this video very useful for all chemistry related exam like jee main+adwance bsc msc iit jam CSIR gate du bhu hcu in this we discuss stereochemistry of substitute cyclohexane. we discuss about 1,3 diaxal interaction 1,2 interaction mono substitute cyclohexane Conformation of Cyclohexane Stereochemical configuration of cyclohexane Newman projection of cyclohexane Repulsion energy of substituent in cyclohexane

1. STUDY OF CONFORMERS OF
SUBSITUTED CYCLOHEXANE

2. Simple representation
Conformation of Cyclohexane
1. Chair Conformation
2. Boat Conformation
3. Twist Conformation or Skew Boat Conformation
4. Half Chair Conformation
Cyclohexane is a cycloalkane with the molecular formula C6H12.
Cyclohexane is non-polar. Cyclohexane is a colourless, flammable liquid
with a distinctive detergent-like odour.
2

3. stability order: chair > twist-boat > boat> half-chair
Energy diagram of
Conformation of
cyclohexane
3

4. In Cyclohexane every carbon have 2 more groups which one is above
the plane and another one is below the plane
In chair form there are
3 axial-up group
3 axial-down group
3 Equatorial-Up group
3 Equatorial-down group
4

5. 5

6. Most stable conformer of cyclohexane is chair form.
So generally cyclohexane exist as chair form
If cyclohexane have one or more than one group which larger than hydrogen
so they fell some repulsion which decrease stability
1
2
3
4
5
6
Generally this repulsion is 2 type
1,3-Diaxial Interactions
1, 2 repulsion
6

7. 1,3-Diaxial Interactions
A
H
H
H
H
When any group which larger then hydrogen place in axial position
then it feel interaction with 3ed position carbons axial hydrogen .
This is type gauche interactions in Newman projection.
7

8. 8

9. Q
A
1 and 4 is 1,3 repulsion
2 is 1, 2 repulsion
This repulsion due to attraction of adjacent substituent
9

10. A
H
H
H
H
CH3
• When the cyclohexane ring bears a substituent, the two chair
conformers are not the same. In one conformer the substituent
is axial, in the other it is equatorial.
• Due to steric hindrance In the axial location, substituent
groups prefer to be equatorial and that chair conformer
predominates in the equilibrium.
1, 3 repulsion
10

11. Why Iodine bromine chlorine etc. have low repulsion energy
In mono-substituted cyclohexane, the energy difference between
axial and equatorial conformers with a given substituent is known
as its A-value.
A value useful to find repulsion energy of substitute cyclohexane
Generally A value is additive in nature
11

12. We know that conformational isomers normally interconvert
rapidly so both form exist together but one form is more
compare to other
A value
12

13. An ethyl group is one carbon larger than a methyl group, About double in
size. it must be produce twice steric hindrance, and the energy difference
would be twice as big.
But the observable value similar to methyl group because The ethyl group
can rotate such that the CH3 points away from the ring, where it does not
lead to any significant increase in strain
13

14. the isopropyl group represents a secondary carbon attached to the
cyclohexane. There is a relatively small but significant increase in strain
to 2.15 kcal/mol .
This is because the isopropyl group can still adopt a conformation where
the C-H bond lies over the cyclohexane ring, which does not bring it into
significant contact with the axial C-H bonds.
14

15. 15

16. Given that oxygen has a larger atomic number than carbon,
it’s not unreasonable to think that the OH group might be
“bulkier” than carbon.
But A value of Hydroxyl group is less than CH3 because
less number of hydrogen
16

17. Along similar lines one could be forgiven for thinking that Br,
being such a heavy and large atom, might exert a large
destabilizing influence when in the axial position. However, the
difference is only 0.43 kcal/mol, less than that for OH.
Because there are no hydrogen atom
the A values for Cl, Br, and I are all roughly similar (about 0.43
or so). This is because the increased size is balanced by the
increased bond lengath – the halogens might be increasing in size
along Cl <Br < I – but they are also getting farther away.
17

18. Q
A
In disubstituted cyclohexane's the steric effects of both substituents are
different in both conformations.
18

19. Substitute cyclohexane also show cis-trans isomers
19

20.  Compounds with 1,2-ax,ax-substitutions are always trans.
 Compounds with 1,2-eq,eq-substitutions are always trans as well.
 Compounds with 1,2-ax,eq- or 1,2-eq,ax-bonds are always cis.
 Compounds with 1,3-ax,ax-substitutions are always cis.
 Compounds with 1,3-eq,eq-substitutions are always cis as well.
20

21.  Compounds with 1,3-ax,eq- or 1,2-eq,ax-bonds are
always trans.
 Compounds with 1,4-ax,ax-substitutions are always trans.
 Compounds with 1,4-eq,eq-substitutions are always trans as
well.
 Compounds with 1,4-ax,eq- or 1,4-eq,ax-bonds are always cis
21

22. Stereo of 1,2 substitute cyclohexane
cis
trans
R
2
R
1
R
2
R
1
R
1
R
2
R
2
R
1
22

23. R
1
R
2
R
1
R
2
R
2
R
2
cis
cis
R
1
R
2
R
1
R
2
R
1
R
2
R
1
R
2
R
1
R
2
Stereo of 1,3 substitute cyclohexane
Stereo of 1,4 substitute cyclohexane
23

24. 24

25.  1,1-dimethylcyclohexane: Both ring flip isomers identical: +1.8
kcal/mol.
 cis-1,2-dimethylcyclohexane: Both ring flip isomers identical: one axial
methyl group = +1.8 kcal/mol. There is a gauche interaction between the
two methyl groups that raises this by 0.9 kcal/mol
Both energy is 1.8kcal/mol
Both energy is 2.7kcal/mol
25

26.  trans-1,2-dimethylcyclohexane: Diaxial form would be 2*(+1.8)
kcal/mol = +3.6 kcal/mol, but the diequatorial form would be 0.
However, we should add a little bit back because the two methyl groups
are gauche to each other (this is about +0.9 kcal/mol
 cis-1,3-dimethylcyclohexane: The diaxial form would be more than 2 x
1.8 kcal/mol, because the two methyl groups would run into each other
(rather than just into an axial hydrogen). The diequatorial form would
have no steric interactions, so it would have a strain energy of 0.
+3.6 kcal/mol
Some about 0 kcal/mol
more than 3.6 kcal/mol
0 kcal/mol
26

27.  trans-1,3-dimethylcyclohexane: Both forms identical, with one
axial methyl group. +1.8 kcal/mol.
 trans-1,4-dimethylcyclohexane: The diaxial form would be 2 x
1.8 kcal/mol = +3.6 kcal/mol; the diequatorial form would have
no interactions and be 0.
 cis-1,4-dimethylcyclohexane: Both ring flip isomers identical:
one axial methyl group = +1.8 kcal/mol.
3.6 kcal/mol 0kcal/mol
27

28. 28

29. Newman presentation of Cis 1,2 dicyclohexanol
29

30. There 2 different energy's conformers
Ax- ax conformer which hydroxy group in
axial position and have four 1,3diaxial
intrection
Newman presentation of stable form of trans 1,2 dicyclohexanol
30

31. There 2 different energy's conformers
Ax-ax conformer which hydroxy group
in axial position and have two 1,3 diaxial
intrection and one hydrogen bond.
Newman presentation of Cis 1,3 dicyclohexanol
31

32. Newman presentation of trans 1,3 dicyclohexanol
32

33. Trans cyclohexan-1,4 dial
33

34. 34

35. There are 1,3 diaxial interaction present
35

36. 36

37. 37

38. 38

39. 39

41. Substitute cyclohexane exist as different conformer with different
interaction, orientation, and energy.
So they so different reaction in different conformation or some time only
one conformer is reactive and another is inactive.
This is because some time substituent is an axial position and
some time equatorial position
41

42.  SN1 reaction is 2 and more than 2 step reaction
 RDS step is first step in which carbocation is form duiring removing groups
In 1st reaction repulsion between
Ots group and axial hydrogen so
Ots group remove easily to
remove repulsion fources
There ots group in equitorial
position so there are no 1,3
repulsion so Ots can’t remove
easily
42

43. In this reaction Nearby groups can evidently increase the rate
of substitution reactions significantly
43

44. 44

45. 45

46. 46

47. E1 and E2 are
mechanism path way of
elimination reaction
Reaction goes to which
path way decide strio
and orientation of
substituent cyclohexane
47

48. If removing group is
axial position it remove
easily because 1,3
repulsion
48

49.  E2 mechanism have
single step
 This occurs only anti
elimination
If L.G. and Hydrogen
are axial and anti
position then it remove
easily
49

50. 50

51. 51

52. 52

53. 53

54. Pinecol pinecolon rearrangement is adjacent diol reaction.
In cyclohexane this occurs three types.
54

55. 55

56. In trans isomer there are no
1,3 diaxial intrection between
hydroxyl and hydrogen atom
so this reaction rate is slow
In cis isomer hydroxy group
faces 1,3 diaxial interaction
56

57. 57

58. Path A 1,3 repulsions
occurs between ester
group and axial
hydrogen. So OH- ion
cant join easily so RDS
step is slow
Path B there are no 1, 3
interaction so RDS step
is fast
RDS step Base catalyze hydrolysis of ester is attacking of OH- ion on
C=O bond and attach there
58

59. 1, 3 repulsion
Rate of reaction
Compound b have fast rate
59

60. 60

61. mechanism
61

62. 62

63. 63

64. 64

65. The effect of conformation on E2 reactions is demonstrated by the difference
in rates of the dehydrobromination of cis- and trans-1-bromo-4-tert-
butylcyclohexane. The axial bromine atom of the cis isomer is eliminated
about 500 times faster than the equatorial atom of the trans isomer.
65

66. Reason of question
66

67. Which of the following compounds will react faster in an E2
reaction; trans-1-bromo-2-isopropylcyclohexane or cis-1-bromo-
2-isopropylcyclohexane?
Answer
The cis isomer will react faster than
the trans. The cis isomer has two possible
perpendicular hydrogen in which it can eliminate
from.
67

68. Ans is c
68

69. Ans is c
69

70. Ans is (d)
70

71. Ans is (b)
71

72. ans is (c)
72