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. Cycloalkanes 1-ethyl-3-methylcyclohexane
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. 3 1 2 4 5 6
1-ethyl-3-methylcyclohexane

6. Cycloalkanes 3-cyclopropyl-2,6-dimethylheptane
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 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
If cyclohexane 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. Cycloalkane Conformations
axis
axis
Equitorial bonds
Axial 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 cis-1,2-dimethylcyclopropane
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-1-ethyl-3-methylcyclobutane
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.