8 Alkene Reaction and Synthesis

8 Alkene Reaction and Synthesis - Based on McMurry’s...

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Unformatted text preview: Based on McMurry’s Organic Chemistry , 8 th edition Diverse Reactions of Alkenes Alkenes react with many electrophiles to give useful products by addition (often through special reagents) 2 Why this chapter? To begin a systematic description of major functional groups Begin to focus on general principles and patterns of reactivity that tie organic chemistry 3 Preparation of Alkenes: A Preview of Elimination Reactions Alkenes are commonly made by elimination of HX from alkyl halide ( dehydrohalogenation ) Uses heat and KOH elimination of H-OH from an alcohol ( dehydration ) require strong acids ( sulfuric acid, 50 ºC ) 4 5 6 Addition of Halogens to Alkenes Bromine and chlorine add to alkenes to give 1,2- dihaldes, an industrially important process F 2 is too reactive and I 2 does not add Cl 2 reacts as Cl + Cl- Br 2 is similar 7 Get a room from a large house is much more easy than get a room from a small house. 8 Addition of Br 2 to Cyclopentene Addition is exclusively trans 9 Mechanism of Bromine Addition Br + adds to an alkene producing a cyclic ion Bromonium ion , bromine shares charge with carbon Gives trans addition 10 Bromonium Ion Mechanism Electrophilic addition of bromine to give a cation is followed by cyclization to give a bromonium ion This bromoniun ion is a reactive electrophone and bromide ion is a good nucleophile 11 The Reality of Bromonium Ions Bromonium ions were postulated more than 60 years ago to explain the stereochemical course of the addition (to give the trans-dibromide from a cyclic alkene Olah showed that bromonium ions are stable in liquid SO 2 with SbF 5 (lewis acid ) and can be studied directly 12 13 Which ion was added first and why? 14...
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8 Alkene Reaction and Synthesis - Based on McMurry’s...

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