Dicyandiamide cyanamide dimerization

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. 

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. 

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. 

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. 

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. 

If there is functionality directly attached to the carbon atom of the tetrazole, a problem may seem to arise. Thus many drugs are made from 5-aminotetrazole 97 and our disconnection requires the unlikely looking molecule H2N-CN. This is in fact cyanamide available as a mixture with water and stabilisers to prevent dimerisation to dicyandiamide 98. The dimer is 50 times cheaper and can be used in the 1,3-dipolar cycloaddition under slightly different conditions.10... 

The efficient conversion of cyanamide to the dimer depends on the contrd of the pH and the use of relatively concentrated solutions. The effect of hydrogen-ion concentration on the formation of dicyandiamide has been studied by numerous investigators, and it is generally agreed that the pH should be between 8.6 and 9.9. The maximum reaction rate appears to be at pH 9.2 at 82°C and slightly higher at lower temperatures. 

Pinck and Hetherington showed that the maximum conversion of cyan-amide to dicyandiamide is obtained when the starting solution contains at least 20 per cent cyanamide and 2 per cent ammonia (Table 8-6). Cyan-amide is quantitatively pol3rmerized to the dimer in liquid ammonia solution upon spontaneous evaporation of the solvent. The use of ammonia is, however, not essential inasmuch as cyanamide polymerizes readily in alkaline solution and in commercial practice, the batch is set at the optimum pH (viz., about 9.2 at 180 C). 

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, 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. 

In 2011, Woo and collaborators prepared ORR catalysts from the pyrolysis at 700, 800, and 900 °C of a mixture of iron oxide supported on Vulcan and dicyandiamide (C2H4N4 a dimer of cyanamide) [111]. TEM of the catalysts revealed that at 700 °C metal particles were encapsulated with a carbon layer, while they were mostly in carbon tubes at 900 °C. The total N content was 2.2,3.5, and 6.6 at.% for the catalysts heat treated at 700,800, and 900 °C, respectively. This N content was broken down as 54 % pyridinic and 0 % graphitic nitrogen atoms at 700 °C, while it was 61.4 % pyridinic and 10.7 % graphitic at 900 C. The Fe content was also measured in these catalysts and also for the catalyst iron... 

One of the most commonly used amine curatives in one-part heat cure paste and film epoxy adhesives is dicyandiamide, 10, prepared by dimerization of cyanamide under relatively mild conditions (Eq. 20). Its utility is... 

Under basic conditions, cyanamide ugdgrgoes a dimerization and forms dicyandiamide at room temperature. Based on this fact, a monomer terminated with two N-cyanourea groups would be expected to polymerize and form a thermoplastic at room temperature. 

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