Question

How does aromaticity affect acidity?

Answer 1

Compare the acidity of phenol and cyclohexanol.

Explanation:

`pK_a`, `C_6H_5OH` `9.96`. There would be little acid base behaviour of cyclohexanol in water (hence I could not find a value!). This is probably best described as an entropy phenomenon. The phenoxide anion is resonance stabilized, and the negative charge is distributed around the ring to some. Phenoxide is thus less charge dense, and not so polarizing as the alkoxide anion of cyclohexanol, where the negative charge is localized on the 1 oxygen atom. Because phenoxide causes less solvent order, its acidity is favoured by entropy.

Benzoic acid has `pK_a=4.76`; formic acid, `4.16`. How can we use the ideas above to rationalize the different acidities.