Cyanuric acid, tautomerism

IR spectroscopy has proved to be particularly valuable in the study of tautomeric forms of the triazines. Thus, cyanuric acid was shown to exist in the keto form by the presence... 

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... 

Cyanuric acid exists in two tautomeric forms corresponding to keto-enol tautomerism in carbonyl compounds. The keto form predominates, and most of the reactions of cyanuric acid have their counterparts in the chemistry of the cyclic imides. Many of the reactions involve the replacement of all three imido hydrogens (Scheme 31). Usually, the reaction cannot be controlled to produce the mono- or di-substituted isocyanurates specificially, but there are exceptions, e.g. the reaction between cyanuric acid and aziridine (Scheme 31) (B-79MI22001, 63JOC85, 63AHC(2)245). 

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. 

Melamine is a very thermoresistant aromatic heterocyclic compound, with three -NH2 groups, which makes it very attractive for use as a starter for polyol synthesis. Unfortunately, melamine is very difficult to directly alkoxylate with PO or EO. This difficulty is because of the amidic structure (melamine is the amide of cyanuric acid) and because of the tautomeric forms (characteristic to all amides) [31] ... 

Similarly to melamine, cyanuric acid has two tautomeric forms [40] ... 

The IR spectra of unsubstituted 1,3,5-triazine and a large number of substituted derivatives have been reported.34,35 Symmetrically substituted triazines25 show characteristic bands (Table 4). Dichloro-1,3,5-triazines and cyanuric chloride36 can be identified by a characteristic C-Cl stretching band at 840cm"1. Triazines with substituents which give rise to tautomeric forms can be readily studied by IR spectroscopy. Cyanuric acid, for instance, exists in the oxo form because peaks at 1719, 3060 and 3210cm I are observed.37... 

The chemistry of 1,3,5-triazinols has been reviewed.10,14,18,21 Cyanuric acid (1,3,5-tri-azine-2,4,6-triol) where the oxo form of the oxo-enol tautomerism predominates, is a thermally very stable compound (up to 500 JC). Thus, the chemistry is mostly based on the cyclic imide portion and the replacement reactions of the imido hydrogen atoms (halogenation, alkylation). 

The prototropic tautomerism of 1,3,5-triazinones, 1,3,5-triazinethiones, and amino-1,3,5-triazines has been discussed in detail . Cyanuric acid was shown to exist as the trioxo structure by UV and x-ray data. l-Methyl-l,3,5-triazine-2,4,6-(l//,3/f,5/f)-trione has the trioxo form as preferred tautomer (see Section 6.12.3.5) <86CB83>. Compounds (19), (20) and (21) were also shown by IR spectroscopy to show the oxo forms as preferred tautomers carbonyl bands were found at 1677-1730 cm-. ... 

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 ... 

The trihydroxy compound rearranges to the tautomeric tri-oxo form, again a predictable property as the compound is derived from a pi-deficient ring system. The tri-oxo compound has measurable acidity and is known as cyanuric acid. Although it can be prepared by hydrolysis of cyanuric chloride, a better process involves urea as a starting material. Thermolysis causes loss of ammonia and gives isocyanic acid, which trimerizes to cyanuric acid (Scheme 9.81). 

The starting product for the manufacture of N-chloro isocyanuric acid derivatives (III) is isocyanuric acid (II) which is in a tautomeric equilibrium with cyanuric acid (I). 

It is difficult to trace when and how exactly tautomerism was discovered. In the first tautomeric book, written by Baker [25], the priority is given to Berzelius, who in 1832 used the term metamerism to explain reciprocal conversion of cyanic and cyanuric acid. In Ingold s [[26], Chapter 11] review on tautomerism, ethyl acetoacetate, discovered in 1863 by Geuther [27], is mentioned as the first tautomeric compound described (Figure 1.7). 

Trithiocyanuric acid exists mainly in the trithione tautomeric structure. IR bands at 1115 cm-, 1330 cm, and 1540 cm- were assigned to —N—C=S and bands at 2900 cm- and 3200 cm" to NH <66JCS(C)909>. The work of Tosato (see Section 6.12.3.2, Table 1) has provided C NMR data for the many possible fixed forms of cyanurates and thiocyanurates. 

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