Acidity function in organic solvents (acetonitrile, phenol, acetone and their mixtures)

Abstract

Acidity function (A.F.) measured by protonation equilibria of indicators of the nitroanilines family, is under investigation for H 2SO 4 (CF 3SO 3H) solution (10 −4 10 −1 M) in organic solvents. The chosen systems are known to be much more acidic than the aqueous ones [1, 2] but no general A.F. has been described like the well known Hammett function for the aqueous system. Some results are reported for the solvent system Phenol—Acetone with a water content variable from 0.5 to 4%. Table I shows the pK S a of the two indicators, o-nitroaniline and p-nitroaniline, determined by the extrapolation method in the system S (65% Phenol–35% Acetone) at variable amount of water. The reported pK S a values are higher than those in water ideal state (pK W a), in agreement with the greater protonating ability of the system compound with the aqueous state. The variation of the pK a values with water concentration points out that water should be considered a strong base in Phenol—Acetone solvent system. It is not possible to derive an acidity function using the ideal solution in water as standard state, since there are specific effects probably due to the low dielectric constant of the medium. As it is known [1, 2] it is possible to define an ‘operational’ acidity function H S o using an ideal solution in the solvent itself, as reference state. Assuming as reference state the solvent system 65% Phenol/35% Acetone with 0.5% water, it is possible to obtain consistent data of the acidity function H S o. Fig. 1 shows the continuous curves at different water content. The t001 pK a of o- and p-Nitroaniline. pK S a pK W a Solvent: Phenol—Acetone (65–35) solvent: Water % water p-nitroaniline o-nitroaniline p-nitroaniline o-nitroaniline 0.55 4.27 2.50 1.00 −0.29 1.05 3.55 1.80 2.05 2.77 1.07 4.05 1.91 0.23 ▪ Fig. 1. Acidity function H S o for the system Phenol—Acetone 65 35 . variation of H S o with the log of sulphuric acid concentration is a linear function with the expected slope −1 at low concentration. The variation of H S o with solvent composition from 40% to 85% Phenol in Acetone with water concentration from 0.5 to 4%, can be expressed by an empirical relation. The relationship points out that the acidity grows with the content of phenol and decreases with the content of water and acetone. The H S o parameter has been correlated with the kinetic constant of some scission reaction related to the synthesis of phenol from cumenehydroperoxide.