Guanidines from cyanamides

Guanidine from Cyanamide. Ammonia adds to cyanamide to form guanidine. It is presumed that addition occurs at the —C=N group with the formation of the amidine. 

At the same time a small amount of hydrogen sulphide is also eliminated. The main product of this reaction, carbodiphenylimide (di-phenylcyanamide), an extremely reactive substance, combines with the aniline present in the solution to form triphenylguanidine in the same way as, from cyanamide itself and ammonia, the unsubstituted guanidine is formed. 

Odo studied the formation of methylguanidine from cyanamide and aqueous mixtures of methylamine and methylamine hydrochloride in various proportions [ 110]. He concluded that the reaction occurred by a reversible nucleophilic attack of the free amine on cyanamide, and that an acid was required to shift the equilibrium in the direction of the guanidine. 

CA 43, 5416(1949) (Improved process for the prepn of GuN from Amm thiocyanate and AN. As the direct fusion of these two substances produces an expin, the new process calls for dissolving the first in liq ammonia, and then heating with AN at 120° in a closed vessel to produce GuN, ammonia and hydrogen sulfide) 9) Cyanamid s Nitrogen Chemical Digest, Vol IV, Chemistry of Guanidine , American Cyanamid Co, NV(1950)... 

Changing the nature of the triazine substitution pattern to the l,3,5-triazine-2,4-dione system leads to another series of herbicides, of which the best example is hexazinone (1) (75USP3902887). It is built up from cyanamide which is converted in several steps to the guanidine (2). Reaction with cyclohexyl isocyanate then gives an intermediate which can be cyclized to hexazinone (Scheme 2). 

The nitroamines are substituted ammonias, substances in which a nitro group is attached directly to a trivalent nitrogen atom. They are prepared in general either by the nitration of a nitrogen base or of one of its salts, or they are prepared by the splitting off of water from the nitrate of the base by the action of concentrated sulfuric acid upon it. At present two nitroamines are of particular interest to the explosives worker, namely, nitro-guanidine and cyclotrimethylenetrinitramine (cyclonite). Both are produced from synthetic materials which have become available in large commercial quantities only since the first World War, the first from cyanamide, the second from formaldehyde from the oxidation of synthetic methyl alcohol. 

Large quantities of guanidine are made from cyanamide. Two methods are available for this... 

In practice, the ammonolysis is carried out in the presence of ammonium salts—preferably ammonium nitrate—and the comparatively stable guanidine nitrate is formed. Such a technique is employed to prevent the relatively active guanidine from combining with unreacted cyanamide to form melamine. The reaction is carried out by heating calcium cyanamide, cyanamide, or dicyandiamide with ammonia and ammonium nitrate for 2 hr at 160°C under autogenic pressure. During the course of the reaction, dicyandiamide is formed from cyanamide in the presence of ammonia and, in all probability, is the primary product. The monomeric reaction is... 

In the commonly used Welland process, calcium cyanamide, made from calcium carbonate, is converted to cyanamide by reaction with carbon dioxide and water. Dicyandiamide is fused with ammonium nitrate to form guanidine nitrate. Dehydration with 96% sulfuric acid gives nitroguanidine which is precipitated by dilution. In the aqueous fusion process, calcium cyanamide is fused with ammonium nitrate ia the presence of some water. The calcium nitrate produced is removed by precipitation with ammonium carbonate or carbon dioxide. The filtrate contains the guanidine nitrate that is recovered by vacuum evaporation and converted to nitroguanidine. Both operations can be mn on a continuous basis (see Cyanamides). In the Marquerol and Loriette process, nitroguanidine is obtained directly ia about 90% yield from dicyandiamide by reaction with sulfuric acid to form guanidine sulfate followed by direct nitration with nitric acid (169—172). 

Guanidine dearranges less readily than urea and yidds ammonia and cyanamide as would be expected from the familiar method for its synthesis. If it is heated with aniline, the cyanamide shows no tendency to combine with that substance but combines with itself, as it does in the absence of aniline, to form melamine. 

Guanidines have been prepared by a wide variety of methods, of which two are of much greater importance than the others. These two methods, (a) the addition of amines to cyanamides and (b) the displacement of an alkylmercaptan by an amine from an alkylisothiouronium salt, together with close variants, are discussed first, and this discussion is followed by a survey of less frequently used procedures. 

According to equation (19) decomposition also occurs on boiling nitroguanidine in an aqueous solution of ammonium carbonate, with liberation of nitrous oxide and ammonia. The latter combines with the cyanamide also resulting from reaction (19) and guanidine carbonate is formed in almost quantitative yield. 

Cyanamides like 4 (from amines and cyanogen bromide) provide access to guanidines. This approach allows for introduction of different substituents, and alkylating intermediates can further increase the diversity of products produced. However, high temperatures are required, especially with aromatic amines, for the final addition to give the guanidine products (Scheme 5).9... 

The guanidine sulfate, without removal from the mixture, is then nitrated to nitroguanidine.7 The process yields a nitroguanidine which is suitable for use in nitrocellulose powder, but it suffers from the disadvantages that the dicyandiamide, which corresponds after all to two molecules of cyanamide, yields in theory... 

Dicyandiamide, which is readily obtainable from calcium cyanamide and is a stable commercial product, may be converted into guanidine nitrate by the action of ammonium nitrate, either in concentrated aqueous solution under pressure12,13 or by a fusion reaction.13-16 The latter procedure gives a very pure product and is to be preferred for laboratory practice. Directions for its use are given under procedure A. 

Cyanamide is also acted upon directly by ammonium salts in aqueous and non-aqueous solutions.17-20 Since it is of decided advantage to avoid the isolation of pure cyanamide, methods have been developed in which aqueous solutions of cyanamide are prepared from calcium cyanamide and are then permitted to react with ammonium salts. However, guanidine nitrate may also be prepared directly from commercial calcium cyanamide by the action of ammonium nitrate. This reaction takes place readily in the fused state, in concentrated aqueous solution,21 or by direct admixture of solid ammonium nitrate with calcium cyanamide and subsequent heating to 100°C.22 The latter procedure is to be preferred, since better yields are obtainable and the hazard of fusion with ammonium nitrate is obviated. The directions given for procedure B require the use of a fairly good grade of unoiled calcium cyanamide containing approximately 65 per cent CaNCN. 

B. GUANIDINE NITRATE FROM CALCIUM CYANAMIDE AND AMMONIUM NITRATE... 

Guanidine nitrate, 94, 96, 97 from calcium cyanamide, 97 from dicyandiamide, 96... 

---