The formaldehyde-ammonia reaction has been examined in DzO solvent with the aid of 'H and 13C NMR spectroscopy. Reaction intermediates, including 1,3,5-hexahydrotriazine (2) and 1,3,5,7-tetraazabicyclo[3.3.l]nonane (6), are observed, and the mechanisms of their formation and conversion into hexamine are discussed. Solutions previously reported to contain 2 alone have been shown to be complex mixtures.The formaldehyde-ammonia reaction under appropriate conditions leads quantitatively to hexamine (1, eq lL3 Despite HN-O n n HN-NH 1 its importance, it has received limited detailed study; understanding of this complex reaction sequence is tentative.In the present work 'H and 13C NMR spectroscopy have been employed to follow the reaction and identify some intermediates.The most extensive recent study of the formaldehydeammonia reaction is that of Richmond, Myers, and Wright.4 From chemical evidence they concluded that 1,3,5-hexahydrotriazine (2) is a reaction intermediate. This result is in I I H H 2 3, R = alkyl 3a, R = CH, agreement with the early reports of Duden and Scharff,j who first correctly assigned structure 1 to hexamine. The present study provides NMR evidence for formation of 2. Our previous studies on reaction of rz-alkanals with ammonia showed that the ultimate reaction products are usually 2,4,6-trialkyl-1,3,5-hexahydrotriazines (3).'s6 The kinetics of the formaldehyde-ammonia reaction have been examined by several workers.7
Results and Discussion 1H and I3C NMR Spectra of Formaldehyde-AmmoniaSolutions. In the present work lH and 13C NMR spectra of various D20 solutions of ammonium-d4 hydroxide and formaldehyde a t 25 "C were determined a t intervals. The 'H spectra clearly reveal the rapid formation of hexamine and the presence of reaction intermediates (Figure 1). The methylene signals are grouped in two principal regions. Rather broad signals near 6 4.5 are attributed to NCHPO-type methylenes; compare these signals with those of 1,3-perhydrotriazine (4) in which the C-2 methylenes appear at 6 4.558a (Table I). Sharper, more defined signals appear near 6 3.5-4.0 and represent NCHPN-type methylenes (with the exception of hexamine itself which appears as a singlet at 6 4.75).9 Altering the formaldehyde-ammonia molar ratio from 1:4 to 3:2 provides the same products in each experiment, and their relative distribution at a given time is changed but slightly. 4 5 1,3,5-Hexahydrotriazine (2) exhibits a sharp singlet at 63.95. This value may be compared with the C-2 methylene signal of 1,3-hexahydrodiazine (5, 6 3.73)8b and the ring methine signal of 2,4,6-trimethyl-1,3,5-hexahydrotriazine [3a, 6 3.80 (q)] in D20 solvent (Table I). Hexahydrotriazine forms rapidly and ultimately is the principal species present other than hexamine. Initially, its concentration is much greater than that of hexamine, a fact supported by low temperature (-10 O C ) I3C NMR spectra observations; e.g., the peak height ratio of 2/1 equals 3 after 1 h.Signals attributed to 1,3,5,7-tetraazabicyclo[3.3.l]nonane (6) persist in the lH NM...
