lecture 07 - 7. Alkenes: Reactions and Synthesis Based on...

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7. Alkenes: Reactions and Synthesis Based on McMurry’s Organic Chemistry , 7 th edition
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2 Diverse Reactions of Alkenes Alkenes react with many electrophiles to give useful products by addition (often through special reagents)
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3 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
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4 7.1 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)
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5 7.2 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
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6 Addition of Br 2 to Cyclopentene Addition is exclusively trans
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7 Mechanism of Bromine Addition Br + adds to an alkene producing a cyclic ion Bromonium ion, bromine shares charge with carbon Gives trans addition
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8 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 electrophile and bromide ion is a good nucleophile
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9 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 and can be studied directly
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10 7.3 Addition of Hypohalous Acids to Alkenes: Halohydrin Formation This is formally the addition of HO-X to an alkene to give a 1,2-halo alcohol, called a halohydrin The actual reagent is the dihalogen (Br 2 or Cl 2 in water in an organic solvent)
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lecture 07 - 7. Alkenes: Reactions and Synthesis Based on...

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