Chapter 3 Conformations of Alkanes and Cycloalkanes

1. Chapter 3Conformations of Alkanes and Cycloalkanes

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

3. eclipsed conformation Ethane

4. Ethane eclipsed conformation

5. Ethane staggered conformation

6. Ethane staggered conformation

7. Projection Formulas of the Staggered Conformation of Ethane

8. Anti Relationships Two bonds are anti when the angle between them is 180�.

9. Gauche Relationships Two bonds are gauche when the angle between them is 60�.

10. An important point: The terms anti and gauche apply only to bonds (or groups) on adjacent carbons, and only to staggered conformations.

12. 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 bonds. Torsional strain

13. 3.2Conformational Analysis of Butane

15. 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. van der Waals strain

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

17. 3.3Conformations of Higher Alkanes

18. The most stable conformation of unbranchedalkanes has anti relationships between carbons. Unbranched alkanes

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

20. 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. Adolf von Baeyer (19th century)

21. � 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 Types of Strain

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. 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 Heats of Combustion in Cycloalkanes

25. 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. Heats of Combustion in Cycloalkanes

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. Cyclopropane Cyclobutane 3.5Small Rings

28. sources of strain torsional strain angle strain Cyclopropane

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

30. 3.6Cyclopentane

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

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

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

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

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

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

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

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

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

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

43. 6 Bonds are equatorial

44. 3.9Conformational Inversion(Ring-Flipping) in Cyclohexane

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

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

49. 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. Methylcyclohexane

50. 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. Methylcyclohexane

51. Crowding is less pronounced with a "small" substituent such as fluorine. Size of substituent is related to its branching. Fluorocyclohexane

52. Crowding is more pronounced with a "bulky" substituent such as tert-butyl. tert-Butyl is highly branched. tert-Butylcyclohexane

53. tert-Butylcyclohexane

54. 3.11Disubstituted Cycloalkanes:Stereoisomers Stereoisomers are isomers that have same constitution but different arrangement of atoms in space

56. 1,2-Dimethylcyclopropane There are two stereoisomers of 1,2-dimethylcyclopropane. They differ in spatial arrangement of atoms.

57. 1,2-Dimethylcyclopropane cis-1,2-Dimethylcyclopropane has methyl groupson same side of ring. trans-1,2-Dimethylcyclopropane has methyl groupson opposite sides.

60. 3.12Conformational Analysisof Disubstituted Cyclohexanes

61. Trans stereoisomer is more stable than cis, but methyl groups are too far apart to crowd each other. 1,4-Dimethylcyclohexane Stereoisomers

62. Conformational analysis ofcis-1,4-dimethylcyclohexane

63. Conformational analysis oftrans-1,4-dimethylcyclohexane

64. Analogous to 1,4 in that trans is more stablethan cis. 1,2-Dimethylcyclohexane Stereoisomers

65. Conformational analysis ofcis-1,2-dimethylcyclohexane

66. Conformational analysis oftrans-1,2-dimethylcyclohexane

67. Unlike 1,2 and 1,4; cis-1,3 is more stable than trans. 1,3-Dimethylcyclohexane Stereoisomers

68. Conformational analysis ofcis-1,3-dimethylcyclohexane

69. Conformational analysis oftrans-1,3-dimethylcyclohexane

70. Compound Orientation -?H� cis-1,2-dimethyl ax-eq 5223trans-1,2-dimethyl eq-eq 5217* cis-1,3-dimethyl eq-eq 5212*trans-1,3-dimethyl ax-eq 5219 cis-1,4-dimethyl ax-eq 5219trans-1,4-dimethyl eq-eq 5212* Table 3.2 Heats of Combustion ofIsomeric Dimethylcyclohexanes

71. 3.13Medium and Large Rings

72. More complicated than cyclohexane. Common for several conformations to be of similar energy. Principles are the same, however.Minimize total strain. Cycloheptane and Larger Rings

73. contain more than one ring bicyclic tricyclic tetracyclic etc 3.14Polycyclic Ring Systems

74. spirocyclic fused ring bridged ring Types of Ring Systems

75. one atom common to two rings Spirocyclic

76. adjacent atoms common to two rings two rings share a common side Fused Ring

77. nonadjacent atoms common to two rings Bridged Ring

78. equals minimum number of bond disconnectionsrequired to give a noncyclic species Number of Rings

79. requires one bond disconnection Monocyclic

80. requires two bond disconnections Bicyclic

81. requires two bond disconnections Bridged Bicyclic

82. carbon skeleton is tetracyclic Steroids

83. 3.15Heterocyclic Compounds

84. a cyclic compound that contains an atom other than carbon in the ring (such atoms are called heteroatoms) typical heteroatoms are N, O, and S Heterocyclic Compound

85. Oxygen-containing Heterocycles

86. Nitrogen-containing Heterocycles

87. Sulfur-containing Heterocycles