Spectrophotometric Acidity Scale of Strong Neutral Brønsted Acids in Acetonitrile
Citations Over TimeTop 12% of 1998 papers
Abstract
A continuous self-consistent quantitative UV−vis spectrophotometric pKa 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 pKa 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 pKa values around 4 in acetonitrile and can be regarded as true superacids. The scale is anchored to the pKa value 11.0 for picric acid. The status of 2,4,6-(SO2CF3)3C6H2OH as the most acidic phenol presently known (pKa = 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 (pKa = 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 pKa units and set solid ground for pKa measurements of strong acids in acetonitrile.
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