The uncatalysed and nickel(II) catalysed decarboxylation of dihydroxyfumaric acid

Abstract

The kinetics of decarboxylation of dihydroxyfumaric acid have keen studied in aqueous solution at 25°, over a range of pH. At 25°, the thermodynamic ionization constants are 2.7 x 10^-2 = and 4.36 x 10^-4, indicating the trans enol structure in aqueous solution. The decarboxylation occurs in two consecutive steps to give glycolaldehyde as the final product. The monoanion of dihydroxyfumaric acid with k = 2.69 x 10^-2 min^-1 is the active species in the reaction. The decarboxylation is catalysed by nickel(II) ions at low metal ion concentrations, but at high metal ion concentrations there is evidence for the formation of catalytically inactive metal complexes. The application of computer techniques to consecutive first-order reactions is discussed. The enol + keto equilibrium of dihydroxyfumaric acid has been studied. In aqueous solution the ultraviolet spectrum indicates that the acid is completely enolized, but exists almost exclusively as the keto form in alkaline solution. The rate constant for the process enol + OH^- → keto + OH' is 2.9 l^-1 mole sec^-1 at 25°.