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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(1Δg)] 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(1Δg)] 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 neighboring 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.