A Comprehensive Self-Consistent Spectrophotometric Acidity Scale of Neutral Brønsted Acids in Acetonitrile

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Abstract
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 pKa 3.7 to 28.1 in AN. The scale includes 93 acids that are interconnected by 203 relative acidity measurements (ΔpKa 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 ΔpKa measurements. The consistency standard deviation of the scale is 0.03 pKa 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.

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