11-01 - 11/1/2010 11/1/2010 1 Alkenes: Addition Reactions O...

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Unformatted text preview: 11/1/2010 11/1/2010 1 Alkenes: Addition Reactions O O H2C O C C H H H H 1 HO H H H CH O H2C O O H H 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 first step of this reaction? Which substrate acts as a base in the second 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 LUMO HBr HBr An acid is an electron An acid is an electron * acid acid base base 2e 2e- MO description 4 An acid is an electron An acid is an electron acceptor. So the most acceptor. So the most important orbital important orbital is the LUMO is the LUMO The The alkene alkene acts as a base. acts as a base. So the most important orbital is the HOMO So the most important orbital is the HOMO HOMO HOMO base base H H H H 3 C H Br + H What if we use What if we use propene propene instead instead? Unstable protonated alkene collapses to a more stable configuration Which is the preferred Which is the preferred 5 Br H 3 C H H H Which is the preferred Which is the preferred carbocation carbocation? A B less stable less stable more stable more stable 6 < < < least stable least stable most stable most stable but still reactive but still reactive 1 2 3 11/1/2010 11/1/2010...
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11-01 - 11/1/2010 11/1/2010 1 Alkenes: Addition Reactions O...

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