The inductive effect does not explain electron density in haloacetates: are our textbooks wrong?

Abstract

The inductive effect is a central concept in chemistry and is often exemplified by the pK_a values of acetic acid derivatives. The reduction in pK_a 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(N-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 ¹³C NMR spectroscopy and gas phase acidity of polyatomic acids. In summary, these independent results show that the induction effect does not explain pK_a trends across the haloacetic acids.