CH264
1
CH264/3
Organic Chemistry II
Cyclohexane Rings
Dr Andrew Marsh C515
a.marsh@warwick.ac.uk
Dr David J Fox B510
d.j....
CH264
2
Today’s Lecture
1. Cyclohexane conformation
2. Drawing cyclohexane chairs
3. Monosubstituted cyclohexanes
4. Disub...
CH264
3
Angle strain in rings
24.5° 9.5° 0.5° -5° "angle strain" (divided between two ring bonds)
9.17 6.58 1.24 0.02
38.4...
CH264
4
Cyclohexane is ‘strain free’
A planar arrangement of the six methylene groups in cyclohexane does not
give a tetra...
Conformational analysis
CH264
5
Nobel Prize 1969
CH264
6
Cyclohexane is ‘strain free’
HeqHeq
Heq
Hax
Ha
He
Hax
Hering flip
Heq Heq
HaxHax
view as Newman
projection
1,3-dia...
CH264
7
Substituents on cyclohexane
a a
aa
a
e
ee
e
e
e
axial substituents
equatorial substituents
a
CGW p. 371
CH264
8
Ring Flip
Ha
He
He
Ha
Ha
He
He
Ha
Ha
He
Ha
He
Ha
He
Ha
He
twist
boat
half chair
Ha
He
Ha
He
Ha
He
Ha
He
Ha
He
Ha
H...
CH264
9
Chair Conformer
Ha
He
He
Ha
Ha
He
Ha
He
view Ha
He
He
Ha
staggered
Ha
He
He
Ha
Ha
He
Ha
He
view Ha
He
He
Ha
CH264
10
Boat Conformer
Ha
He
Ha
He
1,4-transannular interaction
H
H
H
H
He
Ha Ha
He
• eclipsing C-C bonds & C-H
• no C-H ...
CH264
11
Substituted Cyclohexanes
HeqHeq
He
Ha
Ha
He
Ha
H3C CH3
CH3
CH3
CH3
CH3
Hering flip
favoured conformer
Heq Heq
Hax...
Substituted cyclohexanes:
energy difference
Substituent K Axial – equatorial
energy difference
kJ mol-1
% equatorial
H 1 0...
CH264
13
Disubstituted cyclohexanes
CH3
CH3
CH3
CH3
anti-1,3-dimethylcyclohexane
syn-1,3-dimethylcyclohexane
H3C
CH3
CH3
C...
CH264
14
The tert-butyl group is a
conformational ‘lock’
OH
OH
OH
H
H
cis-4-t-butylcyclohexanol
trans-4-t-butylcyclohexano...
Decalins
H
H
H
trans-decalin
conformationally locked
Ha
cis-decalin
H
H
H
He
CGW p. 378
Steroids: cholestanol
CH264
16
Conformationally locked A–B–C–D rings
HHO
H
Me
Me
Me
HO
H
H
H
H
H
A B
C D
CGW p. 379
CH264
17
You should be able to:
(i) Draw cyclohexane as chair conformers
(ii) Ring-flip monosubstituted cyclohexane
(iii) ...
Upcoming SlideShare
Loading in …5
×

Year 2 Organic Chemistry Conformational Analysis of Cyclohexane Rings

526 views

Published on

Year 2 module on mechanism and stereochemistry. Conformational analysis of cyclohexane.

Published in: Science, Technology, Business
0 Comments
1 Like
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total views
526
On SlideShare
0
From Embeds
0
Number of Embeds
3
Actions
Shares
0
Downloads
17
Comments
0
Likes
1
Embeds 0
No embeds

No notes for slide

Year 2 Organic Chemistry Conformational Analysis of Cyclohexane Rings

  1. 1. CH264 1 CH264/3 Organic Chemistry II Cyclohexane Rings Dr Andrew Marsh C515 a.marsh@warwick.ac.uk Dr David J Fox B510 d.j.fox@warwick.ac.uk
  2. 2. CH264 2 Today’s Lecture 1. Cyclohexane conformation 2. Drawing cyclohexane chairs 3. Monosubstituted cyclohexanes 4. Disubstituted cyclohexanes, decalins and steroids
  3. 3. CH264 3 Angle strain in rings 24.5° 9.5° 0.5° -5° "angle strain" (divided between two ring bonds) 9.17 6.58 1.24 0.02 38.4 27.5 5.19 0.09 strain per CH2 group (kcal mol-1) strain per CH2 group (kJ mol-1) "angle strain" is (109° - ring angle)/2
  4. 4. CH264 4 Cyclohexane is ‘strain free’ A planar arrangement of the six methylene groups in cyclohexane does not give a tetrahedral shape for every carbon atom - this is achieved by puckering the ring. Cyclohexane does this by adopting mainly two conformations the CHAIR and the BOAT. chair boat chair CGW p.371
  5. 5. Conformational analysis CH264 5 Nobel Prize 1969
  6. 6. CH264 6 Cyclohexane is ‘strain free’ HeqHeq Heq Hax Ha He Hax Hering flip Heq Heq HaxHax view as Newman projection 1,3-diaxial interaction HeqHeq Heq HaxHax Heq view as Newman projection gauche interaction 109° angle allows near strain free cyclic molecule
  7. 7. CH264 7 Substituents on cyclohexane a a aa a e ee e e e axial substituents equatorial substituents a CGW p. 371
  8. 8. CH264 8 Ring Flip Ha He He Ha Ha He He Ha Ha He Ha He Ha He Ha He twist boat half chair Ha He Ha He Ha He Ha He Ha He Ha He boat 4 kJ mol-1 43 25 21 0
  9. 9. CH264 9 Chair Conformer Ha He He Ha Ha He Ha He view Ha He He Ha staggered Ha He He Ha Ha He Ha He view Ha He He Ha
  10. 10. CH264 10 Boat Conformer Ha He Ha He 1,4-transannular interaction H H H H He Ha Ha He • eclipsing C-C bonds & C-H • no C-H HOMO C-C LUMO donation view
  11. 11. CH264 11 Substituted Cyclohexanes HeqHeq He Ha Ha He Ha H3C CH3 CH3 CH3 CH3 CH3 Hering flip favoured conformer Heq Heq Hax H3C CH3 CH3 view as Newman projection unfavourable gauche interaction 1,3-diaxial interaction HeqHeq Heq HaxHax CH3 CH3 CH3 conc equatorial conformer conc axial conformer K = >3000, >99.9% equatorial, >20 kJ mol-1 difference
  12. 12. Substituted cyclohexanes: energy difference Substituent K Axial – equatorial energy difference kJ mol-1 % equatorial H 1 0 50 OMe 2.7 2.5 73 Me 19 7.3 95 Et 20 7.5 95 iPr 42 9.3 98 tBu >3000 >20 >99.9 110 110 11.7 99 CH264 12 CGW p. 375
  13. 13. CH264 13 Disubstituted cyclohexanes CH3 CH3 CH3 CH3 anti-1,3-dimethylcyclohexane syn-1,3-dimethylcyclohexane H3C CH3 CH3 CH3 H3C CH3 CH3 CH3 favoured cis- trans- H H H H
  14. 14. CH264 14 The tert-butyl group is a conformational ‘lock’ OH OH OH H H cis-4-t-butylcyclohexanol trans-4-t-butylcyclohexanol H OH H OH H H H OH H
  15. 15. Decalins H H H trans-decalin conformationally locked Ha cis-decalin H H H He CGW p. 378
  16. 16. Steroids: cholestanol CH264 16 Conformationally locked A–B–C–D rings HHO H Me Me Me HO H H H H H A B C D CGW p. 379
  17. 17. CH264 17 You should be able to: (i) Draw cyclohexane as chair conformers (ii) Ring-flip monosubstituted cyclohexane (iii) Show which conformer is favoured in mono- and di- substituted cyclohexanes Outputs

×