Investigation in the Series of Substituted Butan- and Butenolides: XV. Transformations of 4-Hydroxy-2-butenolide in Water Media at Various pH Values

The 4-hydroxy-2-butenolide was established by polarographic method to undergo in water solutions fast tautomeric and acid-base transformations. Depending on pH of the medium the compound is present either as a cyclic irreducible form (at pH 0-4) or in the open-chain (carbonyl-containing) reducible forms (at pH > 4 and < 0). It is presumed that in going from the basic to strongly acidic medium and backwards occur consecutive equilibrium transitions of four species: cis--formylacrylic acid anion (A^-), its neutral molecule (HA) existing as linear and cyclic tautomers, and protonated forms (H₂A⁺ and H₃A⁺). The formation mechanism thereof is considered.