Hydroxamic acids as weak base indicators: protonation in strong acid media.
The protonation equilibria of N-phenylbenzohydroxamic, benzohydroxamic, salicylhydroxamic, and N-p-tolylcinnamohydroxamic acids have been studied at 25 degrees C in concentrated sulfuric, hydrochloric, and perchloric acid media; the UV-vis spectral measurements were analyzed using the Hammett equation and the Bunnett-Olsen and excess acidity methods. The medium effects observed in the UV spectral curves were corrected with the Cox-Yates and vector analysis methods. The H(A) acidity function based on benzamides provided the best results. The range of variation of the solvation coefficient m is similar to that of amides, this indicating similar solvation requirements for amides and hydroxamic acids. For the same substrate, the observed variations of pK(BH)(+) with the mineral acid used was justified by formation of solvent-separated ion pairs; for the same mineral acid, the observed changes in pK(BH)(+) can be explained by the solvation of BH(+). The change of the pK(BH)(+) values was in reasonably good agreement with the sequence of the catalytic efficiency of the mineral acids used, HCl > H(2)SO(4) > HClO(4).
García
B
B
Dipartimento di Chimica e Chimica Industriale dell'Università di Pisa, Via Risorgimento 35, 56126 Pisa, Italy. begar@ubu.es
Ibeas
S
S
Hoyuelos
F J
FJ
Leal
J M
JM
Secco
F
F
Venturini
M
M
D000136:Acid-Base Equilibrium; D002614:Chelating Agents / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D006877:Hydroxamic Acids / Q000737:chemistry*; D008024:Ligands; D013056:Spectrophotometry, Ultraviolet
D002614:Chelating Agents; D006877:Hydroxamic Acids; D008024:Ligands