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Solutions_Manual_for_Organic_Chemistry_6th_Ed 497

Solutions_Manual_for_Organic_Chemistry_6th_Ed 497 - 20-44...

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20-44 continued (c) The simpler case first-induction only: o II o II CH30-CH2 -C-O-H is a stronger acid than H-CH2 -C-O-H There is no resonance overlap between the methoxy group and the carboxyl group, so this is a pure inductive effect. The methoxy substituent increases the acidity, so methoxy must be electron-withdrawing by induction. This should come as no surprise as oxygen is the second most electronegative element. The anomaly comes in the decreased acidity of 4-methoxybenzoic acid: o 0 CH30 -< >- C -0 - H is a weaker acid than H -< >- C - 0 - H Through resonance, a pair of electrons from the methoxy oxygen can be donated through the benzene ring to the carboxyl group-a stabilizing effect. However, this electron donation destabilizes the carboxylate anion as there is already a negative charge on the carboxyl group; the resonance donation intensifies the negative charge. Since the product of the equilibrium would be destabilized relative to the starting material, the
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