Hydrolysis and ammonolysis of EDTA in aqueous solution

Abstract

The hydrolysis and ammonolysis of EDTA were studied in aqueous solution over a range of temperatures and at various pH values with the aid of nmr, gc, and gc – mass spectroscopic techniques. At high pH in the presence of ammonia, both ammonolysis and hydrolysis occur with the production of N-(2-aminoethyl)iminodiacetic acid (UEDDA), N-(2-hydroxyethyl)-iminodiacetic acid (HEIDA), and iminodiacetic acid (IDA) in molar ratios such that [IDA] = [UEDDA] + [HEIDA]. The first-order rate constant for the disappearance of EDTA at 175 °C in dilute aqueous ammonia is 8.6 × 10−5 s−1 whereas in the absence of ammonia its hydrolysis constant is 4.2 × 10−5 s−1. The value of ΔH0 for this reaction is approximately 35 kcal/mol. When methylamine replaces ammonia, the UEDDA is replaced by N-(2-methylaminoethyl)iminodiacetic acid. The rate of hydrolysis is increased by the presence of a tertiary amine but the latter does not become incorporated into the reaction products. A reaction mechanism is proposed involving bimolecular SN2 attack by base on a carbon atom of the ethylene bridge adjacent to a protonated nitrogen atom of EDTA with concomitant displacement of iminodiacetic acid.