ANGEL-transparancies-part-3 - Chem 210 1001 reactions...

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Chem 210 175 1001 reactions Chemical transformations of functional groups are central to organic synthesis. Organic chemists make their own stuff!
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Chem 210 176 Electrophilic addition of H–X to π bonds protonation of a π bond starts the reaction carbocation intermediates Markovnikov's rule applies (regioselective) carbocation rearrangements possible reagents: HCl, HBr in ether or AcOH; KI in H 3 PO 4 R H X R X H alkyl cations as intermediates, no relative stereochemical preference (rsp) R H X R X H H X R X H X H vinyl cations as intermediates, first addition trans preferred, no rsp in the second addition, less reactive than alkenes H X H X X H + 1 2 1 4 resonance-stabilized cations as intermediates, no rsp, 1,4-adduct (more stable in this case; E isomer preferred) and 1,2-adduct are produced in comparable amounts, more reactive than alkenes (more info TBA)
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Chem 210 177 Electrophilic addition of X 2 to π bonds Br Br Br + Br bromonium ion (halonium ion) LUMO of the bromonium ion no-bond resonance structures showing delocalization of positive charge over the whole ring of the ion
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Chem 210 178 Stereochemical results: anti addition Br + Br anti As the result of the anti addition stereochemistry, trans products (two enantiomers) form from cyclic alkenes.
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Chem 210 179 Stereochemical consequences What are the products (including stereochemistry) of addition of Br 2 to trans and cis -2-butenes?
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Chem 210 180 Unsymmetrical bromonium ions With alkyl substituents the bromonium ion resembles more a “normal” carbocation and has only weak bonding to the more substituted carbon. The structure is still “bridged” (cyclic), and all stereochemical consequences ( anti addition) remain. LUMO with hyperconjugation “wings” In the no-bond resonance structures, C contributes more than A to the description of the bromonium ion.
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Chem 210 181 Swimming lessons: bromohydrin bromonium ion forms in a standard way as two enantiomers* (one on the top, and one on the bottom face of the double bond) the bromonium ion is attacked in the anti fashion on the more substituted carbon (where there is more positive charge) by an ubiquitous nucleophile (in aqueous solution) the oxonium ion (protonated alcohol) is deprotonated by water in a acid-base reaction the only bromohydrin regioisomer formed (Markovnikov’s rule) is a racemic mixture of just two enantiomers (because of anti addition) *Note: only one enantiomer shown throughout
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Chem 210 182 Bromohydrin formation: alphabet soup To improve solubility of alkenes in the reaction medium another set of reagents is commonly used: NBS = N -bromosuccinimide, a source of Br 2 in polar hydroxylic solvents (solvents with –OH group) DMSO = dimethylsulfoxide, CH 3 –SO–CH 3 , a good polar solvent for organic compounds; miscible with water Many more reagents are known by their acronyms or abbreviations. You need to memorize them (know what they are, and what they do).
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Chem 210 183 X 2 addition to other π bonds: alkynes Alkynes react similarly to alkenes, but are somewhat less reactive.
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This note was uploaded on 10/12/2009 for the course CHEM 210 taught by Professor Maslak,przemyslaw during the Fall '08 term at Pennsylvania State University, University Park.

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ANGEL-transparancies-part-3 - Chem 210 1001 reactions...

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