Strengths of Acids in Acetonitrile

Abstract The equilibrium acidity scale (p K a scale) in acetonitrile has been supplemented by numerous new compounds and new Δp K a measurements. It now contains altogether 231 acids – over twice more than published previously – linked by 569 Δp K a measurements and spans between the p K a 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 p K a 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 p K a values for the studied types of compounds in water, DMSO, DMF, and 1,2‐dichloroethane on the basis of p K a values in acetonitrile. These equations enable predicting p K a values with an average error around or less than 1 p K a 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.