Protonated benzofuran, anthracene, naphthalene, benzene, ethene, and ethyne: Measurements and estimates of pK_a and pK_R

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 k_p for the deprotonation and k_H2O 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 pK_R = -6.0 and -9.4 and pK_a = -13.5 and -16.3 for the protonated anthracene and protonated benzofuran, respectively. These pK values may be compared with pK_R = -6.7 for naphthalene hydrate (1-hydroxy-1,2-dihydronaphthalene), extrapolated to water from measurements by Pirinccioglu and Thibblin for acetonitrile-water mixtures, and pK_a = -20.4 for the 2-protonated naphthalene from combining k_p with an exchange rate constant. The differences between pK_R and pK_a correspond to pK_H2O, the equilibrium constant for hydration of the aromatic molecule (pK_H2O = pK_R - pk_a). For naphthalene and anthracene values of pK_H2O = +13.7 and +7.5 compare with independent estimates of +14.2 and +7.4. For benzene, pK_a = -24.3 is derived from an exchange rate constant and an assigned value for the reverse rate constant close to the limit for solvent relaxation. Combining this pK_a with calculated values of pK_H2O gives pK_R = -2.4 and -2.1 for protonated benzenes forming 1,2- and 1,4-hydrates, respectively. Coincidentally, the rate constant for protonation of benzene is similar to those for protonation of ethylene and acetylene (Lucchini, V.; Modena, G. J. Am. Chem Soc. 1990, 112, 6291). Values of pK_a for the ethyl and vinyl cations (-24.8) may thus be derived in the same way as that for the benzenonium ion. Combining these with appropriate values of pK_H2O then yields pKR = -39.8 and -29.6 for the vinyl and ethyl cations, respectively.