Interaction of DL-2,3-diaminopropionic acid and its methyl ester with metal ions. Part II. Hydrolysis kinetics

Abstract

Base hydrolysis of methyl DL-2,3-diaminopropionate (E) has been studied in aqueous solution at I= 0·1M and three temperatures (25, 37, and 50 °C). Specific rate constants have been obtained for the hydrolysis of the unprotonated ester and its monoprotonated (α-amino-group) form (EH⁺), and the corresponding activation parameters determined.

Rate constants for base hydrolysis of the ester function in bis-complexes of copper(II) and mercury(II) with methyl DL-2,3-diaminopropionate have been obtained at 25 °C and I= 0·1M. The hydrolytic behaviour of the bis-complexes is consistent with equations (i) and (ii), where A^– is the anion of 2,3-diaminopropionic acid and M²⁺ is a bivalent metal ion. The metal complexes undergo base hydrolysis considerably more rapidly than does ME₂²⁺+ OH^–→ MEA⁺+ MeOH (i), MEA⁺+ OH^–→ MA₂+ MeOH (ii) the free ligand E. Thermodynamic parameters for hydrolysis of the ester and the copper(II) complexes are compared. Specific rate constants for base hydrolysis of the mono-complex CuE²⁺, the hydroxy-complexes CuEOH⁺ and HgEOH⁺, and the mixed complex CuE(dae)²⁺(dae = 1,2-diaminoethane) have also been obtained at 25 °C and I= 0·1M. The relative values of the various rate constants are discussed.