Johnson Edwin C, Gregory Kasimir P, Robertson Hayden, Gresham Isaac J, Nelson Andrew R J, Craig Vincent S J, Prescott Stuart W, Page Alister J, Webber Grant B, Wanless Erica J
Department of Chemistry, University of Sheffield Dainton Building, Brook Hill Sheffield S3 7HF UK
College of Science, Engineering and Environment, University of Newcastle Callaghan NSW 2308 Australia.
Chem Sci. 2024 Dec 19;16(5):2382-2390. doi: 10.1039/d4sc04832f. eCollection 2025 Jan 29.
The inductive effect is a central concept in chemistry and is often exemplified by the p values of acetic acid derivatives. The reduction in p is canonically attributed to the reduction in the electron density of the carboxylate group through the inductive effect. However, wave functional theory calculations presented herein reveal that the charge density of the carboxylate group is not explained by the inductive effect. For a series of trihaloacetates (trichloro-, chlorodifluoro- and trifluoro-) we find that the trichloro group has the greatest reduction on the charge density of the carboxylate oxygen atoms; change in charge density is inversely related to substituent electronegativity. These puzzling results are experimentally supported by investigating three independent systems: literature gas phase acidities, specific ion effects in a model thermoresponsive polymer system, and nuclear magnetic resonance (NMR) spectroscopy of haloalkanes. Changes in the solubility of poly(-isopropylacrylamide), PNIPAM, due to the presence of different (substituted) acetates allow ionic charge densities to be examined. These studies confirmed the unexpected charge density and substituent-electronegativity relationship. Further analysis of the literature showed anomalous charge densities for haloalkanes with C NMR spectroscopy and gas phase acidity of polyatomic acids. In summary, these independent results show that the induction effect does not explain p trends across the haloacetic acids.
诱导效应是化学中的一个核心概念，通常以乙酸衍生物的pKa值为例进行说明。pKa的降低通常归因于通过诱导效应使羧酸根基团的电子密度降低。然而，本文所呈现的波函数理论计算结果表明，羧酸根基团的电荷密度无法用诱导效应来解释。对于一系列三卤乙酸酯（三氯乙酸酯、氯二氟乙酸酯和三氟乙酸酯），我们发现三氯基团对羧酸根氧原子的电荷密度降低作用最大；电荷密度的变化与取代基的电负性呈反比关系。通过研究三个独立的体系，这些令人困惑的结果得到了实验支持：文献中的气相酸度、模型热响应聚合物体系中的特定离子效应以及卤代烷烃的核磁共振（NMR）光谱。由于存在不同的（取代）乙酸酯，聚（N-异丙基丙烯酰胺）（PNIPAM）的溶解度变化使得离子电荷密度得以考察。这些研究证实了意想不到的电荷密度与取代基电负性的关系。对文献的进一步分析表明，具有碳核磁共振光谱的卤代烷烃以及多元酸的气相酸度存在异常电荷密度。总之，这些独立的结果表明，诱导效应无法解释卤代乙酸的pKa趋势。
