ACID DISSOCIATION IN ACETONE-WATER MIXTURES . AN ANOMALOUS MEDIUM EFFECT WHEN LONDON DISPERSION FORCES ARE LARGE

2,4,6,2′,4′,6′-Hexanitrodiphenylamine (HNDA) undergoes acid dissociation according to HNDA ⇄ H+ + NDA-. At 25°, pKA = 2.75 in water, 2.13 in 12 wt % acetone-water, and 1.26 in 24 wt % acetone-water; 0.1 M p-cyanophenol and 0.1 M p-methoxyphenol likewise raise the dissociation constant. The direction of these changes is contrary to prediction based on the charge-in-dielectric-continuum model, but is consistent with the expected large increase in solvation energy of NDA-, when acetone is added to water, as a result of London dispersion forces.