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. CH264 2 Today’s Lecture 1. Cyclohexane conformation 2. Drawing cyclohexane chairs 3.Monosubstituted cyclohexanes 4.Disubstituted cyclohexanes, decalins and steroids

3. CH264 3 Angle strain in rings

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. CGW p.371

5. Conformational analysis CH264 5 Nobel Prize 1969

6. CH264 6 Cyclohexane is ‘strain free’ 109° angle allows near strain free cyclic molecule

7. CH264 7 Substituents on cyclohexane CGW p. 371

8. CH264 8 Ring Flip

9. CH264 9 Chair Conformer

10. CH264 10 Boat Conformer

11. CH264 11 Substituted Cyclohexanes conc equatorial conformer conc axial conformer K =>3000, >99.9% equatorial, >20 kJ mol -1 difference

12. Substituted cyclohexanes: energy difference SubstituentKAxial – equatorial energy difference kJ mol -1 % equatorial H1050 OMe2.72.573 Me197.395 Et207.595 iPr429.398 tBu>3000>20>99.9 110 11.799 CH264 12 CGW p. 375

13. CH264 13 Disubstituted cyclohexanes

14. CH264 14 The tert-butyl group is a conformational ‘lock’

15. Decalins CGW p. 378

16. Steroids: cholestanol CH264 16 Conformationally locked A–B–C–D rings CGW p. 379

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