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Alcohols (R-OH), phenols (Ar-OH) and ca...

Alcohols (R-OH), phenols (Ar-OH) and carboxylic acids (R-COOH) can undergo ionization of O-H bond to give away proton `H^(+)`, yet they have different `pK_4` values, which are 16, 10 and 4.5 respectively. Explain.

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`pK_a` value is indicative of acid strength. Lower the `pK_a` value stronger the acid. Alcohols, phenols and carboxylic acids, all involve ionization of an O-H bond. But their different `pK_a` values indicate that their acid strength are different. This is because the resulting conjugate bases are stabilized to different extent.

As the conjugate base of carboxylic acid is best stabilized, among the three, carboxylic acids are strongest and have the lowest `pK_a` value. As conjugate base of alcohols is destabilized, alcohols are weakest acids and have highest `pK_a` value. As conjugate base of phenols is moderately stabilized phenols are moderately acidic and have intermediate `pK_a` value.
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