The reactions of ozone with tertiary amines including the complexing agents nitrilotriacetic acid (NTA) and ethylenediaminetetraacetic acid (EDTA) in aqueous solution

Abstract

Using the stopped-flow technique, the rate constants of the reaction of ozone with a number of amines have been determined. While the protonated amines do not react with ozone, the free amines react with rate constants of around 10⁶ dm³ mol⁻¹ s⁻¹ in the case of tertiary and secondary amines, while primary amines react more slowly. Mono-protonated EDTA reacts only with k=1.6×10⁵ and mono-protonated 1,4-diazabicyclo[2.2.2]octane (DABCO) with k = 3.5 × 10³ dm³ mol⁻¹ s⁻¹. In aqueous solution, tertiary amines react with ozone mainly by forming the aminoxide and singlet dioxygen [O₂(¹Δ_g)] and to a lesser extent the secondary amine and the corresponding aldehyde, a reaction which can be partially suppressed by tert-butyl alcohol. These data suggest that O-transfer [aminoxide plus O₂(¹Δ_g)] is in competition with an electron transfer which leads to the amine radical cation and an ozonide radical. In water, the latter gives rise to ˙OH which further reacts with the amine (and ozone). The amine radical cation deprotonates at a neighbouring carbon. The resulting radical adds dioxygen. Subsequent elimination of O₂˙⁻ and hydrolysis of the Schiff-base thus formed leads to the secondary amine and the corresponding aldehyde. In its reaction with ozone, O₂˙⁻ yields further ˙OH. Their reaction with the amines leads to the same intermediate as the free-radical pathway of ozone does, i.e. induces a chain reaction. This is interfered with by tert-butyl alcohol at the OH-radical stage. When complexed to Fe(III), EDTA reacts only very slowly with ozone (k = 330 dm³ mol⁻¹ s⁻¹). This explains why EDTA is not readily removed by ozonation in drinking-water processing.