How Acidic Is Carbonic Acid?

Carbonic, lactic, and pyruvic acids have been generated in aqueous solution by the transient protonation of their corresponding conjugate bases by a tailor-made photoacid, the 6-hydroxy-1-sulfonate pyrene sodium salt molecule. A particular goal is to establish the pK(a) of carbonic acid H2CO3. The on-contact proton transfer (PT) reaction rate from the optically excited photoacid to the carboxylic bases was derived, with unprecedented precision, from time-correlated single-photon-counting measurements of the fluorescence lifetime of the photoacid in the presence of the proton acceptors.

Controlling reactivity by remote protonation of a basic side group in a bifunctional photoacid.

6-Hydroxy-2-naphthoic acid and its sulfonate derivatives belong to a family of bifunctional photoacids where the -OH group acts as a proton donor and the -COO(-) group acts as a proton acceptor. Upon electronic excitation, the -OH group becomes more acidic and the -COO(-) group turns more basic. Change in the ionization state of one functional group causes a change (switch) in the reactivity of the other functional group.

Bifunctional photoacids: remote protonation affecting chemical reactivity.

Reversible protonation (deprotonation) of a side-group is a useful and convenient way to affect the reactivity of large organic and biological molecules. We use bifunctional photoacids to demonstrate how the protonation state of a basic side-group (COO(-)) controls the reactivity of the main acidic group of the photoacid (OH), both in the ground and the electronic excited state of 6-carboxy derivatives of 2-naphthol.