Ch_322a_8.09

Ch_322a_8.09 - in the bromonium ion C=CH 2 CH 3 CH 3 Br 2 H 2 O CH 3 CCH 2 Br CH 3 OH(73 1-bromo-2-methyl-2-propanol Charge Distribution in the

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Halohydrin Formation When the halogenation reaction is carried out in the presence of other nucleophiles that compete with halide ion, other addition products are formed. In water, H 2 O is a competing nucleophile : C=C X 2 H 2 O C C X OH a halohydrin + C C X X vicinal dihalide Example: Reaction of Cyclopentene Br 2 H 2 O HO H H H H Br Br OH + trans -2-bromocyclopentanol (as a racemic form)
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A Mechanism for Bromohydrin Formation Bromohydrin formation is a stereospecific reaction . This reaction specificity is explained by the formation and reaction of a bromonium ion intermediate. Br Br ! + !" Br + H H bromonium ion O H H : : (A) (B) Water attacks the bromonium ion in an S N 2 reaction by paths A and B. (A) (B) H 2 O + Br Br H H H H OH 2 + (-H + ) (-H + ) HO Br H H Br H H OH racemic form
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Regioselectivity in Halohydrin Formation In unsymmetrical alkenes , the major addition product has the halogen attached to the carbon with the greater number of hydrogen atoms. This orientation results from attack by water (the nucleophile) at the more electropositive carbon center
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Unformatted text preview: in the bromonium ion. C=CH 2 CH 3 CH 3 Br 2 H 2 O CH 3 CCH 2 Br CH 3 OH (73%) 1-bromo-2-methyl-2-propanol Charge Distribution in the Unsymmetrical Bromonium Ion An examination of the contributing resonance structures to the hybrid of the bromonium ion offers an explanation for the regioselectivity of the reaction. C C Br CH 3 CH 3 H H + : : : C C Br CH 3 CH 3 H H + : : tertiary carbocation more stable greater contribution primary carbocation less stable lesser contribution C C Br CH 3 CH 3 H H + : : resonance structures C C Br CH 3 CH 3 H H : : ! + ! + ! + O H H : : The nucleophile reacts faster with the more electropositive carbon . Quiz 8.09 Predict the products of the following reactions and provide stereochemical details. CH 3 Br 2 CCl 4 CH 3 Br 2 H 2 O CH 3 Br Br H single diastereomer as a racemic form CH 3 HO Br H single diastereomer as a racemic form...
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This note was uploaded on 07/22/2009 for the course CHEM 322AL taught by Professor Jung during the Summer '07 term at USC.

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Ch_322a_8.09 - in the bromonium ion C=CH 2 CH 3 CH 3 Br 2 H 2 O CH 3 CCH 2 Br CH 3 OH(73 1-bromo-2-methyl-2-propanol Charge Distribution in the

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