Ch_322b_17.02

Ch_322b_17.02 - Chapter 17.2 lecture note

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Carbon Acid Strengths: Stability of Carbanions The term carbon acid refers to the atom to which the proton is attached. Loss of a proton produces a carbanion. C- H C: - -H + carbanion Simple Alkanes Simple alkanes are the l east acidic of organic compounds as shown by the estimated value of the pKa of methane. CH 4 pKa > 50 Aryl Substitution Enhances Acidity C H 3 pKa ~ 41 C H 2 pKa = 33 C H pKa = 31
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Enhanced Stability of Delocalized Carbanions: Increased Acidity The enhanced acidity of the carbon acids with aryl substitution is due to i ncreased stability of the carbanion (conjugate base) because of ! -delocalization as illustrated by the resonance structures below. H-C-H : - H-C-H - H-C-H - etc. . .
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A second important mechanism for dispersing and stabilizing the negative charge is the inductive effect of the aryl ring which is electron-withdrawing . Orbital Hybridization Influences Electrons are stabilized by interaction with the positive nuclear charge. The s-orbital has finite probability, while the p-orbital has
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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_17.02 - Chapter 17.2 lecture note

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