Cyanamides trimerization

Urea is dehydrated to cyanamide which trimerizes to melamine in an atmosphere of ammonia to suppress the formation of deamination products. The ammonium carbamate [1111-78-0] also formed is recycled and converted to urea. For this reason the manufacture of melamine is usually integrated with much larger facilities making ammonia and urea. 

Alkylation with a vatiety of common alkyl halides oi sulfates gives stable dialkylcyanamides. However, the intermediate monoalkylated compounds usually cannot be isolated and cychc trimers or cotrimers with cyanamide ate obtained (13). The reaction can be carried out efficientiy in water or alcohol. Allyl chloride is an especially useful reagent, producing diallylcyanamide [538-08-9J (4). 

It will be noted that dicyanodiamide is the dimer of cyanamide and melamine is the trimer. 

Somewhat above their melting points the aminothiatriazoles decompose more or less violently. When they are heated in aqueous solution nitrogen and sulfur are formed together with a cyanamide (isolated by Freund and Schwarz as the trimeric melamines) [Eq. (18)]. With the unsubstituted 5-aminothiatriazole the reaction... 

Cyanamide itself is not a suitable raw material for the preparation of guanidine salts, for it is difficult to prepare and to purify, and it polymerizes on keeping. The evaporation of an aqueous solution of cyanamide yields the dimer, dicyandiamide, and the heating, or even the long keeping, of the dry substance produces the trimer, melamine. 

Melamine is formed by the trimerization of cyanamide and a hint was given in the chapter as to the mechanism of this process. Expand that hint into a full mechanism. 

Using the hint, we have the dimerization step and then we have a number of options to get to the cyclic trimer. As long as you use amines as nucleophiles and imines as electrophiles, the precise order of events doesn t matter very much. Cyclization to form a six-membered ring will beat addition of a fourth cyanamide molecule and thermodynamically the trimerization is very favourable as melamine is aromatic. 

Similar to urea (see Section 13.2), melamine can react with formaldehyde forming a thermosetting resin. Melamine is a trimer of cyanamide and contains three amino groups that offer six possible points of reaction. Since melamine can be considered a triamine of 1,3,5-triazine, this group of resins can be included in the class of polymers with heterocycles in their structure. The first stage of the reaction of melamine with formaldehyde can be written as follows ... 

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... 

There have been a great number of attempts to make wool resistant to shrinkage by masking the scales with a deposit of an insoluble polymer. Probably the earliest process to achieve a measure of commercial success was based on melamine which is produced by the trimerization of cyanamide ... 

Melamine, a trimer of cyanamide, was first prepared by Liebig in 1834. Melamine has become commercially popular due to its resins, also known as Melmac. ... 

The current standard process route to melamine uses urea as a starting material. Urea is heated in the presence of ammonia at 250—300°C and 4—20 MPa. The reaction probably involves the simultaneous dehydration and hydration of urea to form cyanamide and ammonium carbamate trimerization of the cyanamide then leads to melamine [10]. 

The process used by the former Standard Oil of Ohio (SOHIO currently BP America) involved electrolytic conversion of HCN to cyanogen bromide, and subsequent conversion to cyanamide by ammonolysis, then to melamine by trimerization ... 

Triazine-2,4,6-triamines (melamines) can be obtained by the trimerization of cyanamides. The parent compound, melamine, was first reported in 1834 as the product from heating a mixture of potassium thiocyanate and ammonium chloride.222 Starting from cyanamidc, the synthesis is achieved either in molten cyanamide above 150 °C,10 or by heating in a high boiling solvent.223... 

A proposed trimerization of disubstituted cyanamides occurs during the reaction of cyanogen chloride with diphenylamine the supposed intermediate diphenylcyanamide should trimerize to give hexaphenylmelaminc.232 There is a similar reaction when cyanogen chloride is treated with pyrrole.233... 

Although the reaction mechanism is not yet fully understood, cyanic acid or isocyanic acid seems to be the first decomposition product of urea. Trimerization of cyanic acid leads to cyanuric acid (see section on Cyanuric Acid, p 685). In the presence of catalysts, cyanic acid disproporlionates into carbon dioxide and cyanamide, which then trimerizes to melamine. Thus, decomposition products, and nol urea itself, cyclotrimerize to provide the 1,3,5-triazine rings. These reactions form the basis for the industrial production of cyanuric acid286-301 and melamine.302-315... 

Finally this is the place to mention the hydrogen cyanide derivatives that react with themselves to give dimers, trimers, or polymers. In the presence of acidic catalysts53 cyanogen chloride trimerizes very easily to cyanuric chloride cyanamide is dimerized smoothly by alkali to dicyanodiamide 54 and trimerization to melamine is suitable as a preparative method starting from dicyanodiamide and ammonia under pressure.55... 

Cr3Tstalline cyanamide (mp, 42 C), when heated to 160 C, polymerizes with explosive violence to melamine or 2,4,6-triamino-l,3,5-triazine. In practice, the trimer is obtained from the dimer, dicyandiamide, by heating with ammonia. Scholl and Davis obtained melamine as the principal product by heating dicyandiamide with 2 parts of liquid ammonia at... 

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 ... 

Melamine, dicyandiamide (or cyanoguanidine), and cyanamide are related. The first is considered the trimer and the second the dimer of the third. All contain 66% N and provide fire-retardant properties to resin formulas by releasing nitrogen when burned or charred. 

Melamine (trimerization of cyanamide), formaldehyde Stepwise polycondensation Dinnerware, table tops, coatings... 

The byproducts biuret and cyanuric acid are observed in the largest quantity at low and moderate temperatures up to about 300 °C [36] however, additional byproducts form in smaller amounts as shown in Fig. 16.2 [11, 36, 37]. Substitution of the OH groups of cyanuric acid by NH3 yields the more stable triazines ammelide, ammeline, and melamine [36]. Ammelide may also be formed by the reaction of biuret with HNCO, if water is eliminated instead of NH3 [11]. Melamine may also be produced by trimerization of cyanamide [36] since cyanamide is an intermediate in melamine synthesis [35]. Cyanamide was not observed in a... 

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