(O-HYDROXYPHENYL)METHYLPHOSPHONIC ACIDS - SPECTROPHOTOMETRIC DETERMINATION OF THEIR PKA VALUES AND OF THE DEPROTONATION SEQUENCE

UV/VIS Absorption spectra of nitrosubstituted (o-hydroxyphenyl)methylphosphonic acids (o-(phosphonomethyl)phenols) were measured as a function of pH at 25-degrees in 0.1M NaClO4 solutions. Computational treatment of the whole set of optical density data between 200 and 500 nm resulted in the determination of the dissociation constants of these polyacids and also of the individual electronic spectra of all the species involved in the deprotonation sequence. The spectral behavior gives information on the structure of the anions formed and consequently the order of the subsequent deprotonation steps could be deduced. For the (2-hydroxy-3-nitro(or 5-nitro)phenyl)methylphosphonic acid and the 2-hydroxy-5-nitro-1,3-phenylenebis(methylphosphonic acid), the phenolic proton dissociates in the last step, while, in the case of (2-hydroxy-3,5-dinitrophenyl)methylphosphonic acid, the last dissociating proton comes from a P-OH group. An intermediate situation is found for (3-chloro-2-hydroxy-5-nitrophenyl)methylphosphonic acid. Generally, the deprotonation sequence is governed by intramolecular H-bonds involving the phenolic OH group.