Investigations into an intramolecular proton transfer and solvent dependent acid‐base equilibria in 2,6‐pyridine diacetic acid

Abstract

This study highlights an intramolecular proton transfer and solvent dependent acid-base equilibria in 2,6-pyridine diacetic acid, as contributing factors to a solid state reversible structural transformation and a spontaneous double decarboxylation. Differential scanning calorimetry exhibited several reversible phase transitions, with the most pronounced change occurring at 168 K. A reversible structural transformation, from the monoclinic system to the triclinic one, was confirmed via temperature dependent X-ray diffraction studies. In water, absorbance shifts from 198 to 210 nm were observed, correlating to the deprotonation of the carboxylic groups, synergistically, protonation of the pyridine ring showed shifts from 280 to 270 nm. ¹H NMR provided evidence of the solvent dependent, irreversible conversion to 2,6-dimethylpyridine via sequential decarboxylations. The ΔS^≠ for each decarboxylation were 39.53 ± 3.87 and 23.56 ± 2.84 JK^-1mol^-1 respectively. Finally, the rate constants for observed D₂O exchange with the α-carbon protons were determined as 1.02 ± 0.12 x 10^-3 s^-1 and 6.13 ± 0.12 x 10^-4 s^-1 with a double exponential fit.