Pb87an-09

Pb87an-09 - The 1,2-dibromo compound is more interesting....

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Answers to Problem 87, Chemistry 0243 - 2009 Protonation of 1 can take place in two ways to give carbocations that can lead to the 1,2- and 1,3-dibromo compounds. Br HBr Br Br Br Br 1 2 3 Br Br Br Br cis -1,3-Dibromocyclopentane is a meso compound, so it must be optically inactive; there is no mechanistic information here. trans -1,3 Dibromocyclopentane is formed by capture of the open cation, it must be optically active, as it is. So, this observation is consistent with the mechanism outlined above in which an open cation is the critical intermediate.
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Unformatted text preview: The 1,2-dibromo compound is more interesting. An open cation can t be involved, as shown above, because it must lead to both - and -1,2-dibromocyclopentane, and the trans compound must be optically active if this mechanism is operative. Presumably what happens is that this open cation is captured immediately on formation to give a meso bromonium ion. Now there can be no cis dibromide formed, and the trans 1,2-dibromide must be racemic. Br Br H H Br a b b a Br H H Br Br H H Br a racemic mixture meso S N 2...
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Pb87an-09 - The 1,2-dibromo compound is more interesting....

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