Ch05_Handout2 - Carbocation Rearrangements Rearrangement a...

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1 Carbocation Rearrangements Carbocation Rearrangements Rearrangement: Rearrangement: a change in connectivity of the atoms in a product compared with the connectivity of the same atoms in the starting material Rearrangements are common in additions to alkenes if the initial carbocation can rearrange to a more stable one In electrophilic addition to alkenes, there is the possibility for rearrangement Hydride shift: H on adjacent carbon bonds with C + . Methyl (Alkyl) shift: CH 3 (Alkyl ) moves from adjacent carbon if no H s are available. 3-Methyl-1-butene 2-Methyl-2-butanol (the major product) + HBr Br Br 3-Methyl-2-butanol (the expected product) + H H HCl + + 2-Chloro-3,3-dimethylbutane (the expected product; 17%) 2-Chloro-2,3-dimethylbutane (the major product; 83%) 3,3-Dimethyl- 1-butene Cl Cl Carbocation Rearrangements Carbocation Rearrangements Examples: Examples:
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2 Carbocation Rearrangements Carbocation Rearrangements the driving force is rearrangement of a less stable carbocation to a more stable one the less stable 2° carbocation rearranges to a more stable 3° one by 1,2-shift of a hydride ion + A 2° carbocation intermediate 3-Methyl-1-butene + + C H 3 H C l H C H 3 C H 3 CCH=CH 2 C H 3 C -CHCH 3 slow, rate determining H : : : - C l : : : : + + A 3° carbocation CH 3 H CH 3 H CH 3 C-CHCH 3 CH 3 C-CHCH 3 fast Carbocation Rearrangements Carbocation Rearrangements
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