DEMONSTRATION LESSON IN ENGLISH 4 MATATAG CURRICULUM
Actual shape of alicyclic compounds
1. ACTUAL SHAPE OF
SIX MEMBERED RINGS &
ITS RELATION TO REACTIVITY
DR. Rashmi Saxena
Govt. M H College of
Home Sc & Sciene, Jabalpur
2. Rotation about C-C Single Bonds
Different spatial arrangements of atoms that result from rotation
about carbon-carbon single bonds are known as conformations
3. Baeyer strain theory 1885
• Carbon is tetrahedral (109.5*) any deviation leads to internal strain
• Carbon atoms in alicyclic compounds are planer i.e regular polygons
(wrong assumption)
• In these shapes there is deviation from normal tetrahedral angle,
causing a lot of strain in the molecule
• Ring larger than C-7 should not exist ,if they exist they will be highly
strained
4. Stability of cycloalkanes
• Deviation = 109.5 – bond angle in the planer ring / 2
Deviation
0.7509.7524.75 - 5.25
Heat of
combustion
Kcal/CH2 unit
166.6 164.0 158.7 157.4
For Alkanes Heat of combustion CH2 unit is
fixed quantity 157 – 158 kcal
5. Theory of Puckered Rings : Sachse – Mohr Theory
• Sachse in 1890 pointed out that two puckered i.e. non planer models
could be constructed in which carbon maintain tetrahedral angle 109.5.
• These are Chair & Boat form
• This idea found no acceptance .
6. Theory of Puckered Rings : Sachse – Mohr Theory
• In 1918 Mohr revived the Sachse theory and applied to decalins &
predicted the presence of cis & trans Decalins
• This was experimentally confirmed by W Huckel in 1925.
7. Conformation of Cyclohexane
Planer structure
Angle is 120* ,
which is greater than
normal tetrahedral angle 109.5*
All Hydrogen are
aligned
Angle Strain Torsional Strain
H
HHH
H
H H
H
H
H
H
H
8. Puckered structure of Cyclohexane : Free from Angle Strain
Chair Cyclohexane
Boat Cyclohexane
Angle strain
expansion or compression of bond angles away
from most stable angle109.5*
10. How to Draw Correct Chair Conformation of Cyclohexane ?
Step I Step IIIStep II
Chair is Completed
11. Drawing of axial & Equatorial Bonds in Chair Conformation of
Cyclohexane
axis
a
a
a
a
a
a
e
e
e
e
e
e
Chair Conformation showing axial & Equatorial bonds
Axial Bonds Equatorial Bonds
12. H
H
H
HC H 2
C H 2
H H
H
H
1
2
3
4
5
6
1
2 3
4
56
Newman Projection of Chair Conformation of Cyclohexane
Newman Projection
Equivalent
to H
H
H
H
4
5
Staggered EthaneEnergy zero Kcal/mol
All interactions are staggered
13. axial up
eq. up
Axial Down
Equatorial Down
Flipping of Chair Conformation of Cyclohexane
14. All axial bonds become equatorial
All equatorial bonds become axial
All “up” bonds stay up
All “down” bonds stay down
What Happens on flopping of Chair Conformation of
Cyclohexane
16. H
H
H
H
H
1
2
3
45
6
H
H
Boat Conformation of Cyclohexane
Newman Projection
Flagpole interaction
Eclipsed interaction C-2 ;C-3 & C 5-C 6
Flagpole interaction
Interactions
Staggered = 04
C-1 ; C-2
C-3 ; C-4
C-4 ; C-5
C-6 ; C-1
Eclipsed
C-2 ;C-3
C 5 ; C 6
Flagpole – flagpole
C-1 ; C-4
Energy is 6.8 kcal/mol
17. Boat Conformation of Cyclohexane
Flagpole – flagpole & eclipsed interactions
are reduced
Energy 5.2 Kcal/mole
19. Conformation of Monomethylcyclohexane
1,3 diaxial interactions are equivalent to Gauche
interaction
Energy contribution = 0.9 kcal /mole
axial methylcyclohexane energy 1.8 kcal/mole
Equatorial methyl cyclohexane energy zero kcal/mole
H
H
20. Conformationof1,2dimethylcyclohexane
Cis - 1a 2e dimethylcyclohexane
1,3 diaxial interactions
CH3
H3C
H
H
1 3
5
H C H 3
C H 3
H3
5
5
Total Energy 2.7 Kcal
1,3 diaxial = 02
Gauche = 01
A B
Flip
21. Conformation of 1,2 dimethylcyclohexane
trans – 1e 2e dimethylcyclohexane
CH3
CH3
CH3
CH3
Energy = 0.9 kcal /mole Energy = 3.6 kcal /mole
trans – 1a 2a dimethylcyclohexane
Most stable
Flip
22. Conformation of 1,3-dimethylcyclohexane
Cis - 1a 3a dimethylcyclohexane
Energy = 5.4 kcal /mole
trans - 1a 3e dimethylcyclohexane
CH3
CH3CH3
CH3
CH3
CH3
CH3
Energy = 1.8 kcal /mole
Energy = 1.8 kcal /mole
Energy = zero kcal /mole
Cis – 1e 3e dimethylcyclohexane
CH3
Flip
Flip
23. Conformation of 1,4-dimethylcyclohexane
trans – 1e 4e dimethylcyclohexane
Energy = zero kcal /mole
cis - 1a 4 e dimethylcyclohexane
CH3
CH3
CH3
CH3
CH3
CH3
Energy = 1.8 kcal /mole Energy = 1.8 kcal /mole
Energy = 3.6 kcal /mole
trans – 1a 4a dimethylcyclohexane
CH3
CH3
CH3
Flip
Flip
24. Optical Activity of cis1,2dimethylcyclohexane
CH3
CH3 CH3
CH3
H
H
CH3 CH3
Meso form
CH3
CH3
Rotation by 120 *
Inseparable dl pair
Planer structure
25. Optical Activity of trans1,2dimethylcyclohexane
CH3
CH3
CH3
CH3
CH3
CH3
Optically Active
Both forms are dissymmetric & possess a non identical mirror image
Planer structure
28. Case of steric Hindrance
steric requirements of transition state > steric requirements of ground state
F
FE
*
FA
*
FE
*
FA
*-
E*
A*
E
A
GROUND
STATE
TRANSITION
STATE
Equatorial Isomer
Axial Isomer
Energy Level of Ground State & Transition State
FA
* = FE
*+ (FA
*- FE
*) - F
(FA
*-FE
*) > F
FA
*
> FE
*
Conclusion : Axial isomer
reacts more slowly
Crowing of axial substituent
is greater in Transition state
29. Saponification of 4- t- butylcyclohexyl p –nitrobenzoate &
ethyl 4 - t- butylcyclohexanecarboxylate
CH3)3 C
OCOC6H4NO2-p
H
H
CH3)3 C
OCOC6H4NO2-p
H
H
cis isomer
trans isomer
(CH3)3 C
OH
H
cis Isomer
(CH3)3 C
OH
H
trans isomer
ktrans / kcis = 2.5
30. Saponification of 4- t- butylcyclohexyl p –nitrobenzoate &
ethyl 4 - t- butylcyclohexanecarboxylate
CH3)3 C
CO0CH2CH3
H
H
CH3)3 C
H
H
cis isomer
trans isomer
(CH3)3 C
OH
H
cis Isomer
(CH3)3 C
OH
H
trans isomer
ktrans / kcis = 20
CO0CH2CH3
- C- OR
OH
OH
31. Case of steric Assistance
steric requirements of ground state > steric requirements of transition state
F
FE
*
FA
*
FE
*
FA
*-
E*
A*
E Crowing of axial substituent
is greater in ground state
GROUND
STATE
TRANSITION
STATE
Equatorial Isomer
Axial Isomer
Energy Level of Ground State & Transition State
FA
* = FE
*+ (FA
*- FE
*) - F
(FA
*-FE
*)F >
>
Conclusion : Axial isomer
reacts fast
FA
*FE
*
More crowded substituent
reacts faster
32. Solvolysis of cis & trans – 4 – t - butylcyclohexyl tosylates
( SN 1 Reaction)
CH3)3 C
OTs
H
H (CH3)3 C
cis isomer
trans isomer
(CH3)3 C
+
H
OTs
Solvolysis Product
33. SN 2 Reaction
CH3)3 C
X
H
H
(CH3)3 C
cis isomer
trans isomer
(CH3)3 C
H
Y
H
X
Y
X
Transition State
34. SN 2 Substitution group Y > group x
FX
FE
*
FA
*
FE
*
FA
*-
E*
EGROUND
STATE
TRANSITION
STATE
equatorial isomer
axial isomer
H
X
Y
R
axial isomer
H
Y
X
R
FY
Y = BROMIDE
X = THIOPHENOLATE
equatorial isomer
R X
R R
R
XY
Y
35. SN i Reaction
CH3)3 C
NH2
H
H
(CH3)3 C
trans isomer
trans isomer
(CH3)3 C
H
OH
N2
+
HNO2
• Reaction occurs with the retention of configuration
• Retention is attributed to a cyclic intermediate or a
short lived solvated ion
H
• In Axial amines the predominant reaction
product is olefin. (trans diaxial elimination)
36. SN i Reaction
CH3)3 C
NH2
H
H
cis isomer
(CH3)3 C
N2
+
HNO2
• In Axial amines the predominant reaction product is olefin. (trans
diaxial elimination)
(CH3)3 C
H
37. Elimination Reaction : dehydrohalogination of menthyl chloride by base
(CH3)2 CH
CH3
H
H
menthyl chloride
(CH3)2 HC
• Stereoelectronic requirement
• Groups to be eliminated should be
conformationally anti to each other
• Neomenthyl chloride 3 – menthene
• Menthyl chloride 2 - menthene
- HCl
- HCl
Cl
(CH3)2 CH
CH3
Cl
CH3
neomenthyl chloride
- HCl
- HCl
-OEt
-OEt
3 – menthene
2 - menthene
H
(200 times faster)
H3C
H
Cl
CH(CH3)2
H3C
CH(CH3)2
38. Molecular rearrangement : A concerted Reaction
NH2
H
OH
trans isomer N2
+
HNO2
OH
CHO
Bond is Trans to departing Group
2-aminocyclohexanol
39. Molecular rearrangement : A concerted Reaction
NH2
H
OH
cis isomer
N2
+
HNO2
OH
CHO
2-aminocyclohexanol
NH2
OH
H
N2
OH
H
+
