Melamine-cyanuric acid derivatives

Figure 10.51 Melamine-cyanuric acid derivatives arranged according to their relative stability as measured by the HBI(N — 1) parameter. (Reprinted with permission from Section Key Reference 1995 American Chemical Society).

The complex between melamine and cyanuric acid (1 1) was reported in the literature in the late 1970s, but it was only in the early 1990s that the contributions from Whitesides and the concept of self-assembly popularized these systems [45]. Whitesides and co-workers reported the formation of tapes (Fig. 11.11), crinkled tapes and cyclic hexamers (rosettes) formed between barbituric acid and N,N -bis(p-substituted phenyl)melamine [46], In this they effectively blocked one face of melamine and, by manipulating substituents at the para position, different structures were obtained. Whiteside s putative suggestion that melamine/cyanuric acid formed an extended array (Fig. 11.12) was confirmed recently by Rao et al. with the crystal structure [47]. Hamilton and coworkers reported the crystal structure of a 5-substituted isophthalic acid derivative, which forms a cyclic aggregate held together with six pairs of hydrogen bonds, which in a way resembles the trimesic acid (Fig. 11.13) [48]. 

N. Kimizuka, T. Kawasaki, K. Hirata, T. Kunitake, Supramolecular Membranes. Spontaneous Assembly of Aqueous BUayer Membrane via Formation of Hydrogen Bonded Pairs of Melamine and Cyanuric Acid Derivatives ,]. Am. Chem. Soc., 120,4094 (1998)... 

Figure 5 Possible complementary hydrogen-bond networks formed from melamine and cyanuric acid derivatives, (a) Linear tape, (b) crinkled tape, and (c) circular rosette.

Melamine (2,4,6-triamino-l,3,5-triazine) is a relatively non-toxic compound with acute toxicity observed in rodents, with oral lethal doses (LD50) of 3100 mg kg and 3900 mg kg in male rats and male mice, respectively." In another study, a 125 mg kg oral dose of melamine showed only diuretic effects in dogs." Toxicity studies of cyanuric acid derivatives (e.g. sodium cyanurate) at 700 mg kg and 2200 mg kg doses in rats and mice, respectively, resulted in bladder calculi and bladder epithelial hyperplasia, with no other adverse effects." In other studies, it was observed that dietary monosodium cyanurate administered in dogs for 6 months at 0.8% concentration produced no adverse effects, although at 8% for a longer period of 16 to 24 months it produced kidney fibrosis and fical dilation. This was attributed to the low solubility of sodium cyanurate, due to which it precipitated at higher concentrations in urine, leading to renal dysfunction." ... 

The use of molecular recognition was also extended to silylated melamine or cyanuric acid derivatives that formed complexes through hydrogen bonding [62]. 

Hydrolysis of polyamide-based formulations with 6 N HC1 followed by TLC allows differentiation between a-aminocaproic acid (ACA) and hexamethylenedi-amine (HMD) (hydrolysis products of PA6 and PA6.6, respectively), even at low levels. The monomer composition (PA6/PA6.6 ratio) can be derived after chromatographic determination of the adipic acid (AA) content. Extraction of the hydrolysate with ether and derivatisa-tion allow the quantitative determination of fatty acids (from lubricants) by means of GC (Figure 3.27). Further HC1/HF treatment of the hydrolysis residue, which is composed of mineral fillers, CB and nonhydrolysable polymers (e.g. impact modifiers) permits determination of total IM and CB contents CB is measured quantitatively by means of TGA [157]. Acid hydrolysis of flame retarded polyamides allows to determine the adipic acid content (indicative of PA6.6) by means of HPLC, HCN content (indicative of melamine cyanurate) and fatty acid (indicative of a stearate) by means of GC [640]. Determination of ethylene oxide-based antistatic agents... 

In general, the thiones are not very reactive. Trithiocyanuric acid decomposes on heating above 200 °C, and is hydrolyzed to cyanuric acid and hydrogen sulfide in hydrochloric acid at 200 °C. Melamine is produced in 40% yield on heating trithiocyanuric acid with ammonia under pressure (59HC(l3)l,p.l05). Garmaise has reported the formation of the disulfide derivatives (52) (66CJC1801). 

FIGURE 4.3. Molecular structures and schematic representations of tetra- hexa- and octamelamines 2, 3 and 4, respectively, and di(melamine-cyanurate) 5. The molecular structure of DEB and different isocyanuric acid derivatives (CYA) are also shown. 

Zerkowski, J. A., Seto, C. T. and Whitesides, G. M. (1992). Solid-state structures of rosette and crinkled tape motifs derived from the cyanuric acid-melamine lattice. J. Am. Chem. Soc. 114, 5473. 

The tautomerism of 1,3,5-triazines has been reviewed previously (B-76MI22000, pp.138,152,168). NMR and IR studies have shown that cyanuric acid exists mainly in the oxo form. Although the cyanurates and isocyanurates are the two major derivatives, there is no doubt that compounds with both types of functional group present in the same molecule are possible (e.g. Scheme 36). Trithiocyanuric acid exists predominantly in the thioamido form. In contrast, melamine exists in the amino form. /S-Oxoalkyl-1,3,5-triazines exist mainly in the enol form, and thus they undergo ene type reactions with dienophiles. 

J.A. Zerkowski, C.T. Seto, G.M. Whitesides, Solid State Structure of Rosette and Crinkled Tape Motifs Derived from the Cyanuric Acid and Melamine Lattice , J. Am. Chem. Soc., 114,5473 (1992)... 

Whitesides et al. have prepared a series of large self-assembled structures containing multiple host-guest interactions. These assemblies were built around primary interactions between cyanuric acid (CA) and melamine (M) derivatives as illustrated by 18 (and were designated CA-M) where CA contains similar hydrogen... 

The cyanogen halides polymerize on standing to the trimers, the cyanuric halides. Cyanamide is converted into the corresponding trimer, melamine, on heating to about 150°C. Isocyanic acid, however, polymerizes far more readily. If urea is distilled, isocyanic acid is formed but polymerizes to cyanuric acid, (NCOH)3, a crystalline solid, the vapour of which, on rapid cooling, yields isocyanic acid as a liquid which, above 0 C, polymerizes explosively to cyamelide, a white porcelain-like solid. This latter material is converted into salts of cyanuric acid by boiling with alkalis. These reactions are summarized in Chart 21.1. Cyanuric derivatives... 

Figure 9. Nanorods based on the cyclic hydrogen-bonding motif of cyanuric acid and melamine in bifunctional calixarene derivatives 18 and 19. (Reprinted with permission from ref 106. Copyright 1999 American Chemical Society.)...

Triazines are one of the oldest known compound classes in organic chemistry. First reports date back to 1776 when cyanuric acid (l,3,5-triazine-2,4,6-triol 1,3,5-triazine-2,4,6(l//,3//,5f/)-trione) was obtained by the pyrolysis of uric acid.1 The same method was also utilized in 1820 2 however, triazine compounds were probably first prepared in 1704 upon trimerization of cyanide derivatives when the Berlinerblau -complex salt, the first known cyano compound, was discovered.468 Cyanuric chloride was synthesized in 1828 from hydrogen cyanide and chlorine.3 Another method, discovered in 1834, involves treatment of potassium thiocyanate with chlorine. When heated, cyanuric chloride is obtained.4 Melamine (1,3,5-triazine-2,4,6-triamine) was also prepared in 1834 by heating potassium thiocyanate with ammonium chloride.5 Although 2,4,6-substituted 1,3,5-triazine derivatives were identified very early, the unsubstituted parent compound was not synthesized before 1895.6 At that time, however, the isolated compound was assigned to a dimeric species and not to the trimeric hydrogen cyanide. This was finally proven much later."... 

Triazine derivatives play an important role as polymers, pesticides (herbicides), reactive dyes, optical brighteners and LIV absorbers. Some triazine compounds also exhibit important physiological properties in pharmaceutical applications. The parent compound has no industrial application, although it can be used to prepare a variety of other heterocyclic systems because it is a preparative alternative to hydrogen cyanide. The starting materials for the various applications are often melamine, cyanuric chloride and cyanuric acid. 

The majority of 1,3,5-triazine derivatives are solids, and liquids are seen with the introduction of fluorine into the ring or with other highly fluorinated substituents. Cyanuric fluoride has b.p. 73-74°C cyanuryl chloride is a solid, m.p. 146°C. Cyanuric acid, thiocyanuric acid and melamine have... 

Melamine (2,4,6,-triamino-1,3,5-triazine) is obtained by trimerization of cyanamide or commercially by thermal cyclocondensation of urea at 400°C with elimination of NH3 and CO2. Polycondensation of melamine with formaldehyde produces melamine resins that are used as plastics, glues and adhesives. Cyanuric acid 15 (2,4,6-trihydroxy-1,3,5-triazine) was obtained by Scheele (1776) from pyrolysis of uric acid and was the first 1,3,5-triazine derivative known. Cyanuric acid is synthesized by trimerization of isocyanic acid and is tautomeric with isocyanuric acid 16 ... 

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