SolomonsLKChapter8

SolomonsLKChapter8 - Chapter 8 Alkenes and Alkynes II:...

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Chapter 8 Alkenes and Alkynes II: Addition Reactions
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Generally the reaction is exothermic because one π and one σ bond are converted to two σ bonds Additions to Alkenes
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The π electrons of the double bond are loosely held and are a source of electron density Alkenes are nucleophilic and react with electrophiles such as H + from a hydrogen halide to form a carbocation Alkenes are electron rich X-H < + X - +
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t The carbocation produced is an electrophile l It can react with a nucleophile such as a halide In addition reactions the alkene is a nucleophile in the frst step and an electrophile in the second C C H X
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Addition of HBr to propene occurs to give 2-bromopropane l Markovnikov ` s Rule (Original): addition of HX to an alkene proceeds so that the hydrogen atom adds to the carbon that already has the most hydrogen atoms Addition of Hydrogen Halides to Alkenes: Markovnikov ` s Rule
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t The reaction has a highly endothermic frst step (rate determining) and a highly exothermic second step Mechanism for hydrogen halide addition to an alkene 1. 2.
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Product with the more stable carbocation intermediate predominates Transition state for the rate determining step (Frst step) resembles a carbocation and is stabilized by factors which stabilize carbocations Theoretical Basis of Markovnikov ` s Rule
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t Modern Statement of Markovnikov ` s Rule : In the ionic addition of an unsymmetrical reagent to a double bond, the positive portion of the adding reagent attaches itself to a carbon atom of the double bond so as to yield the more stable carbocation as an intermediate t Regioselective Reaction : When a reaction that can potentially yield two or more constitutional isomers actually produces only one or a predominance of one isomer Addition of HBr to 2-methylpropene gives only tert -butyl bromide
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If the addition of HBr to butene yields a chiral molecule, a racemic mixture is produced because the intermediate carbocation is achiral. Stereochemistry of the Ionic Addition to an Alkene
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What is the major product? HCl ? Cl Cl Cl Cl a. b. c. d.
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Addition of concentrated sulfuric acid to an alkene leads to an alkyl hydrogen sulfate. The addition follows Markovnikov ` s rule t The sulfate can be hydrolyzed by heating with water l The net result is Markovnikov addition of water to the alkene Addition of Sulfuric Acid to Alkenes
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The reaction of alkenes with dilute aqueous acid leads to Markovnikov addition of water The mechanism is the reverse of that for dehydration of an alcohol The Frst step in which a carbocation is formed is rate determining Addition of Water to Alkenes: Acid-Catalyzed Hydration
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This note was uploaded on 12/22/2011 for the course CHEMISTRY 2210 taught by Professor Keller during the Spring '11 term at FIU.

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SolomonsLKChapter8 - Chapter 8 Alkenes and Alkynes II:...

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