Potentiometric study of equilibrium acidities of some carbon acids in acetonitrile

Abstract

The pKa values for 30 C-acids of the type CH·R1·R2·R3 — where the R1, R2, and R3 substituents are H, alkyl, and aryl, and the most common electron withdrawing groups used in studies of proton-transfer reaction — are given herein. The pKa values have been measured potentiometrically in acetonitrile, using a glass electrode. These values were determined in a buffer solution consisting of a C-acid and its tetrabutylammonium salt, at constant ionic strength (0.001 M), at 25°C. The measurements were done under nitrogen, to prevent oxidation of the anion. The influence of water within the sample of the tetrabutylammonium hydroxide base was negligible. The lowest pKa value of 19.1 was found for bis(4-nitrophenyl)cyanomethane, while the largest pKas that could be measured were limited by the basicity of the hydroxide ion, and did not exceed 28. A satisfactory agreement between spectrophotometric and potentiometric acidities was achieved. Key words: C-acids, pKa values, acetonitrile, potentiometry, proton transfer.