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.
methane
-164 oC
water
100 oC
hexane
68.7 oC
1-pentanol
137 oC

3. Intermolecular Forces
Ionic Forces
Hydrogen Bonding
Dipole Dipole Forces
Dispersion Forces
Strength
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. Molecular Structure and Heat of Combustion
Difference in heats of combustion indicates a greater stability of branched structures.
18.8 kJ

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

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

13. Rotational Profile of ethane

14. What are the forces in a molecular structure?
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)

15. Rotation about C2 – C3 in butane
View from here yields view below.
View from here yields view below.
Rotation about C2 – C3 in butane
120 deg.
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 valleys (staggered forms) 120 apart;
Three hills (eclipsed) 120 apart.

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

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

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

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

22. Cyclobutane molecular dynamics

23. Cyclopentane

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

25. Chair Conformation
Axial:
Equatorial:

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
Substituent, R
D G Preference for Equatorial
K at 25 deg
-CH3, methyl
7.28 kJ/mol
18.9
-CH2CH3, ethyl
7.3
19.
-CH(CH3)2, iso propyl
9.0
38.
-C(CH3)3, tert butyl
21.0
4.8 x 103

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

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

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

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

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
E/A
A/E
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