Chem 634 Pre Problem Set KEY February 10 2004 Prof Fox 1 Assign all stereochemistry R or S E or Z a b CH3 H OH d c Br Cl CHO O CH2 Br 1R 6R 6 bromo 2 cyclohexenol 2E 5S 5 methyl 2 heptene O Cl 2R 3 bromo 2 methylpropanal 1R 3 chloro 1 methylpropyl 3R 3 chlorobutanoate 2 For each pair which is more stable c b a t Bu t Bu t Bu t Bu t Bu t Bu Br OH OH Br H Me t B u Me H H t B u Me Me H H H more stable axial methyl H t B u Me H axial methyl costs 1 7 kcal more stable H Me t B u H more stable H t B u HO Br H t B u Br H OH axial Br costs 0 55 kcal axial OH costs more 0 94 kcal 3 Indicate the relationship for each pair below enantiomers diastereomers or meso a CH3 CH3 H OH HO H H OH HO H CH3 CH3 meso b OH S HO S S HO diastereomers H R OH 4 For each pair indicate which is more stable Use a clear picture to explain why Br Br Br Br Both cyclohexanes are locked with equatorial tBu s and axial methyl groups so the answer lies in the double bond t Bu Brax t Bu H H Br eq Breq Breq H H CH3 H sp 2 center There is no axial group here and therefore there is only one 1 3 diaxial interaction make a model to convince yourself H H H H H Brax H Br eq CH3 tBu t Bu most stable form has no axial Br s Brax H Brax two 1 3 diaxial interactions 5 Provide a detailed arrow pushing mechanism for the following reaction Br Bu3 SnH H AIBN O O H H Initiation NC N N CN AIBN N N CN H SnBu3 H atom extraction Bu3Sn SnBu3 Br Br atom extraction O multiple bond addition O H O O H H H SnBu3 Bu3Sn H H atom extraction O H H H multiple bond addition 6 Draw Newman projections for the staggered and eclipsed forms of n butane Give the relative energies of all of the eclipsed and staggered conformers 7 Gauche butane is 0 8 kcal mol higher in energy than anti butane The A strain of a methyl group on cyclohexane is 1 7 kcal mol Use Newman projections to explain the relationship in detail CH3 H H 0 8 kcal mol a gauche butane interaction costs 0 8 kcal mol CH3 H H axial methylcyclohexane has two gauche butane type interactions the first can be seen from this projection gauche CH3 HH C H2 C C H2 H your eye CH3 H H H the second can be seen from this projection C gauche CH3 H H CH3 H2 C H H H H C H2 since each gauche interaction costs 0 8 kcal mol 2 x 0 8 1 6 kcal mol The actual value of 1 7 kcal mol is a close fit Use Models if you are having trouble seeing this 8 9 10 Which reaction is faster A or B Why A Br H B HO HBr HBr Br OH Br Br Br H H O SLOW Remember carbocation formation is rate determining The ideal geometry for a carbocation is planar with 120 angles Thus the carbocation formation is slow because it is severly distorted by the 60 angle of the cyclopropane ring Severe distortions are not required in order to put a cyclopentane in a planar arrangement awith 120 angles This is the faster reaction 11 Which reaction is faster A or B Why The arrow pushing is the same as for question 2 The rate differences again relate to carbocation stability Et B A HBr HO Et HO Br Et Br Et Such resonance structures do not apply for the cation below Et Et Et the orbital of the carbocation cannot overlap with the system of the benzene ring No resonance stabilization side view Et Et Et The carbocation intermediate is resonance stabilized with the benzene ring see below This is the faster reaction Et HBr Et 12 Provide a detailed explanation Br Me X but Me Br Me Me H H eq Br Me eq H E2 elimination can Me occur via an antiperiplanar conformation i e both the hydrogen and the bromine are axial H NaOtBu Br NaOtBu H Br ax H ax the molecule cannot adopt an conformation where Br and H are anti periplanar At least one group must be equatorial O 13 Provide a detailed arrow pushing mechanism OTs H catalytic CH3 H H 2O H CH3 H H H H OTs H alkyl shift CH3 H CH3 CH3 H H H H O H H H CH3 H CH3 H
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