Strengths of Acids in Acetonitrile

• Published in: European journal of organic chemistry 2021-03, Vol.2021 (9), p.1407-1419
• Main Authors: Kütt, Agnes, Tshepelevitsh, Sofja, Saame, Jaan, Lõkov, Märt, Kaljurand, Ivari, Selberg, Sigrid, Leito, Ivo
• Format: Article
• Language: English
• Subjects: Acetonitrile; Acidity; Dichloroethane; Mathematical analysis; Non-aqueous solvents; Organic chemistry; pKa values; Power sources; Solvent effects
• ISSN: 1434-193X; 1099-0690
• DOI: 10.1002/ejoc.202001649

The equilibrium acidity scale (pKa scale) in acetonitrile has been supplemented by numerous new compounds and new ΔpKa measurements. It now contains altogether 231 acids – over twice more than published previously – linked by 569 ΔpKa measurements and spans between the pKa values of hydrogen iodide (2.8) and indole (32.57), covering close to 30 orders of magnitude. Measurement results acquired over the last 15 years were added to the scale and new least‐squares treatment was carried out. The treatment yielded revised pKa values for the compounds published previously, with the root mean square difference between revised and previous values 0.04, demonstrating very good stability of the scale. Correlation equations were developed for estimating pKa values for the studied types of compounds in water, DMSO, DMF, and 1,2‐dichloroethane on the basis of pKa values in acetonitrile. These equations enable predicting pKa values with an average error around or less than 1 pKa unit, which is a sufficient accuracy for many applications. The scale is expected to be a useful tool for the widest possible research areas in organic chemistry, electrochemical power sources, catalysis, etc. Revised pKa scale, containing 231 acids and spanning almost 30 orders of magnitude of acidities in acetonitrile, one of the most useful solvents for non‐aqueous acid‐base chemistry, is presented.