Extension of the Self-Consistent Spectrophotometric Basicity Scale in Acetonitrile to a Full Span of 28 pKa Units: Unification of Different Basicity Scales

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Abstract
The earlier compiled self-consistent spectrophotometric basicity scale in acetonitrile (AN) was expanded to range from 3.8 to 32.0 pKa units, that is 28 orders of magnitude. Altogether 54 new relative basicity measurements (ΔpKa measurements) were carried out and 37 new compounds were introduced to the scale (it now includes altogether 89 bases). The relative basicity of any two bases in the scale can be obtained by combining at least two independent sets of measurements. Multiple overlapping measurements make the results more reliable. The overall consistency (as defined earlier) of the measurements is s = 0.03 pKa units. Thorough analysis of all of our experimental data (ΔpKa values of this and earlier works) and experimental pKa data in AN available in the literature (works from the groups of Coetzee and Padmanabhan, Kolthoff and Chantooni, Jr., the Schwesinger group, Bren' et al. and some others, altogether 19 papers) was carried out. On the basis of this analysis the anchor point of the scalepyridinewas shifted upward by 0.20 pKa units thereby also revising the absolute pKa values of all the bases on the scale. This way very good agreement between our relative data and the absolute pKa values of the abovementioned authors was obtained. The revised basicity scale was interconnected with the earlier published self-consistent acidity scale by ΔpKa measurements between acids and bases. The rms deviation between the directly measured ΔpKa values and the absolute pKa values of the compounds was 0.10 pKa units.