Chapter07a - Diverse Reactions of Alkenes 7 Alkenes...

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1 7. Alkenes: Reactions and Synthesis Based on McMurry’s Organic Chemistry , 6 th edition 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 ) 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) 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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2 5 Addition of Br 2 to Cyclopentene ± Addition is exclusively trans H H Br Br H Br H Br - H Br Br H trans-1,2-dibromocyclopentane cis product is not formed cyclopentene 6 Mechanism of Bromine Addition ± Br + adds to an alkene producing a cyclic ion ± Bromonium ion, bromine shares charge with carbon ± Gives trans addition 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 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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Chapter07a - Diverse Reactions of Alkenes 7 Alkenes...

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