Delocalization of electrons helps to make carbonic acid a stronger acid than boric acid. For benzene, we found that delocalizing the six ring electrons made the molecule 40 kcal mole more stable than it would be otherwise.

The carbonate ion with its two delocalized electrons also is more stable than it would be if the electrons were confined to the two oxygens that released protons during dissociation. Dissociation and association are reversible processes, and both are going on simultaneously:





The actual amounts of carbonic acid and carbonate ion that exist in solution are the result of a balance, or equilibrium, between the forward and reverse processes.

Delocalization, by making the carbonate ion more stable, favors dissociation and discourages association, because the delocalization is destroyed and the electrons are trapped in O-H bonds when the protons rejoin the carbonate ion.

This shifts the balance or equilibrium point to the right, thereby producing more ions, and making carbonic acid a stronger acid than it would be without delocalization.


Another way to look at this behaviour is to recognize that the two negative charges on a carbonate ion would have the strongest attraction for protons if they were tied down on specific oxygens on the outside of the ion.

If the negative charges are spread over the entire ion, the attraction of the ion for protons is blurred. This discourages association, and ensures that more protons remain loose than would be the case if delocalization did not exist.