Cycloalkanes

1. Cycloalkanes • Many organic compounds contain cyclic or ring structures: • carbohydrates • nucleotides in DNA and RNA • antibiotics penicillin G testosterone

2. Cycloalkanes • Cycloalkanes: • alkanes that contain three or more carbons arranged in a ring • CnH2n

3. Cycloalkanes • Cycloalkanes are named using: • prefix “cyclo” • alkane base name • Examples: • a cycloalkane with 5 carbons in the ring: • cyclopentane • a cycloalkane with 10 carbons in the ring: • cyclodecane

4. Naming Substituted Cycloalkanes • To name substituted cycloalkanes: • Use the cycloalkane for the base name • Identify substituents using name and position • no number is needed if only one substituent is present isopropylcyclohexane

5. 3 2 4 1 5 6 Cycloalkanes • For 2 or more substituents, number the ring carbons to give the lowest possible numbers for the substituted carbons • If numbering could begin with either substituent, start with the one that is first alphabetically. 1-ethyl-3-methylcyclohexane

6. Cycloalkanes • When the acyclic portion of the molecule contains more carbons than the cycloalkane, the cyclic portion is named as a cycloalkyl substituent. 3-cyclopropyl-2,6-dimethylheptane

7. Cycloalkanes Example: Name the following cycloalkanes.

8. Cycloalkanes Example: Draw the following cycloalkanes. sec-butylcyclooctane 1,1,3,3-tetramethylcyclohexane

9. Cycloalkane Conformations • Cycloalkanes containing 3 - 20 carbons have been synthesized. • Rings with 5 or 6 carbons are the most common • WHY? • Recall that all alkanes contain C - C single bonds that are formed by the overlap of sp3 hybrid orbitals • tetrahedral geometry • ideal bond angle = 109.5o

10. Cycloalkane Conformations • In cycloalkanes the best overlap (strongest bond) between the sp3 hybrid orbitals will occur when bond angle = 109.5o • In some cycloalkanes, bond angles other than 109.5o lead to angle strain and less than optimum overlap of the sp3 hybrid orbitals • Angle strain: the strain associated with bond angles that are smaller or larger than the ideal value

11. Cycloalkane Conformations • In addition to angle strain, some cycloalkane conformations lead to significant amounts of torsional strain due to eclipsing of bonds. • Ring strain: • the extra strain associated with the ring structure of a compound compared to a similar acyclic compound • angle strain • torsional strain

12. Cycloalkane Conformations • The heat of combustion (DHcomb) is often used to measure the ring strain of a cycloalkane. • The amount of heat released when a substance is burned in the presence of excess oxygen. • As DHcomb increases, a substance contains a greater amount of potential energy. • As potential energy increases, the compound becomes less stable.

13. Cycloalkane Conformations • Cyclopropane: • greatest ring strain per CH2 • 60o bond angles • weaker overlap of sp3 hybrid orbitals • all bonds eclipsed • more reactive than other alkanes

14. Cycloalkane Conformations • Cyclobutane: • second largest amount of ring strain • slightly folded conformation instead of planar and square • Square/planar conformation would have less angle strain but more torsional strain

15. Cycloalkane Conformations • Cyclobutane: • Slightly folded conformation • 88o bond angle • not quite eclipsed • less torsional strain

16. Cycloalkane Conformations • Cyclopentane: • Five membered rings are very important biologically • ribose and deoxyribose have cyclopentane like conformations • If cyclopentane existed as a planar, regular pentagon: • bond angle = 108o • low angle strain • all bonds eclipsed • high torsional strain

17. Cycloalkane Conformations • Cyclopentane: • Actual shape = slightly puckered envelop • reduces eclipsing and torsional strain • Molecule constantly “undulates” • envelop flap moves around ring

18. Cycloalkane Conformations • Cyclohexane: • Most common cycloalkane • Carbohydrates, steroids, and some pesticides contain cyclohexane-like conformations • Ifcyclohexane had a planar, regular hexagonal conformation: • 120o bond angles • high angle strain • adjacent methylene groups eclipsed • high torsional strain

19. Cycloalkane Conformations • Cyclohexane has no ring strain. • Cannot be a planar, regular hexagon • Cyclohexane achieves tetrahedral bond angles and staggered conformation by assuming puckered conformations: • chair conformation • most stable • boat conformation • half-chair conformation • highest energy

20. Cycloalkane Conformations • Chair conformation • most stable • lowest energy • 109.5o • staggered

21. Cycloalkane Conformations • Boat conformation: • 109.5o • torsional strain due to eclipsing of bonds • actually exists as the twist boat conformation in order to eliminate the flagpole effect • Repulsive force between two groups that are in close proximity on opposite ends of a ring

22. Cycloalkane Conformations • The twist boat conformation • reduces flagpole effect • reduces eclipsing of bonds • lower in energy than the boat conformation • higher in energy than the chair conformation • Cyclohexane exists predominantly in the chair conformation • constantly interconverting from chair to half-chair to boat conformations and back

23. Cycloalkane Conformations

24. Cycloalkane Conformations • Each carbon in a cyclohexane ring has two different types of carbon - hydrogen bonds: • axial bond • a bond that is parallel to the axis of the ring • directed up and down • equitorial bond • a bond that is directed along the “equator” of the ring • pointed out from the ring

25. axis axis Cycloalkane Conformations Axial bonds Equitorial bonds Axial and equitorial bonds

26. Monosubstituted Cyclohexanes • Substituents on a cyclohexane ring can occupy either an axial position or an equitorial position. • Chair-chair interconversions that occur at room temperature lead to an equilibrium mixture of both conformations. Axial methyl equitorial methyl

27. Monosubstituted Cyclohexanes • During chair-chair interconversions (ring flip), substituents change from: • axial equitorial • Conformations with the substituent in the equitorial position are lower in energy (more stable) and therefore favored: • “anti” arrangement • no 1,3-diaxial interaction

28. Monosubstituted Cycloalkanes • 1, 3-diaxial interactions • steric hinderance between groups in axial positions on carbons 1 and 3 More stable conformer 1, 3-diaxial interaction

29. Monosubstituted Cyclohexanes Example: Draw the two possible chair conformers of t-butylcyclohexane. Which one is more stable? Why?

30. Cis & Trans Isomers • Cycloalkanes have two distinct faces. • Di-substituted cycloalkanes can exist as cis and trans isomers. • Cis cycloalkane • two identical groups on the same face of the ring cis-1,2-dimethylcyclopropane

31. Cis and Trans Isomers • Trans cycloalkane • two identical groups on opposite faces of the ring trans-1-ethyl-3-methylcyclobutane

32. Cis and Trans Isomers Example: Name the following compound.

33. Cis and Trans Isomers • Drawing cis/trans isomers of disubstituted cyclohexanes: Positions Cis Trans 1,2 a,e or e,a a,a or e,e 1,3 a,a or e,e a,e or e,a 1,4 a,e or e,a a,a or e,e

34. Cis and Trans Isomers • Disubstituted cyclohexanes can also exist as different chair conformations: • Some conformations are lower energy • More stable • Preferred • Some conformations are higher energy • Less stable • Not preferred

35. Cis and Trans Isomers • Points to remember about relative stabilities: • Greatest stability is found in disubstituted conformers with both substituents in equitorial positions. • Disubstituted conformers with both substituents in the axial positions are very unfavorable. • If the isomer requires an a,e conformer, then the most stable conformer will have the largest group in the equitorial position.

36. Cis and Trans Isomers Example: Draw both chair conformations of cis-1,3-diethylcyclohexane. Which one is more stable?

37. Cis and Trans Isomers Example: Draw both possible conformations of trans-1-t-butyl-3-methylcyclohexane. Which one is more stable?

38. Bicyclic Molecules • Two or more rings can be joined into bicyclic or polycyclic molecules. • Fused rings share 2 adjacent carbons and the bond between them.

39. Bicyclic Molecules • Bridged rings share two nonadjacent carbons and one or more carbon atoms between them.