Protonated Benzofuran, Anthracene, Naphthalene, Benzene, Ethene, and Ethyne: Measurements and Estimates of pK a and pK R Aoife C. McCormack, Claire M. McDonnell, Rory A. More O’Ferrall,* AnnMarie C. O’Donoghue, and S. Nagaraja Rao Contribution from the Department of Chemistry and Centre for Synthesis and Chemical Biology, UniVersity College Dublin, Belfield, Dublin 4, Ireland Received November 28, 2001 Abstract: 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 constant. The differences between pKR and pKa correspond to pKH2O, the equilibrium constant for hydration of the aromatic molecule (pKH2O ) pKR - pKa). For naphthalene and anthracene values of pKH2O )+13.7 and +7.5 compare with independent estimates of +14.2 and +7.4. For benzene, pKa )-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 pKa with calculated values of pKH2O gives pKR )-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 pKa 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 pKH2O then yields pKR ) -39.8 and -29.6 for the vinyl and ethyl cations, respectively. Introduction This paper reports measurements of equilibrium constants for the conversion of aromatic molecules and their water adducts (hydrates) to carbocations. The constants of interest, expressed as their negative logarithms, are pK a and pK R for the protonated aromatic species. These are related to the equilibrium constant pK H2O for hydration of the aromatic molecule by a thermody- namic cycle which is illustrated for benzene, benzene hydrate (1), and the benzenonium carbocation (2) in Scheme 1. As in the previous paper, the equilibria are represented by single arrows rather than double arrows to indicate the directions of reaction to which they refer. 1 The paper describes experimental measurements of pK R for hydrates of benzofuran and anthracene. It draws on data from the literature 2-7 and the previous paper 1 to obtain pK a values and to establish the corresponding equilibrium constants for two isomeric hydrates of naphthalene and, more speculatively, the isomeric hydrates of benzene. A pK R value for one of the naphthalene hydrates, 2-hydroxy-1,2-dihydronaphthalene, has already been reported for 90% aqueous acetonitrile by Pirinc- cioglu and Thibblin. 2 An extension of these studies leads to estimates of pK a and pK R for alkyl carbocations and the vinyl cation. * Corresponding author. E-mail: rmof@ucd.ie. (1) Dey, J.; O’Donoghue, A. C.; More O’Ferrall, R. A. J. Am. Chem. Soc. 2002, 124, 8561 (preceding paper in this issue). (2) Pirinccioglu, N.; Thibblin, A. J. Am. Chem. Soc. 1998, 120, 6512. (3) Kelly, S. C.; McDonnell, C. M.; More O’Ferrall, R. A.; Rao, S. N.; Boyd, D. R.; Brannigan, I. N.; Sharma, N. D. Gazz. Chim. Ital. 1996, 126, 747. (4) Katritzky, A. R.; Taylor, R. AdV. Heterocycl. Chem. 1992, 47, 1. (5) Cox, R. A. J. Phys. Org. Chem. 1991 4, 233. (6) Ansell, H. V.; Hirschler, M. M.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1977, 353. (7) Eaborn, C.; Golborn, P.; Spillett, R. E.; Taylor, R. J. Chem. Soc. B 1968, 1112. Scheme 1 Published on Web 06/27/2002 10.1021/ja012613x CCC: $22.00 © 2002 American Chemical Society J. AM. CHEM. SOC. 2002, 124, 8575-8583 9 8575