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Ch_322b_13.03 - Chapter 13.3 lecture note

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The Allyl Cation The allyl cation , CH 2 =CH-CH 2 + , is a very stable carbocation, about equivalent in stability to a tertiary carbocation. Stability Order of Carbocations C=C-C-C + substituted allyl > C-C C C + > CH 2 =CH-CH 2 + > C-C H C + > C-C H H + > CH 2 =CH + tertiary allyl secondary primary vinyl
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Molecular Orbital Description of the Allyl Cation The allyl cation is described in terms of the same set of ! -molecular orbitals as was noted for the allyl radical. However, the molecular orbitals are populated by only two rather than three electrons. ! -electron energy CH 2 CH CH 2 CH 2 CH CH 2 CH 2 CH CH 2 + + + + + + + + - - - - - - - - node nodes " 1 " 2 " 3 Nonbonding MO Antibonding MO Bonding MO allyl radical allyl cation
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Removal of the single electron in ! 2 in the allyl radical produces the allyl cation: ! 1 ! 2 ! 3 ! 1 ! 2 ! 3 CH 2 =CH-CH 2 CH 2 =CH-CH 2 . + -e - allyl radical allyl cation CH 2 CH CH 2 node 1 2 + 1 2 + The distribution of charge in the allyl cation is described by ! 2 , the lowest energy unoccupied MO (LUMO). Since this MO has a node at C 2 , the positive charge is distributed between C 1 and C 3 :
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