Ch_322b_21.02

Ch_322b_21.02 - Chapter 21.2 lecture note

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Phenols as Acids Phenols are much more acidic than alcohols, but are weaker acids than carboxylic acids. RCO 2 H ArO H RO H pK a 4-5 ~10 ~18 These acidities are explained in terms of the different stabilities of the conjugate bases (A: - ) of the acids (HA) that influence the equilibrium: HA + H 2 O H 3 O + + A: - K a K a = [H 3 O + ] [A: - ] [HA] An increase in the stability of A: - drives the equilibrium more to the right increasing the magnitude of K a .
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Cyclohexanol and Phenol Phenol is a much stronger acid than cyclohexanol. This difference in acidity is understandable in terms of the difference in stabilities of the conjugate bases. The conjugate base of cyclohexanol is a localized anion . There is no resonance stabilization. The conjugate base of phenol may be represented through a series of resonance structures that show it is a delocalized anion. The hybrid structure is stabilized by resonance . : O 18 9.89
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Ch_322b_21.02 - Chapter 21.2 lecture note

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