1. Stability of Allylic and Benzylic Cations
An allylic carbon is an sp3 carbon adjacent to an sp2 carbon of an alkene
These carbons can lose a basic substituent and become a carbocation - in the process they become sp2 hybridized
Allylic and benzylic cations have delocalized electrons
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2. Stability of Allylic and Benzylic Cations
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3. Stability of Allylic and Benzylic Cations
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4. Stability of Allylic and Benzylic Cations
Alkyl substitution at the allylic carbon stabilizes allylic and benzylic cations
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5. Stability of Allylic and Benzylic Radicals
Allylic and Benzylic radicals also are stabilized by electron delocalization
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6. Chemical Consequences of Electron Delocalization
Markovnikov’s rule makes no prediction in this case
The stability of the benzylic cation is such as to exclude the other possible product
a secondary benzylic cation
a secondary carbocation
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7. Chemical Consequences of Electron Delocalization
Rearrangements also are driven by electron delocalization
The driving force for the rearrangement is the formation of a more stable secondary benzylic cation
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8. Effect of Delocalization on pKa
Carboxylic acids (RCOOH) are much more acidic than and alcohols (ROH)
pKa = pKa = 15.9
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9. Effect of Delocalization on pKa
Substituents that withdraw electrons from the oxygen of the OH group, such as (C=O), weaken the bond between O and H, and
The resulting carboxylate ion is stabilized by electron delocalization
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10. Effect of Delocalization on pKa
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11. Effect of Delocalization on pKa
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12. Effect of Delocalization on pKa
A protonated aniline is more acidic than a protonated cyclohexylamine
pKa = 4.6
pKa = 11.2
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13. Effect of Delocalization on pKa
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14. Effect of Delocalization on pKa
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