Chapter 8 Notes

Chapter 8 Notes - Chapter8 AlkenesandAlkynesII...

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Chapter 8 Alkenes and Alkynes II: Addition Reactions
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Introduction: Additions to Alkenes Generally the reaction is exothermic because one π and one σ bond are converted to two σ bonds. The π electrons of the double bond are loosely held and are a source of electron density, i.e . they are nucleophilic. Alkenes react with electrophiles such as H + from a hydrogen halide to form a carbocation.
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The carbocation produced is an electrophile. It can react with a nucleophile such as a halide. In addition reactions the alkene changes from a nucleophile in the first step to an electrophile in the second.
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Addition of Hydrogen Halides to Alkenes: Markovnikov’s Rule Addition of HBr to propene occurs to give 2‐bromopropane as the major product. 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.
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Mechanism for Hydrogen Halide Addition to an Alkene
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The reaction has a highly endergonic first step (rate determining) and a highly exergonic second step.
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Theoretical Explanation of Markovnikov’s Rule The product with the more stable carbocation intermediate predominates. The most stable carbocation is formed fastest because it has a lower Δ G . The transition state for the rate determining step (first step) resembles a carbocation and is stabilized by factors which stabilize carbocations.
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Addition of HBr to 2‐methylpropene gives only tert ‐butyl bromide: 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. Regioselective Reaction: When a reaction that can potentially yield two or more constitutional isomers actually produces only one or a predominance of one isomer.
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Stereochemistry of the Ionic Addition to an Alkene Addition of HBr to 1‐butene yields a chiral molecule. A racemic mixture is produced because the intermediate carbocation intermediate is achiral.
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Addition of Sulfuric Acid to Alkenes Addition of concentrated sulfuric acid to alkenes leads to alkyl hydrogen sulfates which are soluble in the acid. The addition follows Markovnikov’s rule. The sulfate can be hydrolyzed by heating with water. The net result is Markovnikov addition of water to an alkene.
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Addition of Water to Alkenes: Acid‐Catalyzed Hydration The reaction of alkenes with dilute aqueous acid leads to Markovnikov addition of water.
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Chapter 8 Notes - Chapter8 AlkenesandAlkynesII...

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