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AUTOOXYDATION OF AMINES: ON INVESTIGATION OF THE MECHANISM OF THE BASE-CATALYZED OXIDATION OF AMINES IN AQUAEOUS SOLUTION WITH MOLECULAR OXYGEN

Objective


The reaction mechanism of the autoxidation and dealkylation reaction of tertiary alkyl amines induced by base in aqueous solution in the presence of molecular oxygen has been studied. Electron paramagnetic resonance (EPR) experiments were carried out to provide direct evidence for the mechanism involved in the dealkylation and oxidation process and the conditions leading to the dealkylation were explored. It was found that the pH has to be greater than, or equal to, 8 for the process to occur. It was proved that molecular oxygen (normally from the atmosphere) is involved. The presence of a beta-methylene group on one of the alkyl chains was found to be a requirement. A qualitative measurement of the rate of reaction was performed by a stop flow experiment and it was shown that the EPR signal reaches its own maximum intensity after a few seconds. Experimental results suggest that the electron withdrawing or donating effect in the beta-substituents is very crucial. Experiments using spin trapping and labelling were inconclusive as were attempts to show the presence of superoxide dismutase. Product studies were carried out and the results suggest that radical decomposition is a very minor pathway.

It is concluded that the mechanism for nitroxide formation probably involves electron transfer from the unprotonated amine to oxygen, to give 2 charged species stabilized in the aqueous environment. It is unlikely that the reaction mechanism involves the prior formation of nitroxides or protonated amines even though these species undergo subsequent attack. Given that a relatively slow, minor reaction of the intermediate radical cation may be involved, 2 potential mechanisms may be suggested to account for most of the observations: reaction with base
It is proposed to determine the mechanism, and the controlling structural and electronic features, of the rapid autooxidation of certain tertiary amines under mild conditions (in oxygenated aqueous solution at pH ca. 10.). The approach will involve both a study of the kinetics and products of reaction of a variety of substrates, as well as a spectroscopic investigation of short-lived intermediates (radicals, anions, peroxy species) with e.s.r. (flow, photolysis, and
spin-trapping).

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

UNIVERSITY OF BOLOGNA
Address
Viale Risorgimento 4
40136 Bologna
Italy

Participants (1)

University of York
United Kingdom
Address
Heslington -
YO1 5DD York