Year 2 Organic Chemistry Mechanism and Stereochemistry Lecture 3 Conformational Analysis of Cyclohexane

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Year 2 undergraduate Organic Chemistry. Mechanism and Stereochemistry Lecture 3 - Conformational Analysis of Cyclohexane.

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Year 2 Organic Chemistry Mechanism and Stereochemistry Lecture 3 Conformational Analysis of Cyclohexane

  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

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