1. Alkanes & Cycloalkanes Cycloalkanes contain rings of carbon atoms.
Chapter 2
Alkanes & Cycloalkanes
Section 2 Cycloalkanes
Cycloalkanes contain rings of carbon atoms.
Ⅰ. Nomenclature
Structure
Properties

2. Classification of cycloalkanes
Monoscyclic:
According to
the number of cycles
Spirocyclic~
Bicyclic ~*
Bridged bicyclic ~
When a bicyclic compound contains 2 rings joined at adjacent carbons, as in bicyclo[4.4.0}decane, it is classified as a fused bicyclic compound,. When the two rings are joined at nonadjacent carbons, as in bicyclo[2.2.1]heptane, the compound is classified as a bridged bicyclic compound.
Polycyclic

3. cyclopropane Small: cyclobutane Common: cyclopentane cyclohexane
According to
number of carbons on the cycle:
Common:
cyclopentane
cyclohexane
Medium:
C7~C12
Large:
≥ C13
Since cycloalkanes consist of rings of CH2 units for monocyclo-alkane, they have the general formula (CH2)n, or CnH2n.

4. Nomenclature
1. Monocycloalkanes
2. Bicyclic bridged hydrocarbons

5. 1. Monocycloalkanes
Prefix: cyclo;
Cyclopropane Cyclobutane Cyclopentane Cyclohexane Cyclooctane
Methylcyclopentane
Ethylcyclopropane
1-Isopropyl-4-methylcyclohexane

6. 1,3-Dicyclohexylpropane
at the lowest sum
1-Ethyl-3-methylcyclohexane (not 1-methyl-3-ethylcyclohexane)
1-Cyclopropylbutane
1,3-Dicyclohexylpropane
5-Cyclopentyl-1-cyclopropyl-2-methylpentane

7. cis-1,2-Dimethylcyclopropane trans-1,2-Dimethylcyclopropane
Exercises:
cis-1,2-Dimethylcyclopropane
trans-1,2-Dimethylcyclopropane

8. 2. Bicyclic bridged hydrocarbons
A bicycloheptane

9. [2.2.1] Bicyclo heptane Bicyclo butane [1.1.0]
The number of C’s on the shortest one
Bicyclo heptane
[2.2.1]
Bicyclo butane
[1.1.0]
Note that a fused-ring system always has a 0 as one of the bracketed numbers.
The number of C’s on the longest bridge (except for the shared one)

10. 7-Ethyl-1,2,4-trimethylbicyclo[3.3.1]nonane
Exercises: 1 2 1 7 3 6 4 5
Bicyclo[3.2.0]heptane
7-Ethyl-1,2,4-trimethylbicyclo[3.3.1]nonane
Also name: Decalin
Bicyclo[4.4.0]decane

11. 1-Chloro-3,7,7-trimethylbicylco[2.2.1]heptane
Q: Give the name of the following . 7 1 2 4 3
1-Chloro-3,7,7-trimethylbicylco[2.2.1]heptane
1-Methylbiicyclo[2.2.0]hexane （1-甲基二环[2.2.0]己烷）
1-Cyclobutyl-1-methylcyclobutane（1-甲基-1-环丁基环丁烷）
1-Methylspiro[3.3]heptane (1-甲基螺[3.3]庚烷)

12. Structure
1. Ring Strain
2. Conformational isomerism in cyclohexane

13. 1. Ring Strain unstable stable [109.5°- 60°] =49.5°
[109.5°- 90°] =19.5°

14. Table Heats of Combustion of Cycloalkanes(kJ·mol-1)
Heat of Combustion
/CH2 Group
Cyclopropane 3
2091.3
697.1
Cyclobutane 4
2744.3
686.1
Cyclopentane 5
3320.0
664.0
Cyclohexane 6
3951.8
658.6
Cycloheptane 7
4636.7
662.4
Cyclooctane 8
5310.3
663.8
Cyclononane 9
5981.0
664.6
Cyclodecane 10
6635.8
663.6
Cyclopentadecane 15
9884.7
659.0
Unbranched alkane

15. (1) Cyclopropane
bent
Banana bond
could not lead
to ring closure

16. The large ring strain : ring-opening reactions
The large ring strain : ring-opening reactions. Easily react with Cl2, Br2, hydrogen halides (HX), H2SO4, etc, to form addition products at room temperature.
Br2
Mechanism: electrophilic addition

17. (2) Cyclobutane (3) Cyclopentane butterfly conformation
“envelope” conformations

18. Chair form of cyclohexane
bond angles of 120o, considerably larger than 109.5o
puckered to chair conformation ,
no angle strain
(b) staggered.
Cyclohexane can’t undergo ring-opening reactions too.

19. 2. Conformational isomerism in cyclohexane
Chair form
Boat form

20. 5 1 2 3 4 6 1 2 3 4 5 6
chair form
boat form 1 2 4 3 5 6

21. (torsional strain) eclipsed Q: Which one is more stable? Why?
Flagpole
Flag H space repulsion
eclipsed H’s
183pm 1 4 2 3 5 6
Gunwale
一些原子和基团的范德华半径（pm）
H 120；C 150；N 150；O 140； Cl 180 ；CH3 200.
Boat form
eclipsed
(torsional strain)
Perspective formula
Newman projection

22. staggered Staggered H’s Chair form Chair form 99.9% Boat form 0.1%
250pm
staggered
Chair form
Chair form 99.9% Boat form 0.1%

23. Boat conformation: Chair conformation:
Characteristics of boat and chair forms:
Boat conformation:
All adjacent C-H bonds are eclipsed
C1-H and C4-H bonds are crowded
Chair conformation:
All adjacent C-H bonds are staggered
Stability: chair’s> boat’s

24. 3. Conformational isomerism of substituted cyclohexane
axis
equator
3. Conformational isomerism of substituted cyclohexane
Equatorial bonds (e bond) and axial bonds (a bond) in chair form
e → a
a →e
Flip up of chair form

25. How to draw chair conformation of cyclohexane?

26. Are they equally stable?

27. equatorial methyl axial methyl
strong repulsion
1,3-diaxial interaction
(steric strain)
- 7.5kJ/mol
weak attraction
equatorial methyl
axial methyl

28. axial t-butylcyclohexane
Conformer of t-butylcyclohexane
Steric strain e a
equatorial
t-butylcyclohexane
axial t-butylcyclohexane
> 99.9% < 0.1%

29. have 2 geometric isomers
Disubstituted Cyclohexanes
There are cis-trans isomers in disubstituted cyclohexane when two substituents are in the different carbon of the ring.
1,4-dimethylcyclohexane
have 2 geometric isomers
cis-
trans-

30. cis-1,4- and trans-1,4-dimethylcyclohexanes
(a,a less stable)
(e,e )

31. cis-1,4-dimethylcyclohexane.
(a,e)
(a,e)

32. Exercises:
1. trans-1,3-dimethylcyclohexane (conformer)
(a,e)
(a,e)

33. More stable (t-Bt,e;Me,a) (a,e)
2. trans-1-tert-butyl-3-methylcyclo-hexane (conformer)
More stable
(t-Bt,e;Me,a)
(a,e)

34. Q3 Draw cis-1-tert-butyl-3-methylcyclohexane in its more stable chair conformation.
Q4 (a) Draw trans-1,2-dimethylcyclohexane in its more stable chair conformation. Are the methyl groups axial or equatorial?
(b) Draw cis-1,2dimethylcyclohexane in its more stable chair conformation. Are the methyl groups axial or equatorial?
(c) Which is more stable, cis-1,2-dimethylcyclohexane or trans-1,2-methylcyclohexane? Explain.

35. Summary: > > >
Stability of cycloalkanes:
> > >
At room temperature, there are many conformations, and 99% conformations are chair’s.
Stability: chair’s > boat’s

36. For the substituted cyclohexane:
(a) The longer the distance between the
larger groups, the more stable the conformation.
(b) For a molecule, the more the large groups on the equatorial bonds, the more stable the conformation.

37. Q5 Point out the order of stability of the following:
Solution: (a) > (b) > (c) > (d)

38. Q6: Draw the most stable conformation of trans-1-methyl-4-sec-butylcyclohexane.
Q7: Draw the most stable conformation of cis-1-methyl-4-sec-butylcyclohexane.

39. H
cis-Decalin
trans-Decalin

40. Properties 1.Free-radical substitution 2.Electrophilic addition
In many chemical respects cycloalkanes are similar to the alkanes, but in others they are different: the lower members form addition products with ring fission.

41. Physical Properties of Cycloalkanes
1. similar to open-chain alkanes
2. higher boiling points than unbranched alkanes
3. much higher melting points than their open-chain counterpart
Name
formula
mp/℃
bp/℃
density（g/cm3）
Cyclopropane
C3H6
-127.6
-32.7
Cyclobutane
C4H8
-80 13
Cyclopentane
C5H10
-94
49.5
0.751
Cyclohexane
C6H12
6.5
80.8
0.779
Cycloheptane
C7H14
-12
117
0.811
Cyclooctane
C8H16
13.5
148
0.834

42. 1. Free-radical substitution
Chlorine and bromine, in the presence of a catalyst, react with cycloalkanes to form halogenocycloalkanes:
excess
excess

44. 2. Addition
(1) Hydrogenation:

45. (2) Electrophilic Addition with X2
Cyclopropane reacts with chlorine (Cl2)or bromine at room temperature giving open-ring addition product:
room temp
CCl4

46. (3) Electrophilic addition with hydrogen halide
20℃

47. (Abide by Markovnikov’s rule)
When the asymmetric substituted cyclopropane reacts with concentrated HBr or HI, this open addition happens according to following way:
(Abide by Markovnikov’s rule)

48. (Abide by Markovnikov’s rule)

49. No reaction occur!
Q8: Compare the following reactions:

50. Q9: Differentiate the following compounds.
1. CH3CH2CH2CH2CH3; CH3CH2CH=CH-CH3 2.
、 &

51. Summary 1.Nomenclature Bicyclic bridged: bicyclo[x.y.z]octane
2.Stability of cycloalkanes:
cyclopropane < cyclobutane < cyclopentane ≈ (≥C6)
3. Conformations of Cyclohexane: chair & boat form
conformer:
4. Properties: substitution; addition(H2、X2、HX)