Chapter 3 Alkanes and Cycloalkanes: Conformations and cis-trans Stereoisomers

1. Chapter 3Alkanes and Cycloalkanes: Conformations and cis-trans Stereoisomers

2. 3.1Conformational Analysis of Ethane • Conformations are different spatial arrangements of a molecule that are generated by rotation about single bonds.

3. Ethane • eclipsed conformation

4. Ethane • eclipsed conformation

5. Ethane • staggered conformation

6. Ethane • staggered conformation

7. H H H H H H H H H H H H Projection Formulas of the Staggered Conformation of Ethane Newman Sawhorse

8. H H H H H H H H H H H H Anti Relationships • Two bonds are anti when the angle between them is 180°. 180°

9. Gauche Relationships • Two bonds are gauche when the angle between them is 60°. H 60° H H H H H H H H H H H

10. An Important Point: • The terms anti and gauche apply only to bonds (or groups) on adjacentcarbons, and only to staggered conformations.

11. 12 kJ/mol 0° 60° 120° 180° 240° 300° 360°

12. Torsional Strain • The eclipsed conformation of ethane is 12 kJ/mol less stable than the staggered. • The eclipsed conformation is destabilized bytorsional strain. • Torsional strain is the destabilization that resultsfrom eclipsed or partially eclipsed bonds.

13. 3.2Conformational Analysis of Butane

14. 14 kJ/mol 3 kJ/mol 0° 60° 120° 180° 240° 300° 360°

15. van der Waals Strain • The gauche conformation of butane is 3 kJ/molless stable than the anti. • The gauche conformation is destabilized byvan der Waals strain (also called steric strain). • van der Waals strain is the destabilization that results from atoms being too close together.

16. van der Waals Strain • The conformation of butane in which the twomethyl groups are eclipsed with each other isthe least stable of all the conformations. • It is destabilized by both torsional strain(eclipsed bonds) and van der Waals strain.

17. 3.3Conformations of Higher Alkanes

18. Unbranched Alkanes • The most stable conformation of unbranchedalkanes has anti relationships between carbons. Hexane

19. 3.4The Shapes of Cycloalkanes:Planar or Nonplanar?

20. Adolf von Baeyer (19th century) • Baeyer assumed cycloalkanes are planar polygons, • and that distortion of bond angles from 109.5° givesangle strain to cycloalkanes with rings eithersmaller or larger than cyclopentane. • Baeyer deserves credit for advancing the ideaof angle strain as a destabilizing factor. • But Baeyer was incorrect in his belief that cycloalkanes were planar.

21. Types of Strain • • Torsional strain strain that results from eclipsed bonds • • van der Waals strain (steric strain) strain that results from atoms being too close together • • angle strain strain that results from distortion of bond angles from normal values

22. Measuring Strain in Cycloalkanes • Heats of combustion can be used to comparestabilities of isomers. • But cyclopropane, cyclobutane, etc. are not isomers. • All heats of combustion increase as the numberof carbon atoms increase.

23. Measuring Strain in Cycloalkanes • Therefore, divide heats of combustion by number of carbons and compare heats of combustion on a "per CH2 group" basis.

24. Heats of Combustion in Cycloalkanes • Cycloalkane kJ/mol Per CH2 • Cyclopropane 2,091 697 • Cyclobutane 2,721 681 • Cyclopentane 3,291 658 • Cyclohexane 3,920 653 • Cycloheptane 4,599 657 • Cyclooctane 5,267 658 • Cyclononane 5,933 659 • Cyclodecane 6,587 659

25. Heats of Combustion in Cycloalkanes • Cycloalkane kJ/mol Per CH2 • According to Baeyer, cyclopentane should • have less angle strain than cyclohexane. • Cyclopentane 3,291 658 • Cyclohexane 3,920 653 • The heat of combustion per CH2 group is • less for cyclohexane than for cyclopentane. • Therefore, cyclohexane has less strain than • cyclopentane.

26. Adolf von Baeyer (19th century) • assumed cycloalkanes are planar polygons • distortion of bond angles from 109.5° givesangle strain to cycloalkanes with rings eithersmaller or larger than cyclopentane • Baeyer deserves credit for advancing the ideaof angle strain as a destabilizing factor. • But Baeyer was incorrect in his belief that cycloalkanes were planar.

27. 3.5Small Rings • Cyclopropane • Cyclobutane

28. Cyclopropane • sources of strain • torsional strain • angle strain

29. Cyclobutane • nonplanar conformation relieves some torsional strain • angle strain present

30. 3.6Cyclopentane

31. Cyclopentane • all bonds are eclipsed in planar conformation • planar conformation destabilizedby torsional strain

32. Nonplanar Conformations of Cyclopentane • Relieve some, but not all, of the torsional strain. • Envelope and half-chair are of similar stabilityand interconvert rapidly. Envelope Half-chair

33. 3.7Conformations of Cyclohexane • heat of combustion suggests that angle strain is unimportant in cyclohexane • tetrahedral bond angles require nonplanar geometries

34. Chair is the most stable conformation of cyclohexane • All of the bonds are staggered and the bond angles at carbon are close to tetrahedral.

35. Boat conformation is less stable than the chair 180 pm • All of the bond angles are close to tetrahedralbut close contact between flagpole hydrogenscauses van der Waals strain in boat.

36. Boat conformation is less stable than the chair • Eclipsed bonds bonds gives torsional strain toboat.

37. Skew boat is slightly more stable than boat Skew boat Boat • Less van der Waals strain and less torsional strain in skew boat.

38. Generalization • The chair conformation of cyclohexane is themost stable conformation and derivativesof cyclohexane almost always exist in the chair conformation.

39. 3.8Axial and Equatorial Bonds in Cyclohexane

40. The 12 bonds to the ring can be divided into two sets of 6.

41. 6 Bonds are axial Axial bonds point "north” and “south"

42. The 12 bonds to the ring can be divided into two sets of 6.

43. 6 Bonds are equatorial Equatorial bonds lie along the “equator.”

44. 3.9Conformational Inversion in Cyclohexane

45. Conformational Inversion • chair-chair interconversion (ring-flipping) • rapid process (activation energy = 45 kJ/mol) • all axial bonds become equatorial and vice versa

46. Half-chair Skewboat

47. 45 kJ/mol 45 kJ/mol 23 kJ/mol

48. 3.10Conformational Analysis of MonosubstitutedCyclohexanes most stable conformation is chair substituent is more stable when equatorial

49. CH3 CH3 Methylcyclohexane Chair chair interconversion occurs, but at any instant 95% of the molecules have their methyl group equatorial. Axial methyl group is more crowded than an equatorial one. 5% 95%

50. Methylcyclohexane Source of crowding is close approach to axial hydrogens on same side of ring. Crowding is called a "1,3-diaxial repulsion" and is a type of van der Waals strain. 5% 95%