Acidities of strong neutral Brønsted acids in different media

TLDR

The study examines acidities of various acid families in water, acetonitrile, 1,2‑dichloroethane, and the gas phase, focusing on strong acids. The authors analyze how acidity trends depend on moving from water to the gas phase, considering the acidity center and molecular structure. The investigation includes OH (carboxylic acids, alcohols, phenols), NH (sulfonamides, imides), CH (phenylmalonitriles, etc.) acids, as well as HCl, HBr, and HI, and analyzes their behavior across media. Acidity orders differ across water, AN, DCE, and gas phase, with AN and DCE showing similar orders; dramatic differences occur in some cases, and the extent of charge delocalization in the anion, quantified by a weighted average positive sigma parameter, largely explains the observed trends. © 2012 John Wiley & Sons, Ltd.

Abstract

Acidities of different families of acids are examined in media of different physical and chemical nature: water, acetonitrile (AN), 1,2‐dichloroethane (DCE) and the gas phase, with special emphasis on strong acids. Included are OH (carboxylic acids, alcohols, and phenols), NH (sulfonamides, imides), and CH (phenylmalonitriles, etc.) acids as well as HCl, HBr, and HI. Dependence of the acidity trends on moving from water to the gas phase on the nature of the acidity center, and the molecular structure are analyzed. The acidity orders are different in water, AN, DCE, and the gas phase. In some cases the differences are dramatic. AN and DCE display similar acidity order in the set of the investigated acids. It is demonstrated that the decisive factor for the behavior of the acids when transferring between different media is the extent of charge delocalization in the anion and that the recently introduced weighted average positive sigma parameter in spite of its simplicity enables interpretation of the trends in the majority of cases. Copyright © 2012 John Wiley & Sons, Ltd.