Ch_322b_14.05

Ch_322b_14.05 - Chapter 14.5 lecture note

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Aromatic Ions Cyclopentadienyl Anion Monocyclic species that carry either a positive or negative charge often show unusual stability when they have closed ! -electron shells of 4n+2 ! -electrons (Huckel's rule). Such systems are "aromatic." : Cyclopentadiene is unusually acidic ( pK a = 16 ) for a hydrocarbon. The acidity is due to the unusual stability of its conjugate base, the cyclopentadienyl anion . H H cyclopentadiene pK a = 16 B: - + H - cyclopentadienyl anion ( unusually stable ) + BH comparable to water
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Resonance Theory: An Inadequate Explanation of Acidity Resonance theory does not adequately explain the unusual stability of the cyclopentadienyl anion . Although a series of resonance structures can be drawn that contribute to and stabilize the hybrid, even more resonance structures can be drawn for the cycloheptatrienyl anion that shows no unusual stability. H H cyclopentadiene pK a = 16 + B: - -BH H : - H - H - etc unusually stable H H cycloheptatriene pK a = 36 + B: - -BH H - H - H - etc not unusually stable
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This note was uploaded on 10/20/2009 for the course CHEM 322BL at USC.

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Ch_322b_14.05 - Chapter 14.5 lecture note

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