Structure biuret

Urea and biuret type linkages were all characterised. Biuret structures were predominant when the moisture content was low, gradually being replaced by urea linkages when there was higher moisture content, the formation of urethane and amine moieties also occurred at intermediate moisture contents. This is in contrast to the previous study [17] where only a small amount of biuret linkages were detected. This may be due to the presence... 

Formation of Substituted Biuret Structures. By analogy with the well-known formation of biuret by heating urea ... 

The analogous biuret structure can be produced when moisture decomposes some isocyanate to the corresponding amine and the latter adds to two intact isocyanates ... 

In most cases, the allophanate reaction is an undesirable side reaction that can cause problems, such as high-viscosity urethane prepolymers, lower pot lives of curing hot-melt adhesives, or poor shelf lives of certain urethane adhesives. The allophanate reaction may, however, produce some benefits in urethane structural adhesives, e.g., additional crosslinking, additional modulus, and resistance to creep. The same may be said about the biuret reaction, i.e., the reaction product of a substituted urea linkage with isocyanate. The allophanate and biuret linkages are not usually as thermally stable as urethane linkages [8]. 

If an isocyanate group reacts with a hydrogen within the polyurea structure, a branching point is formed, a biuret group ... 

Its oxidation to cyanuric acid (10) and its hydrolysis to biuret (11) and formic acid is also in agreement with the new formulation of allantoxaidine as a triazine derivative. A very strained explanation would be required to make these reactions conform to the original structure. 

Final structure determination of the monomethyl derivatives was made possible only on performing straightforward syntheses of the compounds, analogous to syntheses of allantoxaidine. The 1-methyl (19) and 3-methyl (20) derivatives were prepared by cyclization of 1- and 3-methyl biuret,respectively. The 3-methyl derivative was also obtained by cyclization of A ,A -bis(methylcarbamyl)formami-dxne. The 1-methyl derivative was the same as the product obtained by Biltz on methylation of silver salt of allantoxaidine with methyl iodide. The same product is formed primarily during methylation of dioxotriazine with diazomethane. An excess of this agent produces the expected 1,3-dimethyl derivative (21). These results of alkyl-... 

Works on the oxidation of uric acid has unequivocally established the triazine structure > ° (9) of oxonic acid. This is further confirmed by the straightforward synthesis described by Piskala and Gut. ° The reaction of biuret (11) with potassium ethyloxalate yielded a potassium salt (24), that with ethyl oxamate, the amide of oxonic acid (25). Both these compounds were converted to 5-azauracil. An analogous reaction with diethyloxalate which should produce an ester of oxonic acid resulted in a mixture of urethane and parabanic acid, however. 

From these, prepolymers are prepared where the diisocyanates may be completely reacted as in the case of the urethane oils which resemble the oil-modified alkyds but have urethane (—NHCOO—) links in place of the ester (—COO—) links of the alkyds, or where one only of the isocyanate groups is combined, leaving the other to participate in crosslinking reactions. Such a reactive prepolymer is the biuret that may be prepared from hexamethylene diisocyanate, has the following structure ... 

Urea acts not only as an ammonia source but also forms decomposition products, such as biuret and higher condensation products. 14C labeling has indicated that the carbon atom of the urea molecule is not incorporated into the phthalocyanine structure. Employing a phthalic anhydride molecule bearing one radioac-tively labeled carbonyl function affords labeled copper phthalocyanine and phthalimide (as a side product), while the liberated carbon dioxide was found not to show any radioactivity. Labeled carbon dioxide, on the other hand, has been obtained in corresponding experiments using 14C labeled urea. 

Structural Chemistry of the Free Ligands, Biuret, Biguanide... 

Lindoy et al. [16], have recently investigated the perturbation to the biuret lattice (Fig. 1) resulting from the introduction of [Cu(dmca)2]2+ cations [dmca= di(methoxycarbimido)amine], Br anions, MeOH and MeCN molecules. An alternating layered structure is formed in which one layer [stacked parallel to (0 0 1) at zlc=0.00 and 0.50] comprises bu molecules and Br anions (Fig. 9) and the other [stacked parallel to (0 0 1) at z/c=025 and 0.75] comprises [Cu(dmca)2]2+... 

This addition reaction proceeds readily and quantitatively. Side reactions can give amide, urea, biuret, allophanate, and isocyanurate groupings, so that the structure of the product can deviate from that above such side reactions are sometimes desired (see Sect. 4.2.1.2). 

A reaction of protein with the Cu(II) ion, which forms a violet Cu chelate, is well known as a biuret reaction. Studies of this reaction65 show that Cu chelate formation was hindered for polypeptides which assumed a helical or -structure. 

The reaction has been extended to the synthesis of the analogous 3-alkylthio heterocycles (123) from the appropriate dithiobiurets (122).132 1-Substituted 2,4-dithiobiurets (125), unlike their monothio-biuret analogs (118), are unsuitable as precursors of 1,2,4-thiadiazoles, since in any oxidation the two thiol groups may be expected to react preferentially, resulting in the formation of cyclic disulfides (126). The formulation of such oxidation products as 1,2,4-thiadiazoles has indeed been discussed by Bambas1 however, the available evidence133 favors the cyclic disulfide structure.134... 

It follows from the structure of cured KL-3 that secondary reactions are possible in the formation of the network manifested in the formation of allophanate and biuret... 

Chemically and structurally biguanide1 is closely related to biuret, for it may be viewed as derived from the substitution of both the oxygen atoms of the biuret by imino (NH) groups. 

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