Ammonia-dimethylchloramine system: kinetic approach in an aqueous medium and comparison with the mechanism involving in liquid ammonia
Résumé
After an exhaustive study of the system ammonia-dimethylchloramine in liquid ammonia, it was interesting to compare the reactivity of this system in liquid ammonia with the same systeme in an aqueous medium. Dimethylchloramine prepared in a pure state, undergoes dehydrohalogenation in an alkaline medium: the principal products formed are N-methylmethanimine, 1,3,5-trimethylhexahydrotriazine, formaldehyde and methylamine. The kinetics of this reaction was studied by UV, GC and HPLC as a function of temperature, initial concentrations of sodium hydroxide and chlorinated derivative. The reaction is of the second order and obeys an E2 mechanism (k1 = 4.2 x 10-5 M-1 s-1, ΔH°# = 82 kJ mol-1, ΔS°# = - 59 J mol-1 K-1). The oxidation of unsymmetrical dimethylhydrazine by dimethylchloramine involves two consecutive processes. The first step follows a first-order law with respect to the haloamine and the hydrazine, leading to the formation of an aminonitrene intermediate (k2 = 150 x 10-5 M-1 s-1). The second step corresponds to the conversion of aminonitrene into formaldehyde dimethylhydrazone at pH=13). This reaction follows a first-order law (k3 = 23.5 x 10-5 s-1). The dimethylchloramine/ammonia interaction corresponds to a SN2 bimolecular mechanism (k4 = 0.9 x 10-5 M-1 s-1, pH = 13 and T = 25°C). The kinetic model formulated on the basis of the above reactions shows that the formation of the hydrazine in an aqueous medium comes under strong competition from the dehydrohalogenation of DMC and the oxidation of the hydrazine formed by the original chlorinated derivative. A global model which explains the mechanisms both in an anhydrous and in an aqueous medium was elaborated.
Domaines
Chimie inorganique
Origine : Fichiers produits par l'(les) auteur(s)
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