Ch_322a_8.08

Ch_322a_8.08 - cis-2-butene is due to the specificity of...

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Stereospecific Reactions A reaction where one particular stereoisomeric form of the starting material reacts to give a particular stereoisomeric form of the product is a stereospecific reaction. In such reactions, the reaction mechanism introduces stereospecificity. The brominations of cyclopentene and cyclohexene are examples of stereospecific reactions. The brominations of cis - and trans -2-butene are stereospecific reactions. C=C CH 3 CH 3 H H cis-2-butene Br 2 CCl 4 C C Br Br CH 3 CH 3 H H + C C Br Br CH 3 CH 3 H H 2S,3S 2R,3R racemic form of chiral diastereomer C=C CH 3 CH 3 H H C C Br Br CH 3 CH 3 H H trans-2-butene Br 2 CCl 4 C C Br Br CH 3 CH 3 H H 2R,3S meso diastereomer (achiral)
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Stereospecificity in the Bromination of cis- 2-Butene The formation of only a single diastereomer in the bromination of
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Unformatted text preview: cis-2-butene is due to the specificity of formation and reaction of the bromonium ion: C=C CH 3 CH 3 H H Br Br ! + !" syn-addition C C Br + CH 3 CH 3 H H Br-(A) (B) Attack by bromide ion occurs by the equivalent paths A and B. (A) C C Br Br CH 3 CH 3 H H C C Br Br CH 3 CH 3 H H 2S,3S 2R,3R (B) A racemic form of the chiral diastereomer is produced . The mechanism does not allow formation of the meso-diastereomer from the cis-2-butene. This reaction is stereospecific . By a parallel mechanism, only the meso-diastereomer is formed in the bromination of trans-2-butene....
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Ch_322a_8.08 - cis-2-butene is due to the specificity of...

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