11-01 - Alkenes Addition Reactions Alkene Additions O O H2C...

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11/1/2010 1 Alkenes: Addition Reactions O O H 2 C C C H H H H 1 HO H H H CH O H 2 C O O Alkene Additions 2 A B B Electrophilic addition to an alkene 3 The The alkene alkene acts as a base. acts as a base. The The HBr HBr is is an an acid. acid. Which substrate acts as a base in the first step of this reaction? Which substrate acts as a base in the second step of this reaction? A The The bromide acts bromide acts as a base. as a base. The The carbocation carbocation is is an an acid. acid. LUMO HBr An acid is an electron σ * acid base 2e 2e- MO description 4 acceptor. So the most important orbital is the LUMO The The alkene alkene acts as a base. acts as a base. So the most important orbital is the HOMO HOMO H H H H 3 C H Br + H What if we use propene instead instead? Unstable π –protonated alkene collapses to a more stable configuration Which is the preferred 5 Br H 3 C H H H carbocation carbocation? A B less stable more stable 6 < < < least stable most stable but still reactive
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11/1/2010 2 H H H H 3 C H Br + H H 3 C H H H H Br 7 Br H 3 C H H H This isomer does not form most stable Markovnikov’s Rule Two ways for HX to add to an asymmetric alkene 8 If one orientation predominates, the reaction is described as regiospecific . Markovnikov observed in the 19 th century that in the addition of HX to alkene, the H forms a bond with the C with the most H’s and X bonds to the other C (to the one with the most alkyl substituents) This is known as Markovnikov’s rule Vladimir Markovnikov (1838-1904) Kazan University
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