CATALYZED NITRATION OF AMINES: III. THE EASE OF NITRATION AMONG ALIPHATIC SECONDARY AMINES

A series of secondary amines, the proton-attracting ability of which had previously been determined, have been converted to their nitramines with nitric acid and acetic anhydride. The gradation in ease of nitration has been found to vary inversely with the proton-attracting ability of the amine. Nitration becomes so difficult at an amine strength corresponding to that of diethanolamine that nitric acid and acetic anhydride alone are ineffective; a chloride catalyst must be used. The amount of this catalyst must be increased as the proton-attracting ability of the amine becomes greater until a full equivalent is required for adequate yield from the strongest amine in the series, diisopropylamine. As the nitration in the series becomes more difficult, side reactions become apparent such as nitrosation, acetylation, and fission of the secondary amine to primary amine and aldehyde. The extent of nitrosation is dependent on the concentration of catalyst, although nitrosation is not catalyzed by presence of chloride. This implies that hydrogen chloride generates nitrous acid in the reaction mixture. Acetylation is independent of presence or concentration of catalyst, but it does not occur during the formation of dicyclohexylnitramine or diisopropylnitramine. This is thought to be owing to steric hindrance from the secondary alkyl groups in these amines. Since nitracidium perchlorate has been found to be ineffective as a catalyst for this nitration, it is doubtful that nitryl chloride is the active form of the catalyst except in so far as it exists in the form of chlorine nitrite. Evidence has accumulated to show that electropositive chlorine is the effective catalyst, and that it is formed by a modification of the aqua regia reaction.