Chapter 3 Conformations of Alkanes and Cycloalkanes

1. Chapter 3Conformations of Alkanes and Cycloalkanes • Conformations or Conformers or Rotamers; • Different spatial arrangements of a molecule that are generated by rotation about single bonds.

2. Ethane • eclipsed conformation

3. Ethane • staggered conformation

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

5. H H H H H H 180° H H H H H H Anti Relationships • Two bonds are anti when the angle between them is 180°. • The terms anti and gauche apply only to bonds (or groups) on adjacentcarbons, and only to staggered conformations.

6. H 60° H H H H H H H H H H H Gauche Relationships • Two bonds are gauche when the angle between them is 60°. • The terms anti and gauche apply only to bonds (or groups) on adjacentcarbons, and only to staggered conformations.

7. 12 kJ/mol 0° 60° 120° 180° 240° 300° 360°

8. 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 bonds.

9. 14 kJ/mol 3 kJ/mol 0° 60° 120° 180° 240° 300° 360°

10. 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.

11. van der Waals strain • 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.

12. Line Notation is Obtained from the most stable conformer (anti-staggered conformation)

13. Bond Angle for Cycloalkanesifthey are PlanarBond Angle =(n-2)/n*180°

14. Types of Strain in Cycloalkanes • • 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

15. Experimentally Measuring Strain in Cycloalkanes • Heats of combustion can be used to comparestabilities of isomers. • Since cycloalkanes are not isomers, divide • DH by the number of C in ring

16. 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

17. Heats of Combustion in Cycloalkanes • If rings were planar,cyclopentane should • have less angle strain than cyclohexane. • Cyclopentane 658 • Cyclohexane 653 • Therefore, cyclohexane has less strain than • cyclopentane.

18. Small Rings; Cyclopropane • sources of strain • torsional strain • angle strain

19. Small Rings; Cyclobutane • nonplanar conformation relieves some torsional strain • angle strain present

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

21. 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

22. 3.7Conformations of Cyclohexane • heat of combustion suggests that angle strain is unimportant in cyclohexane • tetrahedral bond angles require nonplanar geometries

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

24. 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 Waalsstrain in boat.

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

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

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

28. 3.8Axial and Equatorial Bonds in the Chair Conformation ofCyclohexane The 12 bonds to the ring can be divided into two sets of 6.

29. 6 Bonds are axial 6 Bonds are equatorial • Axial bonds point "north and south" • Equatorial bonds lie along the equator

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

31. Half-chair Skewboat

32. 45 kJ/mol 45 kJ/mol 23 kJ/mol