Lecture 6 Bromination of Stilbene022510

Lecture 6 Bromination of Stilbene022510 - Lecture #6 A...

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Lecture #6 – A Greener Bromination of Stilbene Br 2 + Br Br
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Announcements An interesting thing happened this  week… The three (+) strikes policy Product from next week will be used the  week after Spring Break
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What is Stilbene used for? Optical brightener Fluorescent color Intermediate to make dyes and pigments Conjugated systems Color = electrons Synthetic labs Intermediates in chiral synthesis H H
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Stilbene Bromination Reaction ( E )-1,2-Diphenylethene trans -Stilbene Electrophilic addition reaction Halogenation of Alkene Test for alkenes and alkynes Adds bromine across a double bond Produces two chiral centers H Br Br H Br 2 + * *
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New Chiral Centers Three potential isomers (1 R , 2 S ) meso Mirror plane in the molecule   (1 R , 2 R ) and (1 S , 2 S ) Can be complicated to separate them * * R S Meso (2R, 3S)-Dibromostilbene H Br Br H Br Br H H * * * R R S S (2R, 3R)-Dibromostilbene (2S, 3S)-Dibromostilbene H H Br Br
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Typical Synthetic Methods Use of pyridinium hydrobromide perbromide  Br 3 -  as Br 2  source Acetic acid Solid, safer to handle Use of Br in CCl 4 Dangerous!  Volatile liquid Corrosive CCl 4  is carcinogenic N + H Br- . Br 2
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Greener Synthesis Ethanol as solvent No acetic acid No Br to transfer In situ  generation of bromine Hydrogen peroxide as oxidizing agent to make  Br 2 Aqueous HBr  Neutralize acidic ethanol with NaHCO 3
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Mechanism of Stilbene Bromination 1. Bromonium ion formation – Br + 2. Nucleophilic attack of Br - Viscinal dibromide  trans  to each other Br Br
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Enantiomers vs. Diastereomers Enantiomers Non-superimposible mirror images Identical physical properties Solubility in typical solvents Identical bp, mp Give same standard proton NMR Behave identically on most GC columns Cannot be separated by conventional means Cl OCH 3 Me H Cl H H 3 CO Me
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Enantiomers vs. Diastereomers
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Lecture 6 Bromination of Stilbene022510 - Lecture #6 A...

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