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10-04 - Cyclohexane is the most stable of the rings H C H H...

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Unformatted text preview: 10/5/2010 Cyclohexane is the most stable of the rings. H C H H C C H H H H H C C H H C C H H H H H H H H H C C H C C C H H H H H H H H C C H C H H H H C C C H H H Cyclohexane is the most stable of the rings. H C H H C C H H H H H C C H H C C H H H H H H H H H C C H C C C H H H H H H H H C C H C H H H H C C C H H H cyclopropane cyclobutane cyclopentane cyclohexane What is the indicated C-C-C bond angle in cyclohexane? What is the indicated C-C-C bond angle in cyclohexane? A. 90˚ B. 109.5˚ C. 120˚ D. 180˚ Cyclohexane is the most stable of the rings. H C H H C C H H H H H C C H H C C H H H H H H H H H C C H C C C H H H H H H H H C C H C H H H H C C C H H H Cyclohexane is the most stable of the rings. H C H H C C H H H H H C C H H C C H H H H H H H H H C C H C C C H H H H H H H H C C H C H H H H C C C H H H What is the indicated C-C-C bond angle in cyclohexane? What is the indicated C-C-C bond angle in cyclohexane? How can we draw a cyclohexane ring? What are the factors affecting conformational stability? H Steric Effects H C H H H H C H H steric effects! eclipsed A B staggered If steric effects are important what structure would be more stable? 1 10/5/2010 What are the factors affecting conformational stability? anti energy Which is the antibonding MO for the C-H bond? energy Electronic Effects (bonding) LUMO C H bonding LUMO C H Electronic Effects (bonding) LUMO C H anti bonding bonding LUMO C H HOMO bonding HOMO HOMO HOMO Hs Csp3 eclipsed A B staggered A B C What are the factors affecting conformational stability? Electronic Effects (bonding) Can these two isomers be separated from each other? A = yes B = no bonding MO H C H C HH eclipsed bonding MO H C HH C H MO anti bonding HH A HH B staggered MO anti bonding If electronic effects are important what structure would be more stable? Can these two isomers be separated from each other? A = yes B = no Can these two isomers be separated from each other? A = yes B = no energy 12.6 kJ (3 kcal)/mol rotation 2 10/5/2010 Can these two different structures be separated from each other under standard conditions? an energy maximum (eclipsed) torsional strain What are Newman Projections? H H H H H H H H H H H H H H H an energy minimum (staggered) an energy minimum (staggered) energy 12.6 kJ (3 kcal)/mol energy 12.6 kJ (3 kcal)/mol rotation rotation What are sawhorse Projections? What are sawhorse Projections? HH H H C C H HH H C HH C H H H H torsional strain HH C HH HH C C H C H energy 12.6 kJ (3 kcal)/mol energy HH 12.6 kJ (3 kcal)/mol rotation rotation Draw the sawhorse projection of the following Newman projection. Cl Cl H H H H Cl H C Cl H C H H Cl Cl Cl H C C H H Cl Cl C HH H Cl Cl C H Cl Cl C HH Cl Cl H C H Cl Cl H C C H Which conformation has the lowest energy (most stable)? Which conformation has the highest energy (least stable)? Cl Cl Cl Cl Cl Cl H H H Cl Cl H C Cl Cl H C H Cl H H H Cl Cl Cl H C C H H Cl Cl C HH H H H H Cl H Cl Cl C H Cl Cl C HH Cl Cl H H Cl Cl H H Cl Cl H C H H H a partial structure H H A B C D A B C D What sawhorse projection is identical to the above Newman projection? 3 10/5/2010 Do C and D have the same energy? A = yes B = no Are C and D identical? A = yes B = no Cl Cl H H H Cl Cl H Cl H H Cl Cl H H H H Cl Cl Cl H H Cl Cl H H H H Butane has two staggered conformations. Which is more stable? H HH C H H C H C C HH H H H H HH C C H C D H Cl Cl H C Cl Cl H C H H Cl Cl Cl H C C H H Cl Cl C C A H Cl Cl C H Cl Cl B Cl Cl H C C H A CH H HHH B staggered anti staggered gauche HH HH A B C D Butane has two staggered conformations. Which is more stable? H HH C H H C H C C HH H H H H HH C C H C In contrast to ethane butane has two staggered conformations. Which is more stable? H HH C H H C H C C HH H H H H HH C C H C CH H H HH CH H H HH staggered anti staggered gauche staggered anti staggered gauche In contrast to ethane butane has two staggered conformations. Which is more stable? H HH C H H C H C C HH H H H H HH C C H C In contrast to ethane butane has two staggered conformations. Which is more stable? H HH C H H C H C C HH H H H H HH C C H C CH H HHH CH H HHH staggered anti staggered anti staggered gauche 4 10/5/2010 In contrast to ethane butane has two staggered conformations. Which is more stable? CH CH3 CH CH 2 CH CH 2 CH CH3 How many stable staggered conformations exist for butane? A. 1 CH CH3 CH CH 2 B. 2 CH CH 2 CH CH3 C. 3 D. 4 E. 5 F. 7 energy 21 kJ /mol 14.6 kJ /mol 3.8 kJ /mol energy 21 kJ /mol 14.6 kJ /mol 3.8 kJ /mol rotation rotation How many stable staggered conformations exist for butane? What is major factor responsible for the energy difference between the staggered and eclipsed conformers? A. Steric Effects (non bonding) B. Torsional Effects (electronic) Although the two gauche conformations have the same energy they have different structures. energy 21 kJ /mol 14.6 kJ /mol 3.8 kJ /mol energy 14.6 kJ /mol 3.8 kJ /mol rotation rotation What is major factor responsible for the energy difference between the staggered and eclipsed conformers? torsional strain 12.6 kJ /mol steric strain 14.6 kJ /mol energy 21 kJ /mol 14.6 kJ /mol 3.8 kJ /mol rotation 5 ...
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  • Fall '08
  • Fowler/Sampson
  • Conformational isomerism, Cl Cl, H H H Cl H C Cl H C H H Cl Cl Cl H C C H H Cl Cl C HH H Cl Cl C H Cl Cl C HH Cl Cl H C H Cl Cl H C C H, H H Cl Cl H C Cl Cl H C H Cl H H H Cl Cl Cl H C C H H Cl Cl C HH H H H H Cl H Cl Cl C H Cl Cl C

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