 kJ (gauche) 0.0 kJ equatorial 7.3 kJ (axial) 0.0 kJ + 3.6 kJ = 3.6 kJ 14.6 kJ + 0.0 kJ = 14.6 kJ diequatorial diaxial

33. When does the gauche interaction occur?

34. Translate ring planar structure into 3D E/A A/E A/E Assume the tert-butyl group is equatorial. E/A A/E E/A Energy accounting No axial substituents One 1,2 gauche interaction between methyl groups, 3.6 kJ/mol Total: 3.6 kJ

35. Problem: Which has a higher heat of combustion per mole, A or B? 7.3 3.6 3.6 3.6 7.3 18.2 7.2 More repulsion, higher heat of combustion by 11.0 kJ/mol

36. Trans and Cis Decalin Now build cis decalin, both same side. Build trans decalin starting from cyclohexane, one linkage up, one down Trans sites used on the left ring Cis sites used on left ring. Trans sites used on the right ring Cis sites used on right ring. Trans decalin Locked, no ring flipping Cis decalin, can ring flip

37. Trans fusions determine geometry What is the geometry of the OH and CH3? E/A A/E A/E E/A E/A A/E Trans fusions, rings must use equatorial position for fusion. Rings are locked. The H’s must both be axial Work out axial / equatorial for the OH and CH3. OH is equatorial and CH3 is axial