1. Chapter 3 Alkanes and Cycloalkanes: Conformations and cis-trans Stereoisomers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

3. eclipsed conformation Ethane

4. eclipsed conformation

5. Ethane staggered conformation

6. Ethane staggered conformation

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

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

9. H H HH HH H H H H H H 60° Gauche Relationships in Ethane Two bonds are gauche when the angle between them is 60°. The terms anti and gauche apply only to bonds (or groups) on adjacent carbons, and only to staggered conformations.

10. 0° 60° 120° 180° 240°300°360° 12 kJ/mol Energy Relationships in Ethane

11. The eclipsed conformation of ethane is 12 kJ/mol less stable than the staggered. The eclipsed conformation is destabilized by torsional strain. Torsional strain is the destabilization that results from eclipsed or partially eclipsed bonds. Torsional Strain

12. 3.2 Conformational Analysis of Butane

13. 0° 60° 120° 180° 240°300°360° 3 kJ/mol 14 kJ/mol

14. The gauche conformation of butane is 3 kJ/mol less stable than the anti. The gauche conformation in butane is destabilized by van 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

15. The conformation of butane in which the two methyl groups are eclipsed with each other is the least stable of all the conformations. It is destabilized by both torsional strain (eclipsed bonds) and van der Waals strain (close together). van der Waals Strain

16. 3.3 Conformations of Higher Alkanes

17. The most stable conformation of unbranched alkanes has anti relationships between carbons. Hexane Unbranched Alkanes

18. 3.4 The Shapes of Cycloalkanes: Planar or Nonplanar ?

19. Adolf von Baeyer (19th century) Baeyer assumed cycloalkanes are planar polygons. Also, that distortion 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. But Baeyer was incorrect in his belief that cycloalkanes were planar.

20. 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 (lack of colinear orbital overlap). Types of Strain

21. Measuring Strain in Cycloalkanes Heats of combustion can be used to compare stabilities of isomers. However cyclopropane, cyclobutane, etc. are not isomers. Heats of combustion of compounds increase as the number of carbon atoms increase. Therefore, the heats of combustion are divided by number of carbons and compared on a "per CH 2 group" basis.

22. CycloalkanekJ/molper CH 2 Cyclopropane2,091697 Cyclobutane2,721681 Cyclopentane3,291658 Cyclohexane3,920653 Cycloheptane4,599657 Cyclooctane5,267658 Cyclononane5,933659 Cyclodecane6,587659 Heats of Combustion in Cycloalkanes

24. According to Baeyer, cyclopentane should have less angle strain than cyclohexane. kJ/molper CH 2 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. Heats of Combustion in Cycloalkanes

25. 3.5 Small Rings: Cyclopropane Cyclobutane

26. Sources of strain: torsional strain, angle strain Cyclopropane

27. Nonplanar conformation relieves some torsional strain but angle strain is present. Cyclobutane

28. 3.6 Cyclopentane

29. In a planar conformation, all bonds are eclipsed. The planar conformation destabilized by torsional strain. Planar Representation of Cyclopentane

30. EnvelopeHalf-chair These relieve some, but not all, of the torsional strain. Envelope and half-chair are of similar stability and interconvert rapidly. Nonplanar Conformations of Cyclopentane

31. 3.7 Conformations of Cyclohexane Heat of combustion suggests that angle strain is unimportant in cyclohexane. Tetrahedral bond angles require nonplanar geometries.

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

33. All of the bond angles are close to tetrahedral but close contact between flagpole hydrogens causes van der Waals strain in boat. 180 pm Boat conformation is less stable than the chair

34. Eclipsed bonds bonds gives torsional strain to boat. Boat conformation is less stable than the chair

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

36. 3.8 Axial and Equitorial Bonds in Cyclohexane

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

38. Axial bonds point “up” and “down”. 6 Bonds are axial

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

40. 3.9 Conformational Inversion in Cyclohexane

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

42. Half- chair Skew boat

43. 45 kJ/mol 23 kJ/mol

44. 3.10 Conformational Analysis of Monosubstituted Cyclohexanes The most stable conformation is chair. A substituent is more stable when equatorial.

45. 5%95% Chair chair interconversion occurs, but at any instant 95% of the molecules have their methyl group equatorial. Equilibrium favors equitorial. Axial methyl group is more crowded than an equatorial one. Methylcyclohexane CH 3

46. 5% 95% 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

47. 40% 60% Crowding is less pronounced with a "small" substituent such as fluorine. Size of substituent is related to its branching. F F Fluorocyclohexane

48. Less than 0.01% Greater than 99.99% Crowding is more pronounced with a "bulky" substituent such as tert-butyl. tert-Butyl is highly branched. C(CH 3 ) 3 t-Butylcyclohexane

49. van der Waals strain due to 1,3-diaxial repulsions t-Butylcyclohexane

50. 3.11 Disubstituted Cyclohexanes: Cis-trans Stereoisomers Stereoisomers are isomers that have same constitution but different arrangement of atoms in space.

51. Isomers Constitutional isomers Stereoisomers

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

53. cis-1,2-Dimethylcyclopropane has methyl groups on same side of ring. trans-1,2-Dimethylcyclopropane has methyl groups on opposite sides. 1,2-Dimethylcyclopropane

54. 3371 kJ/mol 3366 kJ/mol Van der Waals strain makes cis stereoisomer less stable than trans. Relative stabilities of stereoisomers may be determined from heats of combustion.

55. 3.12 Conformational Analysis of Disubstituted Cyclohexanes

56. cistrans CH 3 5219 kJ/mol 5212 kJ/mol less stable more stable Trans stereoisomer is more stable than cis, but methyl groups are too far apart to crowd each other. H3CH3C H H H3CH3C CH 3 H H 1,4-Dimethylcyclohexane Stereoisomers

57. CH 3 H3CH3C H H Two equivalent conformations; each has one axial methyl group and one equatorial methyl group H CH 3 H H H3CH3C H Conformational analysis of cis-1,4-dimethylcyclohexane

58. CH 3 H3CH3C H H Two conformations are not equivalent; most stable conformation has both methyl groups equatorial. H H3CH3C H CH 3 H H3CH3C H Conformational analysis of trans-1,4-dimethylcyclohexane

59. cistrans 5223 kJ/mol 5217 kJ/mol less stable more stable Analogous to 1,4 in that trans is more stable than cis. CH 3 H H H3CH3C H H 1,2-Dimethylcyclohexane Stereoisomers

60. CH 3 H H Two equivalent conformations; each has one axial methyl group and one equatorial methyl group. H CH 3 H H H Conformational analysis of cis-1,2-dimethylcyclohexane

61. CH 3 H3CH3C H H Two conformations are not equivalent; most stable conformation has both methyl groups equatorial. H CH 3 H H H3CH3C H Conformational analysis of trans-1,2-dimethylcyclohexane

62. cistrans 5212 kJ/mol 5219 kJ/mol more stable less stable Unlike 1,2 and 1,4; cis-1,3 is more stable than trans. H3CH3C CH 3 H H H3CH3C H H 1,3-Dimethylcyclohexane Stereoisomers

63. CH 3 H3CH3C H H Two conformations are not equivalent; most stable conformation has both methyl groups equatorial. H3CH3C H H CH 3 H H Conformational analysis of cis-1,3-dimethylcyclohexane

64. Two equivalent conformations; each has one axial and one equatorial methyl group. H3CH3C H H CH 3 H H3CH3C H H3CH3C H H Conformational analysis of trans-1,3-dimethylcyclohexane

65. CompoundOrientation-  H° (kJ/mol) cis-1,2-dimethylax-eq5223 trans-1,2-dimethyleq-eq5217* cis-1,3-dimethyleq-eq5212* trans-1,3-dimethylax-eq5219 cis-1,4-dimethylax-eq5219 trans-1,4-dimethyleq-eq5212* *more stable stereoisomer of pair Table 3.2 Heats of Combustion of Isomeric Dimethylcyclohexanes

66. 3.13 Medium and Large Rings

67. 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

68. 3.14 Polycyclic Ring Systems contain more than one ring: bicyclic tricyclic tetracyclic etc

69. spirocyclic fused ring bridged ring Types of Ring Systems

70. Adamantane: A Tricyclic Compound Three bond cleavages are needed to create an open-chain structure.

71. one atom common to two rings Spirocyclic

72. Have adjacent atoms common to two rings and two rings share a common side Fused Ring

73. Has nonadjacent atoms common to two rings Bridged Ring

74. The carbon skeleton is tetracyclic. Steroids

75. 3.15 Heterocyclic Compounds Contain an atom other than C.

76. 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

77. Ethylene oxide Tetrahydrofuran Tetrahydropyran Oxygen-containing Heterocycles

78. Pyrrolidine Piperidine Nitrogen-containing Heterocycles

79. Lipoic acid Lenthionine CH 2 CH 2 CH 2 CH 2 COH O Sulfur-containing Heterocycles

80. End of Chapter 3 Alkanes and Cycloalkanes: Conformations and cis-trans Stereoisomers