Study of Protonation Behaviour and Distribution Ratios of Hydroxamic Acids in Hydrochloric and Perchloric Acid Solutions Through Hammett Acidity Function, Bunnett-Olsen and Excess Acidity Method

Abstract

The protonation parameters, dissociation constants (pKBH+) of conjugate acid, slope values (m, φ and m*) and correlation coefficients (r) of hydroxamic acids were determined by Hammett acidity function method, Bunnett- Olsen method and excess acidity method in hydrochloric and perchloric acid solutions. Effect of acid concentration on partition and percentage protonation was also studied. pKBH+ values show that hydroxamic acids do not behave as Hammett bases, but hydroxamic acids behave as weak bases in strong acidic solutions. The values of pKBH+ obtained through Bunnett-Olsen method and excess acidity method were compared with the Hammett acidity function. ChemAxon’s MarvinSketch 6.1.5 software was also used for determining pKa, pI and microspecies distribution (%) of hydroxamic acids with pH. Hydrogen donor and acceptor values and logD were also obtained. The results show that N-p-chlorophenyl-4- bromobenzohydroxamic acid has the highest pKa and lowest logD values. On the contrary, N-phenyl-3,5-dinitrobenzohydroxamic acid has lowest the pKa and highest logD values.