Protonated benzofuran, anthracene, naphthalene, benzene, ethene, and ethyne: Measurements and estimates of pKa and pKR

Aqueous solvolyses of acyl derivatives of hydrates (water adducts) of anthracene and benzofuran yield carbocations which undergo competitive deprotonation to form the aromatic molecules and nucleophilic reaction with water to give the aromatic hydrates. Trapping experiments with azide ions yield rate constants kp for the deprotonation and kH2O for the nucleophilic reaction based on the “azide clock”. Combining these with rate constants for (a) the H+-catalyzed reaction of the hydrate to form the carbocation and (b) hydrogen isotope exchange of the aromatic molecule (from the literature) yields pKR = −6.0 and −9.4 and pKa = −13.5 and −16.3 for the protonated anthracene and protonated benzofuran, respectively. These pK values may be compared with pKR = −6.7 for naphthalene hydrate (1-hydroxy-1,2-dihydronaphthalene), extrapolated to water from measurements by Pirinccioglu and Thibblin for acetonitrile−water mixtures, and pKa = −20.4 for the 2-protonated naphthalene from combining kp with an exchange rate con...