CH264 1 CH264/3 Organic Chemistry II Cyclohexane Rings Dr Andrew Marsh C515 Dr David J Fox B510

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Presentation transcript:

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 disubstituted cyclohexanes Outputs