At temperatures above 100 °C iron(III) oxidizes coordinated EDTA to ethylenediaminetriacetic acid in aqueous solution in the absence of molecular oxygen. The reaction proceeds with an activation energy of 28.6 kcal/mol, and its rate is directly proportional to the concentration of Fe(III) and inversely proportional to pH. At 125 °C, the halflife of Fe(III) in the presence of excess EDTA is about 3 h at pH 9.3, but increases to >70 h at pH 5.4. The reaction is stoichiometric and no other reaction products or by-products were detected by nmr, gc, and gc – mass spectroscopy. In the presence of oxygen iron catalyzes quantitative oxidation of ethylenediamine-N,N,N′,N′-tetraacetic acid (EDTA) to ethylenediaminetriacetic acid. The copper(II)–EDTA chelate undergoes a similar reaction but higher temperatures [Formula: see text] are required. Iron(III) also oxidizes nitrilotriacetic acid (NTA) to iminodiacetic acid (IDA) and glycine. The hydrolyzed species Fe(OH)EDTA is shown to be the reactive intermediate, and the well-known (Fe–EDTA)2O4− μ-oxo dimer is shown not to exist at elevated temperatures (above 100 °C). Probable mechanisms are proposed for these reactions and comparisons are made with earlier work.
The Thermal and Radiation Oxidation of Benzene to Phenol in Aqueous Solutions Containing Metal Ions at Elevated Temperatures. VI. The By-products of Benzene Oxidation