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Portmess Homework Q&S 120

Portmess Homework Q&S 120 - cis-4-nonene and anti...

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5.39 The two starting materials are stereoisomers of each other, and so it is reasonable to begin by ex- amining each one in more stereochemical detail. First, write the most stable conformation of each isomer, keeping in mind that isopropyl is the bulkiest of the three substituents and has the greatest preference for an equatorial orientation. The anti periplanar relationship of halide and proton can be achieved only when the chlorine is axial; this corresponds to the most stable conformation of neomenthyl chloride. Menthyl chloride, on the other hand, must undergo appreciable distortion of its ring to achieve an anti periplanar Cl @ C @ C @ H geometry. Strain increases substantially in going to the transition state for E2 elim- ination in menthyl chloride but not in neomenthyl chloride. Neomenthyl chloride undergoes E2 elimination at the faster rate. 5.40 The proton that is removed by the base must be anti to bromine. Thus, the alkyl groups must be gauche to one another in the conformation that leads to
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Unformatted text preview: cis-4-nonene and anti to one another in the one that leads to trans-4-nonene. Br H H H CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH 2 Anti conformation of 5-bromononane E2 transition state trans-4-Nonene CH 3 CH 2 CH 2 CH 2 Br H H H O CH 3 CH 2 CH 2 CH 2 CH 3 d 2 d 2 H CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH 2 H Br H H H CH 2 CH 2 CH 3 CH 2 CH 2 CH 2 CH 3 Gauche conformation of 5-bromononane Br H H H O CH 3 CH 2 CH 2 CH 2 CH 3 CH 2 CH 2 CH 2 CH 3 E2 transition state cis-4-Nonene d 2 d 2 H CH 2 CH 2 CH 3 CH 2 CH 2 CH 2 CH 3 H CH(CH 3 ) 2 CH(CH 3 ) 2 Cl H 3 C H H H Cl Menthyl chloride Most stable conformation of menthyl chloride: none of the three b protons is anti to chlorine H 3 C CH(CH 3 ) 2 CH(CH 3 ) 2 Cl H 3 C H H Cl Neomenthyl chloride Most stable conformation of neomenthyl chloride: each b carbon has a proton that is anti to chlorine H 3 C 114 STRUCTURE AND PREPARATION OF ALKENES: ELIMINATION REACTIONS...
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