Ch_322a_7.010

Ch_322a_7.010 - Carbocation Stability and Molecular...

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Carbocation Stability and Molecular Rearrangements Carbon skeletal rearrangements are commonly observed with carbocation intermediates. The guiding principle is that a less stable carbocation transforms into a more stable carbocation . Example: The Dehydration of 3,3-Dimethyl-2-butanol CH 3 -C-CHCH 3 OH CH 3 CH 3 85% H 3 PO 4 heat CH 3 C=CCH 3 CH 3 CH 3 + CH 2 =C-CHCH 3 CH 3 CH 3 2,3-dimethyl-2-butene 2,3-dimethyl-1-butene (major) (minor) products of skeletal rearrangement
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CH 3 -C OH CH 3 CH 3 CHCH 3 : : + H-O-H + fast H + H 2 O CH 3 -C OH 2 CH 3 CH 3 CHCH 3 + CH 3 -C OH 2 CH 3 CH 3 CHCH 3 + slow step Bond Heterolysis CH 3 -C CH 3 CH 3 CHCH 3 + + H 2 O secondary carbocation Rearrangement Step CH 3 -C CH 3 CH 3 CHCH 3 + secondary carbocation less stable A methyl group moves to the carbocation carbon with its pair of electrons a 1,2-shift of H 3 C: CH 3 -C-CH-CH 3 CH 3 CH 3 ! + ! + transition state for rearrangement step CH 3 -C-CH-CH 3 CH 3 CH 3 + tertiary carbocation more stable
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Ch_322a_7.010 - Carbocation Stability and Molecular...

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