Ch_322a_8.04

Ch_322a_8.04 - Markovnikov's Rule and Carbocation Stability...

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Markovnikov's Rule and Carbocation Stability When H + or any electrophile adds to an unsymmetrical alkene, there are competing pathways that involve different carbocation intermediates: CH 3 CH=CH 2 + H + Path 1 Path 2 CH 3 CH 2 CH 2 + CH 3 CHCH 3 + propyl carbocation isopropyl carbocation (less stable) (more stable) Because Step 1 (formation of the carbocation intermediate) is the rate-determining step , the products are determined by the relative rates of Paths 1 and 2. Path 2 is faster because it leads to a more stable carbocation.
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Overall Regiochemistry of the Hydrobromination of Propene Paths 1 and 2 compete. Since path 2 leads to a more stable carbocation intermediate (with a lower free energy of activation), it proceeds faster than path 1. Since more of the secondary propyl cation is formed, more of the isopropyl bromide product is formed. CH 3 CH=CH 2 + HBr H + H + CH 3 CH 2 CH 2 + CH 3 CHCH 3 + very little dominates slow step path 1 path 2 Br - CH 3 CH 2 CH 2 Br very little Br - CH 3 CHCH 3 Br dominates fast step
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Ch_322a_8.04 - Markovnikov's Rule and Carbocation Stability...

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