Conformational Analysis of Substituted Cyclohexanes

1
Conformational Analysis of Substituted
Cyclohexanes
2
Methylcyclohexane
5
95

• 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.

3
Methylcyclohexane
5
95

• Source of crowding is close approach to axial
  hydrogens.
• Crowding is called a "1,3-diaxial interaction"
  and is a type of van der Waals strain.

4
Fluorocyclohexane
F
F
40
60

• Crowding is less pronounced with a "small"
  substituent such as fluorine.

5
tert-Butylcyclohexane
Less than 0.01
Greater than 99.99

• Crowding is more pronounced with a "bulky"
  substituent such as tert-butyl.

6
tert-Butylcyclohexane
7
1,2-Dimethylcyclopropane
cis
trans
8
Isomers
9
1,2-Dimethylcyclopropane

• There are two stereoisomers of 1,2-dimethylcyclop
  ropane.
• They differ in spatial arrangement of atoms.

10
1,2-Dimethylcyclohexane Stereoisomers
cis
trans
11
Conformational analysis ofcis-1,2-dimethylcyclohe
xane
Two equivalent conformations each has one axial
methyl group and one equatorial methyl group
12
Conformational analysis oftrans-1,2-dimethylcyclo
hexane
CH3
H
H3C
H
CH3


H
H
CH3
H
H3C
CH3
H
Two conformations are not equivalent most
stableconformation has both methyl groups
equatorial.
13
1,2-Dimethylcyclohexane Stereoisomers
cis
trans
5223 kJ/mol
5217 kJ/mol
less stable
more stable
14
1,3-Dimethylcyclohexane Stereoisomers
15
Conformational analysis ofcis-1,3-dimethylcyclohe
xane
Two conformations are not equivalent most
stableconformation has both methyl groups
equatorial.
16
Conformational analysis oftrans-1,3-dimethylcyclo
hexane
CH3
H3C
H
H
Two equivalent conformations each has one
axialand one equatorial methyl group.
17
1,3-Dimethylcyclohexane Stereoisomers
18
1,4-Dimethylcyclohexane Stereoisomers
CH3
cis
trans
19
Conformational analysis ofcis-1,4-dimethylcyclohe
xane
CH3
Two equivalent conformations each has one axial
methyl group and one equatorial methyl group
20
Conformational analysis oftrans-1,4-dimethylcyclo
hexane

Two conformations are not equivalent most
stableconformation has both methyl groups
equatorial.
21
1,4-Dimethylcyclohexane Stereoisomers
CH3
cis
trans
5212 kJ/mol
5219 kJ/mol
less stable
more stable
22
Table 3.2 Heats of Combustion ofIsomeric
Dimethylcyclohexanes

• Compound Orientation -?H
• cis-1,2-dimethyl ax-eq 5223trans-1,2-dimethyl eq-
  eq 5217
• cis-1,3-dimethyl eq-eq 5212trans-1,3-dimethyl ax
  -eq 5219
• cis-1,4-dimethyl ax-eq 5219trans-1,4-dimethyl eq-
  eq 5212

more stable stereoisomer of pair