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Chapter_8_Add - Chapter 8 Alkenes and Alkynes II Addition...

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Chapter 8 Alkenes and Alkynes II: Addition Reactions
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Chapter 8 2 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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Chapter 8 3 The carbocation produced is an electrophile It can react with a nucleophile such as a halide » Insert top scheme pg 331 In addition reactions the alkene changes from a nucleophile in the first step to an electrophile in the second
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Chapter 8 4 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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Chapter 8 5 Mechanism for hydrogen halide addition to an alkene The reaction has a highly endergonic first step (rate determining) and a highly exergonic second step
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Chapter 8 6 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 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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Chapter 8 7 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
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