A Comparison between the Acid-Catalysed Reactions of Some Dihydroxamic Acids, Monohydroxamic Acids and Desferal

A kinetic study on the hydrolysis of some dihydroxamic acids HOHNOC-(CH2)n-CONHOH (n = 0, oxalo, [ODHA]; n = 1 malono [MDHA]; and n = 2, succino, [SDHA] dihydroxamic acids) in aqueous mineral acids is reported. A comparison of the kinetic data with those from the hydrolysis of simple monohydroxamic acid, (acetohydroxamic acid [AHA] CH3CONHOH, benzohydroxamic acid [BHA] C6H5CONHOH) and the natural trihydroxamate-based siderophore desferal (DFB) revealed that the hydrolytic stability sequence of the compounds is generally: BHA > ODHA > MDHA > DFB > AHA > SDHA. An excess acidity analysis reveals that the reaction involving a pre-equilibrium protonation was followed by a rate determining A-2 type nucleophilic attack of water molecule on the protonated substrate. An attempt has been made to study protonation equilibria.