Strengths of Acids in Acetonitrile

Abstract

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.

The Reaction of Hydrogen Halides with Tetrahydroborate Anion and Hexahydro-closo-hexaborate Dianion

“…In the case of weak acids like HF (pK a MeCN (estim) = 25.2 [ 44 ]) only one product-like concerted transition state (TS) of proton-hydride transfer and H 2 elimination was found ( TS1 CONC HF , Figure 3 ). In the case of stronger acids HCl (pK a MeCN = 10.3 [ 45 ]) or HBr (pK a MeCN = 5.4 [ 45 ]), two TS’s were observed: a reagent-like transition state of proton transfer ( TS1 PT HCl ) and a product-like transition state of H 2 elimination ( TS1 ELIM HCl ). Both concerted TS of proton-hydride transfer/H 2 elimination TSn CONC HX and TS of H 2 elimination TSn ELIM HX (preceded by proton transfer step TSn PT HX ) exhibited a similar triangle-shaped form of front-side nucleophilic substitution of H 2 by halide-ion X − .…”

“… a An estimated value is taken from [ 44 ]; b Experimental values are taken from [ 45 ]; c The values for TSn PT HX / TSn ELIM HX are given. …”

Rifampicin as an example of beyond-rule-of-5 compound: Ionization beyond water and lipophilicity beyond octanol/water

“…NMR-spectrometric titration with p-toluenesulfonic acid (pK a = 8.5 in MeCN) (Kütt et al, 2021) in deuterated acetonitrile solution was performed to examine the protonation behavior of rifampicin. Bruker Avance III 700 MHz spectrometer was used to collect the 1 H and 13 C NMR spectra.…”

“…Literature analysis shows that acidity and basicity data for different compounds in the pK a range of 2.8 to 32.5 exist in MeCN. (Kütt et al, 2021) (Tshepelevitsh et al, 2019) Because of the complications of measuring the acidity (pH) of the medium, we use a method that eliminates the need for its determination. (Kütt et al, 2006) The method consists of UV-Vis spectrophotometric titration of an anhydrous solution, where both of the acids (or bases) are present, with a transparent acid or base and yields the difference of their pK a values (ΔpK a value).…”

Ionic Liquids based Acid‐base Indicators for Aqueous to the Non‐Aqueous Medium: An Overview

“…[17][18][19] However, plenty of recent literature reports are available that deal with aqueous and non-aqueous pK a data. [20][21][22][23] Nevertheless, the pK a values and pK a scale of a particular compound in various solvents may often be unavailable. From the aforementioned discussions, it is clear that visualization of acid-base equilibria and determination of pK a values have paramount importance for understanding the various chemical, biochemical processes and their applicability in versatile fields such as catalysis, [2] drug delivery, [11] material science [5] and sensing applications.…”