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

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

2. Conformational Analysis Conformations are different spatial arrangements of a molecule that are generated by rotation about single bonds. Conformational analysis is the study of how conformational factors affect the structure of a molecule and its properties.

3. Representing Conformations These are common ways to show conformations.

4. Newman Projections In Newman projections we sight down a C ⎯ C bond. The front carbon by a point and the back carbon by a circle. Each carbon has three other bonds that are placed symmetrically around it. The bonds on the back carbon are shown sticking out from the circle.

5. Newman Projections of Ethane Newman projections differ with respect to the rotation of the front and back carbon atoms relative to each other. The angles H-C-C-H angle is the torsional or dihedral angle.

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

7. Relative Stability of Newman Projections The eclipsed conformation of ethane is the highest energy conformation. Repulsion between bonds destabilizes the eclipsed conformation. The staggered conformation is the most stable. Better electron delocalization stabilizes the staggered conformation. Conformations that are not staggered are said to have torsional strain.

8. Relative Stability of Ethane Conformations Ethane has infinite conformations corresponding to changes in the H-C-C-H torsional angle. Follow the “red” hydrogen atoms. eclipsed staggered

9. Relative Stability of Newman Projections At any instant, almost all of the molecules are in staggered conformations; hardly any are in eclipsed conformations. The difference between these two conformations is 12 kJ/mol.

10. Chapter 3 Propane Conformations • Propane is shown here as a perspective drawing and as a Newman projection looking down the C1—C2 bond.

11. Chapter 3 Propane Conformations • The staggered conformations of propane is lower in energy than the eclipsed conformations. Since the methyl group occupies more space than a hydrogen, the torsional strain will be 0.3 kcal/mol higher for propane than for ethane.

12. Conformations of Butane There are two different staggered conformations for butane. The anti conformation is the most stable and the gauche conformation is higher in energy because the larger CH3 groups are closer. This is called steric strain.

13. Strain in Newman Projections Torsional strain is the strain that results from eclipsed bonds. Steric hindrance results when two atoms are too close together. Also called van der Waals strain. Steric strain is the combination of both of these.

14. Newman Projections of Butane The eclipsed conformation with the CH3 groups eclipsed has the most steric strain and is the highest energy conformation.

15. Conformations of Higher Alkanes The lowest energy conformation of alkanes has all bonds staggered. With simple alkanes this has a zig-zag arrangement of carbon atoms.

16. The Shapes of Cycloalkanes:Planar or Nonplanar?

17. Stabilities of Cycloalkanes • Five- and six-membered rings are the most common in nature. • Carbons of cycloalkanes are sp3 hybridized and thus require an angle of 109.5º. • When a cycloalkane carbon has an angle other than 109.5º, there will not be optimum overlap and the compound will have angle strain. • Angle strain is sometimes called Baeyer strain in honor of Adolf von Baeyer, who first explained this phenomenon. • Torsional strainarises when all the bonds are eclipsed. Chapter 3

18. 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

19. Angle strain in Cycloalkanes Tetrahedral carbons prefer bond angles of 109.5o and angle strain refers to the strain molecules have when this bond angle cannot be matched. Cyclopropane has the highest angle strain since its bond angles are about 60o. Other cycloalkanes try to minimize the angle strain and are therefore not planar.

20. Angle strain in Cycloalkanes Heat of combustion gives a way to measure the relative stability of cycloalkanes. The lowest heat of combustion per CH2 group corresponds to the most stable cycloalkane.

21. Cyclopropane Strong sp3-sp3 s-bonds cannot be formed because of 60o bond angle. The bonds are called bent bonds.

22. Cyclopropane There is also high torsional strain because the C-H bonds are all eclipsed.

23. Nonplanar Cyclobutane • Cyclic compound with four carbons or more adopt nonplanar conformations to relieve ring strain. • Cyclobutane adopts the folded conformation (“envelope”) to decrease the torsional strain caused by eclipsing hydrogens. Chapter 3

24. Cyclopentane Planar cyclopentane has low angle strain since the natural bond angle is 108o. Torsional strain is significant because the C-H bonds are eclipsed.

25. Cyclopentane The envelope and half-chair conformations have similar lower energy and rapidly interconvert. They relieve some, but not all, torsional strain.

26. Conformations of Cyclohexane

27. Cyclohexane The most stable conformation of cyclohexane is known as the chair conformation. The side view shows the bonds across from each other are parallel. One end carbon (1) is up and the other (4) is down.Solid wedges show bonds projecting towards the viewer.

28. Cyclohexane The Newman projection shows that all bonds are staggered minimizing torsional strain.

29. Cyclohexane Other conformations include the skew boat. Most molecules are in the chair conformation and less than 5 molecules in 100,000 are in the skew boat conformation at any point in time at 25 oC. Therefore we concentrate on the chair conformation

30. Axial and Equatorial Bonds in Cyclohexane

31. Definitions The 12 hydrogens of cyclohexane can be divided into two groups: axial and equatorial. Equatorial hydrogens lie around the equator of the molecule. Axial hydrogens are directed alternately up and down.

32. Conformational Inversion in Cyclohexane

33. Ring Inversion There are two chair conformations of cyclohexane that rapidly interconvert. An axial group in the original chair conformation becomes equatorial in the ring-inverted form and vice versa.

34. Ring Inversion Ring inversion procedes through highest energy half-chair conformations and the twist boat and boat conformations.

35. Conformational Analysis of MonosubstitutedCyclohexanes

36. Methylcyclohexane Interconversion between the two chair conformations occurs rapidly and the methyl group is either equatorial or axial. The conformation with an equatorial methyl is favored.

37. Methylcyclohexane Van der Waals strain between the methyl and the axial hydrogen atoms on the same side of the molecule destabilize the conformation with an axial methyl.

38. Chair Inversion of Fluorocyclohexane Crowding is less pronounced with a "small" substituent such as fluorine so the difference in energy is lower than that observed for the methyl substitutent.

39. Chair Inversion of t-butylcyclohexane Crowding is more pronounced with a “larger" substituent such as the tertiary butyl group so the difference in energy is much higher than that observed for the methyl substitutent.

40. DisubstitutedCycloalkanes:cis-trans Stereoisomers

41. Definitions of Isomers Isomers are different compounds that have the same molecular formula. Constitutional isomers differ in connectivity. Stereoisomers have the same connectivity but a different arrangement of the atoms in space.

42. cis and trans Substituents A cycloalkane with two substituents on different carbons in the ring may have two orientations. If the substituents are on the same side of the ring we say they are cis to each other. If the substituents are on the opposite sides of the ring we say they are trans to each other.

43. Relative Energies of Stereoisomers The cis-stereoisomer is higher in energy due to van der Waals strain. The difference in energy is determined by measuring the heat of combustion.

44. Conformational Analysisof DisubstitutedCyclohexanes

45. H H3C H3C CH3 H H H 1,4-Dimethylcyclohexane Stereoisomers CH3 Trans stereoisomer is more stable than cis, but methyl groups are too far apart to crowd each other. cis trans 5212 kJ/mol 5219 kJ/mol less stable more stable

46. H3C H H CH3 CH3 H H CH3 H3C H H Two equivalent conformations; each has one axial methyl group and one equatorial methyl group Conformational analysis ofcis-1,4-dimethylcyclohexane CH3

47. H CH3 H3C H H CH3 H3C H CH3 H H H3C Two conformations are not equivalent; most stableconformation has both methyl groups equatorial. Conformational analysis oftrans-1,4-dimethylcyclohexane

48. CH3 CH3 H H H H3C CH3 H 1,2-Dimethylcyclohexane Stereoisomers Analogous to 1,4 in that trans is more stablethan cis. cis trans 5217 kJ/mol 5223 kJ/mol less stable more stable

49. CH3 H H CH3 CH3 CH3 H CH3 CH3 H H H Two equivalent conformations; each has one axial methyl group and one equatorial methyl group Conformational analysis ofcis-1,2-dimethylcyclohexane

50. CH3 H H CH3 H H3C CH3 H Two conformations are not equivalent; most stableconformation has both methyl groups equatorial. CH3 Conformational analysis oftrans-1,2-dimethylcyclohexane H H3C H