1. 2.6 Cycloalkanes
Cycloalkanes are alkanes that have carbon atoms that form a ring (called alicyclic compounds)
Simple cycloalkanes are rings of CH2 units, (CH2)n, or CnH2n
Structure is shown as a regular polygon with the number of vertices equal to the number of C’s (a projection of the actual structure)

2. Nomenclature of Cycloalkanes
IUPAC naming rules are followed with the addition of the prefix cyclo-
With multiple substituents, use lowest numbering sequence

3. 2.6 Cycloalkanes

4. Complex Cycloalkanes
Naturally occurring materials contain cycloalkane structures
Examples:
chrysanthemic acid (cyclopropane),
prostaglandins (cyclopentane),
steroids (cyclohexanes and cyclopentane)

5. Complex Cycloalkanes

6. Properties of Cycloalkanes
Melting points are affected by the shapes and the way that crystals pack so they do not change uniformly

7. 2.7 Naming Cycloalkanes
Count the number of carbon atoms in the ring and the number in the largest substituent chain. If the number of carbon atoms in the ring is equal to or greater than the number in the substituent, the compound is named as an alkyl-substituted cycloalkane.
For an alkyl- or halo-substituted cycloalkane, start at a point of attachment as C1 and number the substituents on the ring so that the second substituent has as low a number as possible. When two substituents are present, we number the ring beginning with the substituent first in the alphabet.
Wenn three or more substituents are present, we begin at the substituent that leads to the lowest set of locants
Number the substituents and write the name

8. 1. Find the parent:
or butylcyclopropane

9. Number the substituents & write the name:

10. Examples:

11. Problem 2.15: IUPAC names?

12. 1-ethyl-2,3-dimethylcyclopentane4 5

13. cyclopropylcyclohexane
3-ethyl-4-methyl-1-
cyclopropylcyclohexane

14. 5-ethyl-3-cyclohexyl-4-methyloctane

15. 2.8 Cis-Trans Isomerism in Cycloalkanes
Rotation about C-C bonds in cycloalkanes is limited by the ring structure
Rings have two “faces” and substituents are labeled as to their relative facial positions
There are two different 1,2-dimethyl-cyclopropane isomers, one with the two methyls on the same side (cis) of the ring and one with the methyls on opposite sides (trans)

16. Isomerization with Cycloalkanes
Isomers - molecules that are the same but yet different
Constitutional (structural) isomers - same formula but different connectivity
cis -isomer
same side
trans - isomer
opposite sides
Stereoisomers- same formula and connectivity but different spatial arrangement

17. Cis/Trans Isomers Cis-1,2-dimethylcyclohexane
CH3
CH3
Cis-1,2-dimethylcyclohexane
Trans-1,2-dimethylcyclohexane

18. Cis/Trans Isomers Also written as: Cis-1,2-dimethylcyclohexane
CH3
CH3
Cis-1,2-dimethylcyclohexane
Trans-1,2-dimethylcyclohexane

19. 2.8 Cis-Trans Isomerism in Cycloalkanes

20. Stereoisomers
Compounds with atoms connected in the same order but which differ in three-dimensional orientation, are stereoisomers
The terms “cis” and “trans” should be used to specify stereoisomeric ring structures
Recall that constitutional isomers have atoms connected in different order

21. Stereoisomers

22. Practice Prob. 2.4: Name?

23. Problem 2.18: IUPAC Name?

24. Cis and trans isomerization of cycloalkanes is only for disubstitued rings.
Multi-substituted requires additional nomenclature
Examples of disubstituted:
cis-1-ethyl-3-methylcyclohexane
trans-1,2-dimethylcyclopentane

25. Structure of Cycloalkanes
Ring strain accounts for the unstability in certain cycloalkanes
Remember - sp3 has 109.5° bond angle
Ring Strain is combination of eclipsing strain and bond-angle strain

26. Each cycloalkane adopts a conformation that tries to alleviate ring strain as much as possible
cylcopropane
“banana” bonds that do not have end-to-end overlap of typical σ-bonds
Highly unstable ring-system that will “open” or react readily

27. Cyclobutane
Instead of flat conformation, a “puckered” conformation reduces eclipsing strain

28. Cyclopentane
An “envelope” conformation reduces eclipsing strain

29. Ring Conformation
Cyclopentane puckers

30. Cyclohexane
The “chair” conformation eliminates all eclipsing strain and nearly all bond-angle strain

31. Ring Strain vs. Cycloalkane Size
Cyclohexane the most stable
kcal/mol
Cycloalkane Ring Size

32. Axial and Equatorial positions are not energetically equivalent
More Stable
equatorial
1,3-diaxial interaction (steric hinderance)
The larger the group, the greater the 1,3-diaxial interaction
Cyclohexane prefers larger groups at equatorial positions

33. Ball-and-stick Model of Cyclopropane
A ball-and-stick model of cyclopropane shows the ring strain when the bond angles are deformed from their normal 109.5° to 60°.
Ball-and-stick Model of Cyclopropane

34. Cyclobutane
A ball-and-stick model of cyclobutane shows the ring strain when the bond angles are deformed from their normal 109.5° to approximately 90°.

35. Cyclopentane
The ball-and-stick model of cyclopentane reveals a nearly planar molecule with bond angles of 108°.
                                                                        

36. Cyclohexane: Chair and Boat Conformations
The chair conformation of cyclohexane can be twisted into the boat conformation.

37. Cyclohexane: Chair and Boat Conformations
The chair conformation of cyclohexane can be twisted into the boat conformation. Further twisting results in a second chair conformation. By labeling one axial substituent with a white sphere you can see how the axial position of one chair becomes equatorial by twisting into the second chair conformation.

38. Chapter 11 Organic Compounds: Alkanes
11.9
The Shape of Cycloalkanes

39. Bond Angles Ideal bond angle is 109º Cyclopropane - 60º
Cyclobutane - 90º
Strained bonds, unstable
Copyright © by Pearson Education, Inc. Publishing as Benjamin Cumming

40. Free Rotation Free rotation is not possible in cycloalkanes.
Copyright © by Pearson Education, Inc. Publishing as Benjamin Cummings

41. Chair and Boat
Copyright © by Pearson Education, Inc. Publishing as Benjamin Cummings

42. Geometric Isomers
Geometric isomers have restricted rotation around C-C
bonds and differ in 3-dimensional arrangement of atoms.
Named “cis-” and “trans– “

43. Learning Check
Identify each of the following as cis- or trans-

44. Learning Check Draw and name all the isomers of dichlorolcyclobutane.
(There are 5.)

45. Stereoisomers Stereoisomers are compounds with the same structural
formula but different spatial arrangements of atoms
Cannot superimpose.