Hydroxamic acid compounds primarily comprise two major categories: hydroxamic acids and polymeric hydroxamic acids. The latter category, polymeric hydroxamic acids, further subdivides into linear polymeric hydroxamic acids and hydroxamic acid chelating resins featuring three-dimensional cross-linked structures. Hydroxamic acids exist as two tautomers-the hydroxamic acid form and the oxohydroxamic acid form-and are therefore also referred to as oxohydroxamic acids or isohydroxamic acids. These two tautomers coexist simultaneously and are inseparable, with the oxohydroxamic acid form typically predominating.
Hydroxamic acids can be regarded as derivatives of carboxylic acids; consequently, their acidity is weaker than that of their corresponding carboxylic acid counterparts. In acidic media, hydroxamic acids undergo facile hydrolysis to yield hydroxylamine and a carboxylic acid. Polymeric hydroxamic acids contain both carbonyl and oxime groups; within the oxime groups, the oxygen and nitrogen atoms are situated in close proximity to one another, and both atoms possess lone pairs of electrons. This structural feature endows polymeric hydroxamic acids with potent chelating capabilities. They are capable of undergoing chelation reactions with various transition metal ions-such as Pb²⁺, Co²⁺, Ti⁴⁺, La³⁺, and Ta⁵⁺-to form stable, polycyclic metal chelates.
These properties of hydroxamic acid compounds facilitate their widespread application across numerous fields. For instance, hydroxamic acids are utilized in the pharmaceutical sector as antitumor agents; in the beneficiation of oxidized metal ores, they serve as collectors in flotation processes. In the field of metallurgy, linear polymeric hydroxamic acids function as flocculants and precipitants, while polymeric hydroxamic acids generally serve as extractants for non-ferrous metals, and hydroxamic acid chelating resins are employed as adsorbents.
