E N D
1. Chapter 3Conformations of Alkanes and Cycloalkanes
2. 3.1Conformational Analysis of Ethane Conformations are different spatial arrangements of a molecule that are generated by rotation about single bonds.
3. eclipsed conformation Ethane
4. Ethane eclipsed conformation
5. Ethane staggered conformation
6. Ethane staggered conformation
7. Projection Formulas of the Staggered Conformation of Ethane
8. Anti Relationships Two bonds are anti when the angle between them is 180.
9. Gauche Relationships Two bonds are gauche when the angle between them is 60.
10. An important point: The terms anti and gauche apply only to bonds (or groups) on adjacent carbons, and only to staggered conformations.
12. 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. Torsional strain
13. 3.2Conformational Analysis of Butane
15. 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. van der Waals strain
16. 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. van der Waals strain
17. 3.3Conformations of Higher Alkanes
18. The most stable conformation of unbranchedalkanes has anti relationships between carbons.
Unbranched alkanes
19. 3.4The Shapes of Cycloalkanes:Planar or Nonplanar?
20. assumed cycloalkanes are planar polygons
distortion of bond angles from 109.5 givesangle strain to cycloalkanes with rings eithersmaller or larger than cyclopentane
Baeyer deserves credit for advancing the ideaof angle strain as a destabilizing factor.
But Baeyer was incorrect in his belief that cycloalkanes were planar. Adolf von Baeyer (19th century)
21. 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 Types of Strain
22. Measuring Strain in Cycloalkanes Heats of combustion can be used to comparestabilities of isomers.
But cyclopropane, cyclobutane, etc. are not isomers.
All heats of combustion increase as the numberof carbon atoms increase.
23. Measuring Strain in Cycloalkanes Therefore, divide heats of combustion by number of carbons and compare heats of combustion on a "per CH2 group" basis.
24. 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 Heats of Combustion in Cycloalkanes
25. Cycloalkane kJ/mol Per CH2
According to Baeyer, cyclopentane should
have less angle strain than cyclohexane.
Cyclopentane 3,291 658
Cyclohexane 3,920 653
The heat of combustion per CH2 group is
less for cyclohexane than for cyclopentane.
Therefore, cyclohexane has less strain than
cyclopentane. Heats of Combustion in Cycloalkanes
26. Adolf von Baeyer (19th century) assumed cycloalkanes are planar polygons
distortion of bond angles from 109.5 givesangle strain to cycloalkanes with rings eithersmaller or larger than cyclopentane
Baeyer deserves credit for advancing the ideaof angle strain as a destabilizing factor.
But Baeyer was incorrect in his belief that cycloalkanes were planar.
27. Cyclopropane
Cyclobutane 3.5Small Rings
28. sources of strain
torsional strain
angle strain Cyclopropane
29. nonplanar conformation relieves some torsional strain
angle strain present Cyclobutane
30. 3.6Cyclopentane
31. all bonds are eclipsed in planar conformation
planar conformation destabilizedby torsional strain Cyclopentane
32. Relieve some, but not all, of the torsional strain.
Envelope and half-chair are of similar stabilityand interconvert rapidly. Nonplanar Conformations of Cyclopentane
33. heat of combustion suggests that angle strain is unimportant in cyclohexane
tetrahedral bond angles require nonplanar geometries 3.7Conformations of Cyclohexane
34. All of the bonds are staggered and the bond angles at carbon are close to tetrahedral. Chair is the most stable conformation of cyclohexane
35. All of the bond angles are close to tetrahedralbut close contact between flagpole hydrogenscauses van der Waals strain in boat. Boat conformation is less stable than the chair
36. Eclipsed bonds bonds gives torsional strain toboat. Boat conformation is less stable than the chair
37. Less van der Waals strain and less torsional strain in skew boat. Skew boat is slightly more stable than boat
38. the chair conformation of cyclohexane is themost stable conformation and derivativesof cyclohexane almost always exist in the chair conformation Generalization
39. 3.8Axial and Equatorial Bonds in Cyclohexane
40. The 12 bonds to the ring can be divided into two sets of 6.
41. 6 Bonds are axial
42. The 12 bonds to the ring can be divided into two sets of 6.
43. 6 Bonds are equatorial
44. 3.9Conformational Inversion(Ring-Flipping) in Cyclohexane
45. chair-chair interconversion (ring-flipping)
rapid process (activation energy = 45 kJ/mol)
all axial bonds become equatorial and vice versa Conformational Inversion
48. most stable conformation is chair
substituent is more stable when equatorial 3.10Conformational Analysis of Monosubstituted Cyclohexanes
49. Chair chair interconversion occurs, but at any instant 95% of the molecules have their methyl group equatorial.
Axial methyl group is more crowded than an equatorial one. Methylcyclohexane
50. Source of crowding is close approach to axial hydrogens on same side of ring.
Crowding is called a "1,3-diaxial repulsion" and is a type of van der Waals strain. Methylcyclohexane
51. Crowding is less pronounced with a "small" substituent such as fluorine.
Size of substituent is related to its branching. Fluorocyclohexane
52. Crowding is more pronounced with a "bulky" substituent such as tert-butyl.
tert-Butyl is highly branched. tert-Butylcyclohexane
53. tert-Butylcyclohexane
54. 3.11Disubstituted Cycloalkanes:Stereoisomers Stereoisomers are isomers that have same constitution but different arrangement of atoms in space
56. 1,2-Dimethylcyclopropane There are two stereoisomers of 1,2-dimethylcyclopropane.
They differ in spatial arrangement of atoms.
57. 1,2-Dimethylcyclopropane cis-1,2-Dimethylcyclopropane has methyl groupson same side of ring.
trans-1,2-Dimethylcyclopropane has methyl groupson opposite sides.
60. 3.12Conformational Analysisof Disubstituted Cyclohexanes
61. Trans stereoisomer is more stable than cis, but methyl groups are too far apart to crowd each other. 1,4-Dimethylcyclohexane Stereoisomers
62. Conformational analysis ofcis-1,4-dimethylcyclohexane
63. Conformational analysis oftrans-1,4-dimethylcyclohexane
64. Analogous to 1,4 in that trans is more stablethan cis. 1,2-Dimethylcyclohexane Stereoisomers
65. Conformational analysis ofcis-1,2-dimethylcyclohexane
66. Conformational analysis oftrans-1,2-dimethylcyclohexane
67. Unlike 1,2 and 1,4; cis-1,3 is more stable than trans. 1,3-Dimethylcyclohexane Stereoisomers
68. Conformational analysis ofcis-1,3-dimethylcyclohexane
69. Conformational analysis oftrans-1,3-dimethylcyclohexane
70. Compound Orientation -?H
cis-1,2-dimethyl ax-eq 5223trans-1,2-dimethyl eq-eq 5217*
cis-1,3-dimethyl eq-eq 5212*trans-1,3-dimethyl ax-eq 5219
cis-1,4-dimethyl ax-eq 5219trans-1,4-dimethyl eq-eq 5212* Table 3.2 Heats of Combustion ofIsomeric Dimethylcyclohexanes
71. 3.13Medium and Large Rings
72. More complicated than cyclohexane.
Common for several conformations to be of similar energy.
Principles are the same, however.Minimize total strain. Cycloheptane and Larger Rings
73. contain more than one ring
bicyclic
tricyclic
tetracyclic
etc 3.14Polycyclic Ring Systems
74. spirocyclic
fused ring
bridged ring Types of Ring Systems
75. one atom common to two rings Spirocyclic
76. adjacent atoms common to two rings
two rings share a common side Fused Ring
77. nonadjacent atoms common to two rings Bridged Ring
78. equals minimum number of bond disconnectionsrequired to give a noncyclic species Number of Rings
79. requires one bond disconnection Monocyclic
80. requires two bond disconnections Bicyclic
81. requires two bond disconnections Bridged Bicyclic
82. carbon skeleton is tetracyclic Steroids
83. 3.15Heterocyclic Compounds
84. a cyclic compound that contains an atom other than carbon in the ring
(such atoms are called heteroatoms)
typical heteroatoms are N, O, and S Heterocyclic Compound
85. Oxygen-containing Heterocycles
86. Nitrogen-containing Heterocycles
87. Sulfur-containing Heterocycles