The Acidity of Weak NH Acids: Expanding the p<i>K</i>_a Scale in Acetonitrile

Nitrogen heterocycles and aromatic amines are well-known and widely used compounds that are usually not seen as acids, although in organic solvents like acetonitrile (MeCN) or dimethyl sulfoxide (DMSO) their acidic properties can be observed. In this work, the acidities (pKa values) of 37 such weak NH acids were determined in acetonitrile and presented together with computational gas-phase acidities and literature pKa values in DMSO. In the course of the work the weakest acids range (from pKa 29 upward) of the MeCN acidity scale has been revised and expanded to around 33.5 by adding 31 compounds in that specific region and the span of experimental pKa values in MeCN is now more than 30 orders of magnitude. The relations between the structure and acidity of a selection of the studied compounds have been investigated in MeCN and DMSO. The measured pKa values in MeCN and the gathered pKa values in DMSO were used for a correlation analysis between the two solvents and for assessing the quality of pKa conversion equations. A number of pKa values have been predicted in MeCN from pKa values in DMSO via the correlation analysis and pKa conversion equations.氮杂环（nitrogen heterocycles）与芳香胺是一类广为人知且应用广泛的化合物，通常不被视作酸类；但在乙腈（acetonitrile，简称MeCN）、二甲基亚砜（dimethyl sulfoxide，简称DMSO）等有机溶剂中，可观测到它们的酸性性质。本研究测定了37种此类弱NH酸在乙腈中的酸度（pKa值），并结合计算得到的气相酸度与已发表的二甲基亚砜中pKa值一同呈现。研究过程中，我们对乙腈酸度标度中pKa≥29的最弱酸区间进行了修订与拓展：通过在该特定区间新增31种化合物，将标度上限拓展至约33.5，目前乙腈中实验测得的pKa值跨度已超过30个数量级。本研究还针对部分受试化合物，探究了其结构与酸度之间的关联，并分别在乙腈与二甲基亚砜溶剂中开展了相关分析。研究团队利用乙腈中测得的pKa值与收集得到的二甲基亚砜中pKa数据，开展了两种溶剂间的相关性分析，并对pKa转换方程的可靠性进行了评估。基于相关性分析与pKa转换方程，本研究还通过二甲基亚砜中的pKa值预测了乙腈中的部分pKa数值。