Cyanamide dimerization

Marqueyrol and Loriette A process for making nitroguanidine, an explosive. Cyanamide dimer is converted to guanidinium sulfate by heating with sulfuric acid this is then nitrated with nitric acid. See also Welland. 

JMethylphosphonc Acid—Cyanamide System. In another system (65), based on methylphosphoric acid [993-13-5] (MPA) and cyanamide [420-04-2] one or more of the hydroxyls in MPA or in its dimer react with cellulose and the water is taken up by the cyanamide, forming urea ... 

Properties. Cyanamide [420-04-2] also called carbamodiimide or carbamic acid nitrile, crystallises from a variety of solvents as somewhat unstable, colorless, orthorhombic, dehquescent crystals (2). Dimerization is prevented by traces of acidic stabilizers such as monosodium phosphate and by storage at low temperature. 

Reactions. Reactions of cyanamide are either additions to the nitrile group or substitutions at the amino group. Both are involved in the dimerization to dicyandiamide. 

Dimerization involves addition of the cyanamide anion to the nitnle group of an undissociated molecule to give the anion of cyanoguanidine, or dicyandiamide. This reaction takes place most readily at pH 8—10 where the reactants are present in favorable proportion. The product is a weaker acid than cyanamide and is protonated at once with generation of a new cyanamide anion. 

Most of the reactions occurring at the amino group of the cyanamide molecule requite the anionic species, —N=C=N or HN C=N, sometimes in equivalent amount and occasionally as provided by base catalysis. Therefore, the process conditions for dimerization should be created to avoid the use of any metal salt, such as mono sodium phosphate (4). 

Additions. The addition reactions of ammonia and amines to the cyanamide nitrile group have been thoroughly studied (15). For optimum conditions, the reaction should be carried out ia an aqueous medium at about 140°C. Gradual addition of the cyanamide to the amine salt minimises dimerization. 

Handling and Storage. Cyanamide solution dimerizes to dicyandiamide and urea with the evolution of heat and a gradual increase in alkalinity accelerating the reaction. Storage above 30°C without pH stabilizer leads to excessive dimerization and can result in violent exothermic polymerization. Cyanamide should be stored under refrigeration and the pH tested periodically. Stabilized cyanamide can be kept at ambient temperature for a few weeks. 

Properties. Dicyandiamide (2) (cyanoguanidine [461-58-5]) is the dimer of cyanamide and crystallizes ia colorless monoclinic prisms. It is amphoteric, and generally soluble in polar solvents and insoluble in nonpolar solvents. Its properties are Hsted in Table 3. 

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

Hutchins, M. G. K. et al., Tetrahedron Lett., 1981, 22, 4599-4602 Interaction of sodium hydrogen cyanamide and tert-butyl hypochlorite in methanol at —50°C to ambient temperature forms cyanonitrene. Isolation gave an orange dimeric residue which exploded on two occasions. 

Cyanamide and its aromatic derivative, such as 4,4 -methylene bis(phenyl cyanamide), were reported to cure an epoxy resin at elevated temperatures.(4) It is also well known that the dimer of cyanamide (dicyandiamide) is the most important epoxy curing agent in one-package epoxy compounding.(5) Unfortunately, this dimer precipitates from the dispersion causing uneven mixing upon standing. 

At alkaline pH, however, the dimerization of cyanamide to dicyanodiamide is favored. Dicyanodiamide acts as a corrosion inhibitor ... 

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. 

It is made by dimerizing cyanamide in basic aqueous solution, and is a colorless solid melting at 208°C. Dicyandiamide is soluble in polar solvents, but at room temperature is insoluble in bisphenol A epoxy resins. It can be made into a very fine powder and milled into epoxy resins to form stable dispersions. Because the dicy is insoluble in the epoxy, the only possible reaction sites are at the particle surfaces. Although some reaction certainly occurs over a short time, the adhesives easily can have a useful shelf life of six months. On heating to about 150°C, the dicyandiamide becomes soluble in the epoxy resin, and the adhesive polymerizes rapidly. Cure can be accelerated by incorporation of tertiary aromatic amines or substituted ureas. 

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. 

Cyanamide is used in the production of synthetic rubber, cyanide, fumigants, and metal cleaners. It has also found use as an intermediate for pesticides, herbicides, detergents, medicines (e.g., antihistamines, hypertension, sedatives), in the photography industry, as an additive for fuels, lubricants, and cements, and as a paper preservative. The dimer (dicyandiamide) is a raw material for melamine and guanidine. 

The dimer of cyanamide is formed when calcium cyanamide is boiled with water. This also is a colourless crystalline compound, the molecule of which has the structure ... 

---