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

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5/11/2010 1 Chapter 8 Alkenes and Alkynes II: Addition Reactions Chapter 8 2 Addition 2 molecules in, one out. Opposite of elimination: 1 in, 2 out e.g. A-B = H-Br alkene to alkyl halide Reverse is elimination: alkyl halide to alkene Which way will it go? HBr (acid) favours addition KOH (base) favours elimination Chapter 8 3 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 Chapter 8 4 The carbocation produced is an electrophile (like an E1 or SN1 intermediate) It can react with a nucleophile such as a halide » Insert top scheme pg 331 In addition reactions the organic functional group (alkene here) changes from a nucleophile in the first step to an electrophile in the second
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5/11/2010 2 Chapter 8 5 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 Chapter 8 6 Mechanism for hydrogen halide addition to an alkene The reaction has a highly endergonic first step (rate determining) and a highly exergonic second step Chapter 8 7 Theoretical Explanation of Markovnikov s Rule The product with the more stable carbocation intermediate predominates (3 o > 2 o >1 o > Me)
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