Chapter07- Alkenes- Reactions and Synthesis

Chapter07- Alkenes- Reactions and Synthesis - 7 Alkenes...

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7. Alkenes: Reactions and Synthesis Based on McMurry’s Organic Chemistry , 6 th edition ©2003 Ronald Kluger Department of Chemistry University of Toronto
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McMurry Organic Chemistry 6th edition Chapter 7 2 Diverse Reactions of Alkenes Alkenes react with many electrophiles to give useful products by addition (often through special reagents) alcohols (add H-OH) alkanes (add H-H) halohydrins (add HO-X) dihalides (add X-X) halides (add H-X) diols (add HO-OH) cyclopropane (add :CH 2 )
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McMurry Organic Chemistry 6th edition Chapter 7 3 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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McMurry Organic Chemistry 6th edition Chapter 7 4 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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McMurry Organic Chemistry 6th edition Chapter 7 5 Addition of Br 2 to Cyclopentene Addition is exclusively trans +
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McMurry Organic Chemistry 6th edition Chapter 7 6 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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McMurry Organic Chemistry 6th edition Chapter 7 7 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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McMurry Organic Chemistry 6th edition Chapter 7 8 The Reality of Bromonium Ions Bromonium were postulated more than 60 years ago to expain 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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McMurry Organic Chemistry 6th edition Chapter 7 9 7.3 Halohydrin Formation This is formally the addition of HO-X to an alkene (with + OH as the electrophile) 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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McMurry Organic Chemistry 6th edition Chapter 7 10 Mechanism of Formation of a Bromohydrin Br 2 forms bromonium ion, then water adds Orientation toward stable C + species Aromatic rings do not react
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McMurry Organic Chemistry 6th edition Chapter 7 11 An Alternative to Bromine Bromine is a difficult reagent to use for this reaction N-Bromosuccinimide (NBS) produces bromine in organic solvents and is a safer source
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