10 - Chem 10172, Spring 2008 (1) PROBLEM SET 10 SOLUTIONS...

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Chem 10172, Spring 2008 PROBLEM SET 10 SOLUTIONS Due Tuesday, April 8 1 (1) Br Br I: II: (a) Under solvolytic conditions, the rate-determining step in the reation is formation of the carbocation. For I , this results in a serious increase in ring strain as the hybridization changes from sp 3 to sp 2 in the four-membered ring. (Compound II is not highly strained in either the reactant or carbocation.) This would disfavor formation of the carbocation in I and so it would react more slowly. (b) Reaction of these 3º halides in CH 3 OH at room temperature would favor S N 1 reactivity: Br Br CH 3 OH OCH 3 OCH 3 + CH 3 OH OCH 3 OCH 3 + In both cases the nucleophile would react on either face of the planar carbocation, but to an unequal extent (the two faces are not equivalent because of the adjacent carbon with its methyl group pointing up [and not down]). (c) Use of a strong hindered base is clearly going to favor E2 reactions. This requires an anti-periplanar arrangement of the reacting C-H and the C-Br bonds. In the cyclobutane ring, the ring must be nearly planar, which makes the C-Br bond eclipse the neighboring C-H bonds in the ring. Thus none of the hydrogens in the ring will be able to be anti- periplanar with the C-Br bond (one, the upper H on the bottom right corner, would be syn-periplanar, but that is an inferior orientation). The most reactive hydrogen will thus be on the methyl group bonded to the carbon with the Br (of course it can rotate freely to put a C-H bond in the correct orientation, in fact this will be its lowest-energy conformation). The five-membered ring is more flexible, so either of the β hydrogens trans to the Br can be placed into a (nearly) anti-periplanar orientation (build a model and flex it to see). Br Br NaO t Bu + t BuOH NaO t Bu t BuOH + (2) (a)
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Chem 10172, Spring 2008 PROBLEM SET 10 SOLUTIONS Due Tuesday, April 8 2 t Bu OTs NaI Et 2 O Bu NaI Et 2 O OTs Both reactions will proceed via S N 2 (good nucleophile, very weak base, nonpolar
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This note was uploaded on 05/18/2008 for the course CHEM 10172 taught by Professor Brown during the Spring '08 term at Notre Dame.

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10 - Chem 10172, Spring 2008 (1) PROBLEM SET 10 SOLUTIONS...

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