Acid-Base Equilibria in Nonpolar Media. Absolute pK a Scale of Bases in Tetrahydrofuran Gemma Garrido, ² Eve Koort, ²,‡, | Clara Ra `fols, ² Elisabeth Bosch, ² Toomas Rodima, ‡ Ivo Leito, ‡ and Martı ´ Rose ´s* ,² Departament de Quı ´mica Analı ´tica, UniVersitat de Barcelona, Martı ´ i Franque ` s 1-11, 08028 Barcelona, Spain, and Institute of Chemical Physics, UniVersity of Tartu, Jakobi 2, 51014 Tartu, Estonia marti@apolo.qui.ub.es ReceiVed July 10, 2006 The acidity constants (pK a ) of 11 bases (amines, anilines, pyridines, pyrrolidines, and iminophosphoranes) have been determined in tetrahydrofuran by potentiometry, complemented by conductometric measure- ments. The pK a values of the studied bases cover a wide absolute pH range of acidity in tetrahydrofuran, from 7.4 to 21.7. From the pK a values obtained, a scale of absolute acidity in tetrahydrofuran has been established, which has allowed calculation of the absolute pK a values of 77 bases from literature relative pK a data. Introduction Tetrahydrofuran (THF) is a saturated cyclic ether belonging to the class of aprotic dipolar protophilic solvents. 1 It is a widely used solvent in synthetic organic chemistry, chemical analysis, and technology and also in industry, as it dissolves organome- tallic compounds, ionic species, and many polymetric materials. 2-9 From an analytical point of view, mixtures of THF with water are used as mobile phases in liquid chromatography. 10 There are some data on ion-ion and ion-solvent interactions in THF. Several of them have been obtained by means of conductometry 11 and ultrasonic relaxation methods, 12,13 whereas other data were based on potentiometry combined with conductometry. 14-16 It is well-known that in solvents of low permittivity, especially if the solvent is also aprotic, such as THF (ǫ ) 7.39), 17 all chemical reactions involving ionogens or ionophores, including proton-transfer reactions, are greatly complicated by ion association leading to ion pairs and even larger aggregates. * To whom correspondence should be addressed. Phone: +34934039275. Fax: +34934021233. ² Universitat de Barcelona. ‡ University of Tartu. | Present address: University of Tartu. (1) Kolthoff, I. M. Anal. Chem. 1974, 46, 1992-2003. (2) Biellmann, J. F.; Ducep, J. B. Org. React. 1982, 27,1-344. (3) Bors, D. A.; Kaufman, M. J.; Streitwieser, A. J. Am. Chem. Soc. 1985, 107, 6975-6982. (4) Mah, S.; You, D.; Cho, H.; Choi, S.; Shin, J. J. Appl. Polym. Sci. 1998, 69, 611-618. (5) Karunanithi, T.; Thenmozhi, M. Chem. Eng. World 2004, 39, 63- 68. (6) Landrock, A. H. Handbook of Plastic Foams; William Andrew Inc.: New Jersey, 1995. (7) Dreyfuss, P.; Dreyfuss, M. P.; Pruckmayr, G. Encycl. Polym. Sci. Eng. 1989, 16, 649-681. (8) Hanesian, S.; Moitessier, N.; Wilmouth, S. Tetrahedron 2000, 56, 7643-7660. (9) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry, 3rd ed.; Wiley-VCH: Weinheim, Germany, 2003. (10) Meyer V. R. Practical High Performance Liquid Chromatography, 4th ed.; Wiley-VCH: Weinheim, Germany, 2004. (11) Werblan, L.; Suzdorf, A.; Lesinski, J. Bull. Acad. Pol. Sci. 1980, 28, 627-642. (12) Delsignore, M.; Maaser, H. E.; Petrucci, S. J. Phys. Chem. 1984, 88, 2405-2411. (13) Farber, H.; Petrucci, S. J. Phys. Chem. 1976, 80, 327-335. (14) Barbosa, J.; Barro ´n, D.; Bosch, E.; Rose ´s, M. Anal. Chim. Acta 1992, 264, 229-239. (15) Barbosa, J.; Barro ´n, D.; Bosch, E.; Rose ´s, M. Anal. Chim. Acta 1992, 265, 157-165. (16) Barro ´n, D.; Barbosa, J. Anal. Chim. Acta 2000, 403, 339-347. (17) Metz, D. J.; Glines, J. Phys. Chem. 1967, 71, 1158-1159. 9062 J. Org. Chem. 2006, 71, 9062-9067 10.1021/jo061432g CCC: $33.50 © 2006 American Chemical Society Published on Web 10/31/2006