1. Dr. Wolf's CHM 201 & 202 3-1 Chapter 3 Conformations of Alkanes and Cycloalkanes

2. Dr. Wolf's CHM 201 & 202 3-2 3.1 Conformational Analysis of Ethane Conformations are different spatial arrangements of a molecule that are generated by rotation about single bonds.

3. Dr. Wolf's CHM 201 & 202 3-3 Eclipsed conformation Ethane

4. Dr. Wolf's CHM 201 & 202 3-4 Eclipsed conformation Ethane

5. Dr. Wolf's CHM 201 & 202 3-5 Staggered conformation Ethane

6. Dr. Wolf's CHM 201 & 202 3-6 Staggered conformation Ethane

7. Dr. Wolf's CHM 201 & 202 3-7 Projection formulas of the staggered conformation of ethane NewmanSawhorseHH HH HH HH H H H H

8. Dr. Wolf's CHM 201 & 202 3-8 Anti relationships HH HH HH HH H H H H Two bonds are anti when the angle between them is 180°. 180°

9. Dr. Wolf's CHM 201 & 202 3-9 Gauche relationships H H HH HH H H H H H H Two bonds are gauche when the angle between them is 60°. 60°

10. Dr. Wolf's CHM 201 & 202 3-10 An important point: The terms anti and gauche apply only to bonds (or groups) on adjacent carbons, and only to staggered conformations. An important point: The terms anti and gauche apply only to bonds (or groups) on adjacent carbons, and only to staggered conformations.

11. Dr. Wolf's CHM 201 & 202 3-11 0° 60° 120° 180° 240°300°360° 12 kJ/mol

12. Dr. Wolf's CHM 201 & 202 3-12 The eclipsed conformation of ethane is 12 kJ/mol less stable (higher energy) than the staggered.The eclipsed conformation of ethane is 12 kJ/mol less stable (higher energy) than the staggered. The eclipsed conformation is destabilized by torsional strain.The eclipsed conformation is destabilized by torsional strain. Torsional strain is the destabilization that results from eclipsed bonds.Torsional strain is the destabilization that results from eclipsed bonds. The eclipsed conformation of ethane is 12 kJ/mol less stable (higher energy) than the staggered.The eclipsed conformation of ethane is 12 kJ/mol less stable (higher energy) than the staggered. The eclipsed conformation is destabilized by torsional strain.The eclipsed conformation is destabilized by torsional strain. Torsional strain is the destabilization that results from eclipsed bonds.Torsional strain is the destabilization that results from eclipsed bonds. Torsional strain

13. Dr. Wolf's CHM 201 & 202 3-13 3.2 Conformational Analysis of Butane

14. Dr. Wolf's CHM 201 & 202 3-14 Conformational Analysis of Butane: C 2 -C 3 Rotation

15. Dr. Wolf's CHM 201 & 202 3-15 0° 60° 120° 180° 240°300°360° 3 kJ/mol 14 kJ/mol

16. Dr. Wolf's CHM 201 & 202 3-16 The gauche conformation of butane is 3 kJ/mol less stable than the anti.The gauche conformation of butane is 3 kJ/mol less stable than the anti. The gauche conformation is destabilized by van der Waals strain (also called steric strain)The gauche conformation is destabilized by van der Waals strain (also called steric strain) which results from atoms being too close together. The gauche conformation of butane is 3 kJ/mol less stable than the anti.The gauche conformation of butane is 3 kJ/mol less stable than the anti. The gauche conformation is destabilized by van der Waals strain (also called steric strain)The gauche conformation is destabilized by van der Waals strain (also called steric strain) which results from atoms being too close together. van der Waals strain gauche anti

17. Dr. Wolf's CHM 201 & 202 3-17 The conformation of butane in which the two methyl groups are eclipsed with each other is is the least stable of all the conformations.The conformation of butane in which the two methyl groups are eclipsed with each other is is the least stable of all the conformations. It is destabilized by both torsional strain (eclipsed bonds) and van der Waals strain.It is destabilized by both torsional strain (eclipsed bonds) and van der Waals strain. The conformation of butane in which the two methyl groups are eclipsed with each other is is the least stable of all the conformations.The conformation of butane in which the two methyl groups are eclipsed with each other is is the least stable of all the conformations. It is destabilized by both torsional strain (eclipsed bonds) and van der Waals strain.It is destabilized by both torsional strain (eclipsed bonds) and van der Waals strain. van der Waals strain eclipsed

18. Dr. Wolf's CHM 201 & 202 3-18 3.3 Conformational Analysis of Higher Alkanes

19. Dr. Wolf's CHM 201 & 202 3-19 The most stable conformation of unbranched alkanes has anti relationships between carbons Hexane

20. Dr. Wolf's CHM 201 & 202 3-20 3.4 The Shapes of Cycloalkanes: Planar or Nonplanar?

21. Dr. Wolf's CHM 201 & 202 3-21 Adolf von Baeyer (19th century) assumed cycloalkanes are planar polygonsassumed cycloalkanes are planar polygons distortion of bond angles from 109.5° gives angle strain to cycloalkanes with rings either smaller or larger than cyclopentanedistortion of bond angles from 109.5° gives angle strain to cycloalkanes with rings either smaller or larger than cyclopentane Baeyer deserves credit for advancing the idea of angle strain as a destabilizing factor.Baeyer deserves credit for advancing the idea of angle strain as a destabilizing factor. But Baeyer was incorrect in his belief that cycloalkanes were planar.But Baeyer was incorrect in his belief that cycloalkanes were planar.

22. Dr. Wolf's CHM 201 & 202 3-22 Types of Strain Torsional strainTorsional strain strain that results from eclipsed bonds van der Waals strain (steric strain)van der Waals strain (steric strain) strain that results from atoms being too close together angle strainangle strain strain that results from distortion of bond angles from normal values Torsional strainTorsional strain strain that results from eclipsed bonds van der Waals strain (steric strain)van der Waals strain (steric strain) strain that results from atoms being too close together angle strainangle strain strain that results from distortion of bond angles from normal values

23. Dr. Wolf's CHM 201 & 202 3-23 Measuring Strain in Cycloalkanes Measuring Strain in Cycloalkanes Heats of combustion can be used to compare stabilities of isomers.Heats of combustion can be used to compare stabilities of isomers. But cyclopropane, cyclobutane, etc. are not isomers.But cyclopropane, cyclobutane, etc. are not isomers. All heats of combustion increase as the number of carbon atoms increase.All heats of combustion increase as the number of carbon atoms increase.

24. Dr. Wolf's CHM 201 & 202 3-24 Measuring Strain in Cycloalkanes Measuring Strain in Cycloalkanes Therefore, divide heats of combustion by number of carbons and compare heats of combustion on a "per CH 2 group" basis.Therefore, divide heats of combustion by number of carbons and compare heats of combustion on a "per CH 2 group" basis.

25. Dr. Wolf's CHM 201 & 202 3-25 Heats of Combustion of Cycloalkanes CycloalkanekJ/molPer CH 2 Cyclopropane2,091697 Cyclobutane2,721681 Cyclopentane3,291658 Cyclohexane3,920653 Cycloheptane4,599657 Cyclooctane5,267658 Cyclononane5,933659 Cyclodecane6,587659

26. Dr. Wolf's CHM 201 & 202 3-26 Heats of Combustion of Cycloalkanes CycloalkanekJ/molPer CH 2 According to Baeyer, cyclopentane should have less angle strain than cyclohexane. Cyclopentane3,291658 Cyclohexane3,920653 The heat of combustion per CH 2 group is less for cyclohexane than for cyclopentane. Therefore, cyclohexane has less strain than cyclopentane.

27. Dr. Wolf's CHM 201 & 202 3-27 Adolf von Baeyer (19th century) assumed cycloalkanes are planar polygonsassumed cycloalkanes are planar polygons distortion of bond angles from 109.5° gives angle strain to cycloalkanes with rings either smaller or larger than cyclopentanedistortion of bond angles from 109.5° gives angle strain to cycloalkanes with rings either smaller or larger than cyclopentane Baeyer deserves credit for advancing the idea of angle strain as a destabilizing factor.Baeyer deserves credit for advancing the idea of angle strain as a destabilizing factor. But Baeyer was incorrect in his belief that cycloalkanes were planar.But Baeyer was incorrect in his belief that cycloalkanes were planar.

28. Dr. Wolf's CHM 201 & 202 3-28 3.5 Small Rings CyclopropaneCyclobutane

29. Dr. Wolf's CHM 201 & 202 3-29 CyclopropaneCyclopropane sources of strain: torsional strain angle strain

30. Dr. Wolf's CHM 201 & 202 3-30 CyclobutaneCyclobutane nonplanar conformation relieves some torsional strain angle strain present

31. Dr. Wolf's CHM 201 & 202 3-31 3.6 Cyclopentane

32. Dr. Wolf's CHM 201 & 202 3-32 CyclopentaneCyclopentane all bonds are eclipsed planar conformation destabilized by torsional strain

33. Dr. Wolf's CHM 201 & 202 3-33 Nonplanar conformations of cyclopentane EnvelopeHalf-chair Relieve some, but not all, of the torsional strain. Envelope and half-chair are of similar stability and interconvert rapidly.

34. Dr. Wolf's CHM 201 & 202 3-34 3.7 Conformations of Cyclohexane heat of combustion suggests that angle strain is unimportant in cyclohexane tetrahedral bond angles require nonplanar geometries

35. Dr. Wolf's CHM 201 & 202 3-35 Chair is the most stable conformation of cyclohexane All of the bonds are staggered and the bond angles at carbon are close to tetrahedral.

36. Dr. Wolf's CHM 201 & 202 3-36 Boat conformation is less stable than the chair All of the bond angles are close to tetrahedral but close contact between flagpole hydrogens causes van der Waals strain in boat. 180 pm

37. Dr. Wolf's CHM 201 & 202 3-37 Boat conformation is less stable than the chair Eclipsed bonds bonds gives torsional strain to boat.

38. Dr. Wolf's CHM 201 & 202 3-38 Skew boat is slightly more stable than boat Less van der Waals strain and less torsional strain in skew boat. Boat Skew boat

39. Dr. Wolf's CHM 201 & 202 3-39 The chair conformation of cyclohexane is the most stable conformation and derivatives of cyclohexane almost always exist in the chair conformation

40. Dr. Wolf's CHM 201 & 202 3-40 3.8 Axial and Equatorial Bonds in Cyclohexane

41. Dr. Wolf's CHM 201 & 202 3-41 The 12 bonds to the ring can be divided into two sets of 6.

42. Dr. Wolf's CHM 201 & 202 3-42 Axial bonds point "north and south" 6 bonds are axial

43. Dr. Wolf's CHM 201 & 202 3-43 Equatorial bonds lie along the equator 6 bonds are equatorial

44. Dr. Wolf's CHM 201 & 202 3-44 3.9 Conformational Inversion (Ring-Flipping) in Cyclohexane

45. Dr. Wolf's CHM 201 & 202 3-45 Conformational Inversion chair-chair interconversion (ring-flipping) rapid process (activation energy = 45 kJ/mol) all axial bonds become equatorial and vice versa

46. Dr. Wolf's CHM 201 & 202 3-46

47. Dr. Wolf's CHM 201 & 202 3-47 Half- chair

48. Dr. Wolf's CHM 201 & 202 3-48 Half- chair Skew boat

49. Dr. Wolf's CHM 201 & 202 3-49 Half- chair Skew boat

50. Dr. Wolf's CHM 201 & 202 3-50 Half- chair Skew boat

51. Dr. Wolf's CHM 201 & 202 3-51 45 kJ/mol 23 kJ/mol