For the first time, the self-consistent spectrophotometric acidity scale of neutral Brønsted acids in acetonitrile (AN) spanning 24 orders of magnitude of acidities is reported. The scale ranges from pK(a) 3.7 to 28.1 in AN. The scale includes 93 acids that are interconnected by 203 relative acidity measurements (DeltapK(a) measurements) and contains compounds with gradually changing acidities, including representatives from all of the conventional families of OH (alcohols, phenols, carboxylic acids, sulfonic acids), NH (anilines, diphenylamines, disulfonimides), and CH acids (fluorenes, diphenylacetonitriles, phenylmalononitriles). The CH acids were particularly useful in constructing the scale because they do not undergo homo- or heteroconjugation processes and their acidities are rather insensitive to traces of water in the medium. The scale has been fully cross-validated: the relative acidity of any two acids on the scale can be found by combining at least two independent sets of DeltapK(a) measurements. The consistency standard deviation of the scale is 0.03 pK(a) units. Comparison of acidities in many different media has been carried out, and the structure-acidity relations are discussed. The large variety of the acids on the scale, its wide span, and the quality of the data make the scale a useful tool for further acidity studies in acetonitrile.
A continuous self-consistent quantitative UV−vis spectrophotometric pK a scale of strong acids in acetonitrile has been created. The 36 compounds studied include phenols, phenylmalononitriles, sulfonimides, sulfonic acids, and sulfonimides modified with Yagupolskii's superacceptor 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 the need for the direct determination of the acidity of the medium. The most acidic compounds studied have pK a values around 4 in acetonitrile and can be regarded as true superacids. The scale is anchored to the pK a value 11.0 for picric acid. The status of 2,4,6-(SO2CF3)3C6H2OH as the most acidic phenol presently known (pK a = 4.9) is confirmed. It is shown that the replacement of an O fragment with N−SO2CF3 in a −SO2− group in aromatic sulfonimides results in an acidity increase of more than 105 times. The most acidic compound in the scale is 4-ClC6H4SO(NSO2CF3)NHSO2C6H4-4-NO2 (pK a = 3.75). The present results 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 solid ground for pK a measurements of strong acids in acetonitrile.
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