CH264 1 CH264/3 Organic Chemistry II Cyclohexane Rings Dr Andrew Marsh C515 Dr David J Fox B510
CH264 2 Today’s Lecture 1. Cyclohexane conformation 2. Drawing cyclohexane chairs 3.Monosubstituted cyclohexanes 4.Disubstituted cyclohexanes, decalins and steroids
CH264 3 Angle strain in rings
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
Conformational analysis CH264 5 Nobel Prize 1969
CH264 6 Cyclohexane is ‘strain free’ 109° angle allows near strain free cyclic molecule
CH264 7 Substituents on cyclohexane CGW p. 371
CH264 8 Ring Flip
CH264 9 Chair Conformer
CH Boat Conformer
CH Substituted Cyclohexanes conc equatorial conformer conc axial conformer K =>3000, >99.9% equatorial, >20 kJ mol -1 difference
Substituted cyclohexanes: energy difference SubstituentKAxial – equatorial energy difference kJ mol -1 % equatorial H1050 OMe Me Et iPr tBu>3000>20> CH CGW p. 375
CH Disubstituted cyclohexanes
CH The tert-butyl group is a conformational ‘lock’
Decalins CGW p. 378
Steroids: cholestanol CH Conformationally locked A–B–C–D rings CGW p. 379
CH 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