CHARACTERIZATION OF TETRAHYDROFURAN AS A SOLVENT FOR PROTON-TRANSFER REACTIONS

Experiments designed to characterize tetrahydrofuran as a solvent for chemical reactions, particularly those involving proton transfer, are described. Difficulties encountered in the quantitative interpretation of experimental results, caused by the low relative permittivity (7.6) and compounded by the aprotic nature of this solvent, are discussed. Ionization and dissociation constants of perchloric, trifluoromethanesulfonic, and picric acids were determined by a combination of spectrophotometry of an added indicator, conductometry, and potentiometry at the glass electrode. The response of the glass electrode is Nernstian over a wide range of solvated proton activities even in this low permittivity solvent; the same is true for the response of various other indicator electroded to their corresponding ions. Relative strengths of bases including tetrabutylammonium hydroxide, t-butoxide, and fluoride were floride were determined potentiometrically. Picric acid buffers are recommended for calibration of a pH* scale (referred to the standard state in tetrahydrofuran). The accessible range of pH* values for 10(-3)M solutions of acids and bases in the presence of 2.5 x 10(-2)M tetrabutylammonium perchlorate extends from 6 to 28. Some exploratory measurements were carried out in 2-methyltetrahydrofuran as solvent.