Hexamethylenetetramine Formation, mechanism

Fig. 9.24. Mechanism for the formation of hexamethylenetetramine from ammonia and formalin.

Photolysis in the vacuum ultraviolet (1800-2400 A) resulted in the formation of nitrogen, ammonia and hexamethylenetetramine, in yields similar to the thermal decomposition. Hexamethyenetetramine was also a major product in photol-yses above 2400 A and it seems probable that both the thermal and photochemical decompositions proceed by the same mechanism. The solution photolysis of methyl azide has recently been examined and the general kinetic features are similar to the vapor-phase photolysis and thermolysis. Thus increases with CH3N3 concentration to a maximum of 2, and hexamethylenetetramine and N2 were the major products. It is believed that in solution CH3N does not isomerize but condenses with CH3N3 in several steps to produce hexamethylenetetramine. 

In 1913, Marcel Sommelet (1877-1952) of the Faculty de Pharmacie of Paris University published his seminal article in Comptes Rendus with the title Decomposition of Alkyl Halide Addition Products of Hexamethylenetetramine, where he delineated the formation of aldehydes by treatment of benzyl halides followed by hydrolysis.4,5 In 1954, S. J. Angyal published a review in Organic Reactions, where he summarized the mechanism, scope, and limitations of the Sommelet reaction.1 Angyal s review ignited a flurry of research and the Sommelet reaction became a well-known standard for the transformation of benzyl halides to the corresponding benzaldehydes. 

The earlier process for making RDX consisted of reacting hexamine (hexamethylenetetramine) or its dinitrate salt with 98-100 per cent nitric acid. The yields of this method are not good because the formation of RDX is accompanied by the formation of formaldehyde, which is oxidized by the nitric acid. Greatly improved yields (70-80 per cent) can be obtained by means of the Bachman process, which employs hexamine, 98 per cent nitric acid, ammonium nitrate, and acetic anhydride. In the Bachman process a by-product having the structure I is also formed in yields up to 10 per cent. The mechanism of these reactions is not thoroughly understood. 

The only known adamantane containing more than three endocyclic ring nitrogens is hexamethylenetetramine (hexamine, 1,3,5,7-tetraaza-adamantane, 4). It is readily prepared by reaction of formaldehyde and ammonia and was first described by Butlerov in 1859 (Ref. 82). Its preparation and properties have been reviewed (Ref. 83). The mechanism of formation from formaldehyde and ammonia has been studied (Refs. 84,85). Hexamine is employed as a reactant in the synthesis of RDX (3) and HMX (1). 

Figure 7 Mechanism of hexamethylenetetramine decomposition leading to the formation of anion-stabilized reactive iminomethylene bases. The same bases can be formed by reaction of ammonium salts such as ammonium sulfate and formaldehyde and constitute a metastable intermediate between hexamine and final decomposition products, and vice versa (after refs 26, 37-39).

Fe(N03)3 solutions. Precipitates formed from 6.25 x I0 and 6.25 x lO M Fe(N03)3 solutions at 24 and 55 C showed the characteristic pattern of a-FeOOH, whereas in 6.25 x lO M, Fe(N03)3 solution at 24°C an amorphous precipitate was formed. a-Fe203 precipitated at 90 °C. The same authors proposed (Fig. 23.2) the mechanism of hydrolysis-precipitation from Fe(N03)3 solutions, consisting of the hydrolysis to monomers (A) and dimers (B), reversible, rapid growth of small polymers (C), the formation of slowly reacting large polymers (D), and polymerization and oxolation with the formation of a solid phase (E). XRD, FT-IR, and Mbssbauer spectroscopies were used to characterize hydrolytical solid products obtained by the hydrolysis of O.l M Fe(N03)3 aqueous solutions at 90°C in the presence of hexamethylenetetramine (HMTA) [24,25]. HMTA generates OH ions at an elevated temperature in accordance with the chemical reactions... 

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