Chapter 8 - Start of Chapter 8 lecture notes Reactions on Alkenes Many of these reactions occur through an electrophilic addition to an alkene

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Start of Chapter 8 lecture notes
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Reactions on Alkenes Many of these reactions occur through an electrophilic addition to an alkene mechanism
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Reactions on Alkenes Many of these reactions occur through an electrophilic addition to an alkene mechanism E + E + C H E Nuc: Nuc: E Nuc If the electrophile is a small atom (like a H) then a C + intermediate is formed If the electrophile is a larger atom (like a halogen) then a 3-membered ring intermediate is formed. Example: react 2-pentene with HCl and the products are: R or S-2-chloropentane or 3-chloropentane Here the C + intermediate could be on either C that was part of the C=C initially X Nuc: X X X X X 2 X: Example: react 2-pentene with Cl 2 and the products are: All possible stereoisomer mixes of 2,3-dichloropentane
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Alkene Alkyl halide H 2 C CH 2 + HX H 2 C CH 2 H X Get Markovnikov product For I use KI and phosphoric Acid C + int. H 2 C CH 2 + HBr H 2 C CH 2 H Br peroxides Get anti-Markovnikov product Free radical mechanism H 2 C CH 2 + Br 2 or Cl 2 CCl 4 H 2 C CH 2 Br Br Get anti-stereochemistry Bromonium or Cholorinum ion int. H 2 C CH 2 + X 2 H 2 O H 2 C CH 2 Br OH Get Markovnikov product Halonium ion int NBS – is a reagent that is a source of Br 2
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What is a Markovnikov product and why is this often made? When you are making a carbocation intermediate the MOST stable (lowest in energy) intermediate is the one that is made HCl Cl This means that the nucleophile that adds onto one of the C atoms in the alkene goes onto the more highly substituted C atom of the 2 that are Pi bonded together. H Cl H Here I could make a primary C + int. or a secondary C + int. – the lowest energy int. is made by putting the charge on the secondary C Cl This is a Markovnikov product C* C* can be R- or S- Note: rearrangement products are possible here since we are going through a C + int
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Cl Cl + CCl 4 Cl Cl Cl Cl The 2 Cl’s add anti- to one another Halogen addition to an alkene Formation of a chloronium ion intermediate Cl Cl
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Cl Cl + H 2 O Cl O H H Cl O H H O H H Cl O H Halohydrin reaction Gives a Markovnikov product The nucelophile attacks where a C+ would be most stable (if we made a C+ int.) Cl Cl Other products possible here are1,2-dichloro-1-methyl cyclobutane – with the Cl’s trans to one another & enantiomer
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HO OH 2 OH heat Br C Br Br H C Br H + H-Br peroxides Peroxides: H-O-O-H Addition of HBr in a way that leads to a different product Put the free radical on The 3° C atom Get an anti-Markovnikov product OH Br H H 2 O & Br
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This note was uploaded on 02/13/2011 for the course CHEM 2344 taught by Professor Slack,wu during the Spring '11 term at University of Houston.

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Chapter 8 - Start of Chapter 8 lecture notes Reactions on Alkenes Many of these reactions occur through an electrophilic addition to an alkene

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