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Mechanism of permanganate oxidation of aliphatic amines Wei, Min-Min

Abstract

The mechanism of the permanganate oxidation of benzylamine and benzylamine-α-d₂ has been investigated in the pH region 2 to 14. The deuterium isotope effect, kH/kD, was found to be 7-0 from pH 8 to 10.7. The absence of salt effects and the observation that the oxidation rate followed the ionization process of the benzylammonium ion show that benzylamine is oxidized via the neutral molecule. Beyond pH 12 the rate of oxidation was found to be directly proportional to the hydroxyl ion concentration. A study of 11 meta- and para-substituted benzylamines shows excellent Hammett correlation with σ+ for 8 of the compounds, with the p-nitro, m-nitro, and m-trifluoromethyl derivatives deviating considerably from the Hammett plot. ρ+ was -0.28. The correlation with σ+ and the very negative ΔS≠ values obtained for the oxidation reaction show that the transition state must be an ionic one involving extensive charge separation. The rate-determining step is consistent with hydrogen transfer from benzylamine to permanganate but the choice of a hydride ion transfer mechanism or a hydrogen atom transfer mechanism cannot be unequivocally made from the results obtained in this thesis. For the oxidations in the highly alkaline regions beyond pH 12, termolecular mechanisms are suggested. Whereas O-alkylation protects alcohols from oxidation by permanganate, N-alkylation greatly increases the oxidation rate of amines. However, N-acylation tends to protect the amine from oxidation. Limited studies of the kinetics of permanganate oxidation of ammonia, cyclohexylamine, N,N-dimethylbenzylamine, and (-)- α-methylbenzylamine were carried out to compare their rates of oxidation with those for benzylamine. The mechanism of the permanganate oxidation of t-butyl -amine to t-nitrobutane was studied from pH 8 to 12. As in the benzylamine oxidation t-butylamine is oxidized via its neutral molecule. The absence of an α-hydrogen and the absence of a substantial isotope effect in t-butylamine-ND₂ suggest that the mechanism involves an oxidative attack on the nitrogen by permanganate. The formation of a quaternary hydroxylamine derivative between t-butylamine and permanganate has been proposed for the rate-determining step. The kinetics of permanganate oxidation of benzylamines at -10° in frozen H₂O and D₂O has been investigated from pH 7.5 to 9.3. The reaction order remains unchanged from that at 25°. The rates of oxidation are more than 10 times greater in ice at -10° than in liquid water at -10°, and this acceleration increases with decreasing pH. For p-nitrobenzylamine there is only a 3-fold increase in rate in going from liquid to frozen water at -10°. An apparent pKBH+ of 8.70 was obtained for benzylamine at -10° in ice. No change in the rate was observed when the oxidation was carried out in frozen D₂O and when the ionic strength of the medium was increased. The accelerating effects observed in the frozen systems are discussed in terms of pH, concentrating effect of the freezing, and the orientating effect of the ice structure.

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