SPECTROPHOTOMETRIC ACIDITY SCALE OF STRONG NEUTRAL BRONSTED ACIDS IN ACETONITRILE

A continuous self-consistent quantitative UV-vis spectrophotometric pK (a) scale of strong acids in acetonitrile has been created. The 36 com pounds studied include phenols, phenylmalononitriles, sulfonimides, su lfonic acids, and sulfonimides modified with Yagupolskii's superaccept or substituents. The scale spans about 13 pK(a) units and consists of 74 independent equilibrium constant measurements, each describing the relative acidity of two acids. The method of measurements eliminates t he need for the direct determination of the acidity of the medium. The most acidic compounds studied have pK(a) values around 4 in acetonitr ile and can be regarded as true superacids. The scale is anchored to t he pK(a) value 11.0 for picric acid. The status of 2,4,6-(SO2CF3)(3)C6 H2OH as the most acidic phenol presently known (pK(a) = 4.9) is confir med. It is shown that the replacement of an =O fragment with =N-SO2CF3 in a -SO2- group in aromatic sulfonimides results in an acidity incre ase of more than 10(5) times. The most acidic compound in the scale is 4-ClC6H4SO(=NSO2CF3)NHSO2C6H4-4-NO2 (pK(a) = 3.75). The present resul ts together with those from the other authors furnish a unified scale of acidity in acetonitrile ranging from 4 to 27 pK(a) units and set so lid ground for pK(a) measurements of strong acids in acetonitrile.