The inductive effect is a central concept in chemistry and is often exemplified by the pKa values of acetic acid derivatives. The reduction in pKa is canonically attributed to the electron density of the carboxylate group being reduced 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(N-isopropylacrylamide), PNIPAM due to the presence of different (substituted) acetates allows for ionic charge densities to be examined. These studies confirmed the unexpected charge density and substituent-electronegativity relationship. Further analysis of literature showed anomalous charge densities for haloalkanes with 13C NMR spectroscopy and gas phase acidity of polyatomic acids. In summary, these independent results show that the induction effect does not explain pKa trends across the haloacetic acids.

中文翻译：

---

感应效应并不能解释卤酸盐中的电子密度：我们的教科书错了吗？


感应效应是化学中的一个中心概念，通常以乙酸衍生物的 pKa 值为例。pKa 的降低通常归因于羧酸盐基团的电子密度通过感应效应降低。然而，本文提出的波泛函理论计算表明，羧酸盐基团的电荷密度不能用感应效应来解释。对于一系列三卤乙酸酯（三氯-、氯二氟-和三氟-），我们发现三氯基团对羧酸氧原子的电荷密度具有最大的降低;电荷密度的变化与取代基电负性成反比。这些令人费解的结果得到了三个独立系统的实验支持：文献气相酸度、模型热响应聚合物系统中的特定离子效应以及卤代烷烃的核磁共振 （NMR） 波谱。由于不同（取代的）乙酸盐的存在而导致的聚（N-异丙基丙烯酰胺）、PNIPAM 溶解度的变化允许检查离子电荷密度。这些研究证实了意想不到的电荷密度和取代基-电负性关系。对文献的进一步分析表明，用 13C NMR 波谱显示卤代烷烃的电荷密度异常，以及多原子酸的气相酸度。总之，这些独立结果表明，诱导效应并不能解释卤代乙酸的 pKa 趋势。