light-colored resin of unusual properties. Thiourea made from cyanamide has been used in the manufacture of molding resins. The reactions of cyanamide are of industrial importance. In addition, they resemble some other more general reactions and are therefore of scientific interest. They may give some clue to the mechanism of other reactions.
Chemistry of Cyanamide GEORGE BARSKY
BarskyandStraus,Inc.,NEWYORK,N. Y.
Cyanamide, an Ammonoosrbonic
Acid As Franklin (4) pointed out, cyanamide is the analog in the ammonia series of carbon dioxide, and products related to cyanamide may be looked upon as deammonation products of a hypothetical ammono-o-carbonic acid. Figure 1, taken from Franklin's discussion, shows these relations. The structures of calcium cyanamide and calcium carbonate are analogous; both are salts of carbonicacids as shown in Figure 2. The formation of calcium cyanamide from carbide may be considered as a nitrification analogous to a hypothetical oxidation. Moving from the ammonia series to the water series and vice versa is easily accomplished with these carbonates. For example, calcium cyanamide can be hydrolysed to form ammonia and calcium carbonate by autoclaving with water under pressure, as is done extensively on a commercial scale. The reverse reaction, the ammonolysis of calcium carbonate to form water and calcium cyanamide, also takes place, and a considerable amount of experimentation has been done on it. To carry this out it is necessary only to heat
courtesy. AMERICAN CVANAMID CO.
Carbide furnace
T
HE formation of calcium cyanamide from calcium carbide and nitrogen was discovered in 1897 by Frank and Caro who were working on the synthesis of cheap cyanide for use in the extraction of gold and silver from ores. The cyanamide process became one of the early sources of fixed nitrogen, but in recent years its relative importance in that field has diminished as a result of the development of direct ammonia processes. In the first place, the cyanamide process is not economical except forfixednitrogen in the form of calcium cyanamide. When the cost of conversion to ammonia or ammonia compounds is included, the synthetic ammonia process is cheaper. In the second place, calcium cyanamide itself can be used as a fertiliser in only limited quantities for reasons brought out below. However, calcium cyanamide is a new chemical raw material and the cyanamide industry owes much of its position to that value of cyanamide. The original ideas of the inventors finally bore fruit, and cyanide produced from calcium carbide, by a two-step process to be sure, is known in practically every gold and silver mine in the world. Twenty years ago the dimer of cyanamide, dicyanamide, was considered a comparatively rare chemical. Since then its price has come down to 18 cents a pound, and it is sold in carload lots. It is used for the synthesis of barbiturates, for
the production of guanidine and biguanide compounds, and for the production of melamine. Melamine, the trimer of cyanamide, is used in the manufacture of a
HiN
NH.
NH,
C
HN=C
/ \
\
HiN
NHf
HN=»C—NH, NH
/
NHf Guanidine
CN
HN=C—NH, Biguanide
/ HN=C \ NHCN
CN Dicyanimide
Dicyandiamide
CiN*-*rNH0i ^)NH C,N^-=(NH0i Melam
C*NV=NH / N-C,N»=NH \ C,N»=NH Hydromelonic acid
1 Presentcd before a meeting of the New York Section of the American Chemical 8ociety.
NH, / CN.-NH, \ NH,
MeUmine
CNg—(NH,), ^>NH CN*=NH Melem
C.N^=NH ^>NH C,N^=NH Melon N •
CN / N-CN \ CN Carbonic nitride
Figure 1
759
NsaC—NH, Cyanamide
NH« I / \ N N |I | H,N—C C—NH, \ N /
NH,
/ HN \
HN=C=NH
C
C \ / | N N | N»C
NEWS
760 HN—O-NH
EDITION
0*»0*0
TT7
O
f-NHT M~5S,.=
/\/?J" Ca c j ,
IH.I N / \ / \ Ca C \ ^ \ / N O CaCi + Nt — • CaCNi + C CaCi + 3/2 Ot — • CaCOi + C CaCN, + 3H«0 =i CaCOi + 2NH. CaCN. + C ^ Ca(CN)i COi + C ^ 2CO Figure 2 finely divided calcium carbonate in a current of ammonia at about 700-800° C. (5).
creased, larger concentrations of cyanide are required.
Reduction
Polymerization
Calcium cyanamide is reduced by carbon at high temperatures to calcium cyanide. This may be looked upon as analogous to the reduction of carbon dioxide by carbon to carbon monoxide. Both reactions are endothermic, and the influence of high temperature is to promote the reduction. On a commercial scale calcium cyanide is manufactured by the fusion of calcium cyanamide, which contains free carbon, with salt in an electrically heated shaft furnace. The reaction is reversible just as is the carbon monoxide-carbon dioxide reaction, and in order to prevent reversion of the cyanide to cyanamide it is necessary to chill the product quickly. This is done by a water-cooled flaking wheel. The final product consists of thin gray flakes containing about 47 per cent calcium cyanide. The sodium chloride acts as a solvent for the calcium cyanide, and the unconverted cyanamide remains as a separate phase. If the product is cooled slowly, the calcium cyanide is precipitated out and, when in the solid phase, reverts to cyanamide and carbon. This grade of calcium cyanide is extensively used for the extraction of gold and silver from ores, for the manufacture of ferrocyanides and of liquid hydrocyanic acid, and for fumigation. The reverse of the cyanide-forming reaction—that is, the reversion of cyanide to cyanamide and carbon—is a useful reaction in the casehardening of steel and is extensively used. In this case the calcium cyanide is dissolved in a bath of molten salts. In contact with iron, reversion takes place and the carbon combines with the iron. The composition of the salt bath is important, since it affects the solubility of the cyanide and therefore the concentration of cyanide at which reaction with the iron takes place. Thus, with baths high in alkaline-earth chlorides, active casehardening can be obtained with relatively low percentages of cyanide, but as the alkali chloride content of the bath is increased and that of alkali earth de-
When calcium cyanamide is added to water, hydrolysis of the normal cyanamide occurs, with production of a solution of calcium arid cyanamide and precipitation of calcium hydroxide: 2CaCNf + 2H,0-
• Ca(HCN.)t + Ca(OH),
Such a solution is strongly alkaline. Free cyanamide may be recovered from solution by precipitating the calcium with sulfuric acid, vacuum evaporating the solution (the pH of which is maintained at about 5), and crystallizing. In an aqueous solution of cyanamide a number of reactions may occur, of which the two most important are hydrolysis to urea and polymerization to dicyandiamide. Grube and co-workers (5, 6) found that the concentration of the alkali had a marked effect upon the velocity of disappearance of cyanamide, the velocity increasing and then decreasing with increasing alkali concentration. The mechanism of the reaction, according to these workers, is that dicyandiamide is formed by reactiou between an undissociated cyanamide molecule and a cyanamide ion:
Vol. 18, No. IT electrode. At intervals the concentrations of cyanamide, dicyandiamide, and urea were determined. The data obtained may be summarized as follows. 1. At constant hydrogen ion concentration the polymerisation of cyanamide to dicyandiamide proceeds as a reaction of the second order. 2. The velocity of the reaction is a function of the hydrogen ion concentration. 3. The velocity of formation of dicyandiamide is at a maximum at pH 9.6 and decreases rapidly at hydrogen ion concentration above or below this point, as Figure 3 shows. 4. Cyanamide disappearing is quantitatively accounted for by dicyandiamide produced up to about pH 10. Above pH 10 nitrogen accounted for as dicyandiamide and urea is significantly less than the cyanamide disappearing. That this last effect is due to decomposition of dicyandiamide was shown by an investigation of the stability of dicyandiamide at various hydrogen ion concentrations, the data of which are shown in Figure 3. Above pH 10 dicyandiamide is destroyed, the rate increasing with increasing pH. Cyanurea is known to be formed by the action of alkali on dicyandiamide: Nil, NHs / / 0=NH + HA) 0=0 \ \ NH Nil - Nil, i I CN CN A correlation of the velocity constants of dicyandiamide formation at various hydrogen ion concentrations can be found on the basis of the theory that dicyandiamide is formed by reaction between a cyanamide molecule and a cyanamide ion. According to this theory, dx/dt - o(H Cy) (Cy~) where Q — velocity constant II Cy — concentration of undissociated cyanamide Cy" - concentration of cyanamide ion X c o
(NHCN)- + HtNCN — • (NHC—NHHNCN) Morrell and Burgen (9) also studied the kinetics of the formation of dicyandiamide from cyanamide and arrived at the conclusion that the reaction is ionic. Hetherington and Braham (7) showed that in alkaline solution urea is formed as well as dicyandiamide, increased concentrations of alkali resulting in increased urea formation. That effects such as these are controlled by hydrogen ion concentration was shown by a quantitative study of the effect of hydrogen ion concentration upon the reactions of cyanamide (2). The experiments were carried out at 50° C. with cyanamide solutions to which buffer combinations were added. The hydrogen ion concentration was measured with a hydrogen
o
3
I 10
Hydrogen ion concentration, pH Figure 3. Effect of pH on the velocities of formation and decomposition of dicyandiamide {2).
NEWS
September 10, 1940
EDITION
From this it can be shown mathematically that kD - 0
AH*
(tf + H*)«
(1)
where ho - velocity constant at any H ion concentration, H + K — ionization constant of cyanarnidc K is given by Kamayama (8) as 20.1 X 1 0 - " at 50°" (\, or i 0 f 7. Using this value of A', we have the equation giving kD as a function of I P : 10 -» T H + ko - Q (10-1-7 + H*)«
(2)
Fiom this equation we may calculate the constant g, knowing the value of kD for any value of H + . Then knowing gt we may calculate kD for any value of H+. In other words, if we know the velocity constant at any one hydrogen ion concentration, it is possible to calculate it for any other. In Table I we have taken the value of kD for pH 8.4, and from it have calculated the velocity constants for the hydrogen ion concentration at wh*ch the experiments were conducted. That the maintenance of equal concentrations of cyanamide ions and undissociated cyanamide fixes the hydrogen ion concentration is easily proved. In any solution containing cyanamide,
if then
H+(Cy)v HCy " K Cy- - H Cy H~ - K
Tabid. pH
(3)
*Dobsvd. kD calod.
Observed «nd aicvUted CofifUnts
8.4
9.0
9.6
10 5
11.4
7.7 X 10-* ....
21 X 10"' 21 X 10~»
33 X 10~» 38 X 10"«
21 X 10 *» 21 X 10"*
3.1 X 10-« 3.1 X 10-»
Table II.
Effect of Temperature
Temp., ° C.
A' X 10"
pH
0 25 50 80 100
1.15 5.42 20.1 75.6 163
10 4 10 3 9.7 9.1 8.8
The data are the basis for the manufacture of dicyandiamide in high yields and of high purity. These results also explain why calcium cyanamide can be used on the soil only in limited quantities. Dicyandiamide is known to be toxic to plants. When calcium cyanamide is added to the soil in amounts beyond the buffer capacity, the pH rises to such a point that dicyandiamide is formed and poor agricultural results are obtained. When calcium cyanamide is added in small quantities, very dilute solutions are formed and cyanamide probably exists entirely as cyanamide ion. As shown below, this ion hydrolyses to urea. This is undoubtedly the transformation that cyanamide undergoes in the soil to produce assimilable forms of nitrogen. Hydrolysis
This relation may also be derived from Equation 1 by taking the derivative of dkD/dR+ equal to sero and solving for H+. This relation between H*" and K holds, regardless of the temperature. Since Kamayama reported the ionization constants for cyanamide at various temperatures, his values give the optimum hydrogen ion concentration for those temperature*. Values are shown in Table II.
761
Studies on the reaction of cyanamide in highly alkaline solutions showed that at 50° C.: (a) The hydrolysis of cyanamide to urea in alkaline solutions is a reaction of the first order; (6) cyanamide disappearing is quantitatively accounted for by urea formed; (c) the reaction velocity is independent of the concentration of alkali, if the alkali is more than equivalent to the cyanamide.
The data indicate that the hydrolysis of cyanamide in alkaline solutions is a reaction of cyanamide ion, and that the velocity of the hydrolysis is proportional to the concentration of this ion: HNCN- -r H«0 — • HN.CONH," HN.CONHt" + H* — > H,N.CO.NHt If sufficient alkali is already present to convert all the cyanamide into the ion, further addition of alkali has no effect on the rate of hydrolysis. Limited amounts of cyanamide applied to the soil form very dilute solutions which arc highly ionized, hydrolysed to urea, and so utilized as plant food. These data are also of interest in explaining one effect of caustic soda or soda ash (which is converted to caustic by the free lime of the calcium cyanamide) in the industrial autoclaving of calcium cyanamide with water to form ammonia. The strong alkali promotes the formation of urea which is subsequently hydrolysed to ammonia and carbon dioxide. In the absence of strong alkalies, some dicyandiamide is formed and, subsequently from it, cyclic carbon-nitrogen compounds such as melamine which hydrolyse with greater difficulty. In acid solutions, cyanamide forms no appreciable amounts of urea at pH values above 4. In solutions made more strongly acid, the velocity of hydrolysis was found to be proportional to the concentration of acid. The catalytic effect of acid is similar to that in the hydrolysis of esters. The reaction is either one of what we might call a "cyanammonium" ion, HtNCN + H+ — • HiNCN* H.NCN* + H«0 — • H*N CO.NHi* — • HiNCONHt + H* or else of cyanamide hydrochloride, H N = C = N H + HQ — > HN=C—NH,
I
CI H N = C — N H , + H,0
I
— •
CI
HN=C—NH, + HCl
I
O
I
H Reaction with Cyanide
couaruv. AMCMICAA CVAMAMI· to.
Melamine resin kettles
That a reaction takes place between cyanide and cyanamide was discovered during the course of experimental work on the autoclaving of crude calcium cyanide made by fusion of calcium cyanaiaide and salt (/). In addition to calcium formate, small amounts of calcium oxalate were always produced. When pure cyanides
NEWS
762
3
./or*
__
1
1 J
O*** FT
o
0.5
1.0
1.5
Moles of CaNCN per mole of NaCN equivalent Figure 4. Effect of mole ratio on yield (1). were autoclaved, no oxalate was obtained. We recalled that the crude calcium cyanide always contained small amounts of calcium cyanamide. It was then discovered that the addition of calcium cyanamide greatly increased the yield of oxalate, as Figure 4 shows. The total yield of oxalate and formate was less than 100 per cent in all cases, because of loss of hydrocyanic acid by volatilization and polymerization. The oxalate yield cannot be accounted for if we assume that it is derived from cyanide alone or from formate alone. The increasing yield of oxalate with increasing amounts of cyanamide added is additional proof that cyanamide takes part in the reaction. The reaction is simply the addition of hydrocyanic acid formed by hydrolysis to cyanamide, also formed by hydrolysis from the calcium salt. This hypothetical intermediate has not been isolated. It hydrolyzes readily to form oxalate according to the equation: HtN—C=NH
COONH* + 4H,0
CN
•
+NH» COONH4
The existence of an intermediate is shown by the fact that mixtures of cyanamide and cyanide allowed to react at 50° C. in the absence of lime salts must be boiled with hydrochloric acid before precipitating the oxalate if concordant results are to be obtained. When the boiling is omitted, the results are irregular and low. From a study of the effect of pH on the reaction, it was concluded that the mechanism is the addition of a cyanide ion to a cyanamide molecule: HN«=0=NH + C N ~ — • HN«=C—NH~
I
CN
EDITION
Vol. 18, No. 17
The reaction has not been used commercially because of the low yields of oxalate, because of the complications of the formate by-product, and because of the extra step of separating the calcium oxalate from the large proportion of calcium carbonate before acidulating with sulfuric to obtain oxalic acid.
The decomposition of liquid hydrocyanic acid, which is catalyzed by traces of alkali to explosive proportions, is a similar reaction, cyanide ions reacting with undissociated molecules. It can be prevented by the simple procedure of adding acid in sufficient quantity to prevent ionization of hydrocyanic acid to cyanide ions.
Reaction with Hydrogen Sulfide
Literature Cited (1) Bareky, G., and Buchanan, G. H.( / . Am. Chem. Soc., 53. 1270 (1931). (2) Buchanan, G. H., and Bareky, G., Ibid., 52, 105 (1930). (3) Franck, H. H.v U. S. Patent 1,948,106 (1934). (4) Franklin, K. C.t J. Am. Chem. Soc., 44, 486 (1922). (5) Grube, G., and Kroger, J., Z. phytik. Chem., 86, 65 (1913). (6) Grube, G., and NiUche, P., Z. angew. Chem., 27, I, 386 (1914). (7) Hetherington, H. C , and Braham, J. M., J. Am. Chem. Soc., 45, 828 (1923). (8) Kamayama, N., Tram. Am. Electrochem. Soc., 40, 131 (1921). (9) Morrell, G. F , and Bu'gen, P., / . Chem. Soc, 105,576(1914).
The preparation of thiourea from cyanamide by use of ammoniim sidGde is an old reaction. Polysulfides have been considered catalysts. Actually, the addition of hydrogen sulfide to cyanamide is a reaction similar to those described above. It is the addition of an IIS" ion to a cyanamide molecule. Using this theory as a basis for control, the reaction has been carried out successfully on a commercial scale. The operation consists in absorbing hydrogen sulfide gas in a solution of cyanamide, the hydrogen ion concentration of which is maintained at a value sufficiently low to prevent rapid polymerization to dicyandiamiJe and yet high enough to ensure appreciable concentration of HS~ ion. The thiourea is recovered from solution by evaporation and crystallization. Reaction with Ammonia Cyanamide also adds ammonia to form guanidine. Our theory here is that first an ammonium salt is formed and that this then undergoes rearrangement: H N = 0 - NH + >,tf»
> NH,
HN=C=*N—NH«—• H N = C \ NH, Analogous Reactions These reactions of cyanamide in aqueous solution may be looked upon as reactions of the carbonyl group in the ammonia series. They then become analogous to similar reactions of aldehydes. The addition of cyanide to cyanamide or to acetaldehyde is catalyzed by alkali, and in both cases is a reaction of the cyanide ion. The addition of hydrogen sulfide to aldehydes is similar. The formation of aldol from acetaldehyde and diacetone alcohol from acetone are reactions analogous to the formation of dicyandiamide. To carry over the theory from the ammonia series, we may say that, owing to the somewhat acidic nature of the carbonyl group, a dissociation of hydrogen ion occurs, and an aldehyde or ketone ion is formed which then reacts with the undissociated molecule. The requirement that alkali be present for these condensations as well as for many others involving aldehydes may be explained on this basis. There are many reactions of this nature, from those involved in the building up of sugars to those extensively used in industry, particularly in resin formation.
Government Signs Contract (or Smokeless Powder Plant VX^AR DEPARTMENT has announced that
a contract has been signed with the Hercules Powder Co. for the construction of a smokeless powder plant near I kid ford, Ya., and 44 miles from Roanoke, Va. The plant will be owned by the Federal Government, which has retained the Hercules Powder Co. to construct and operate it on a fixed fee basis. Some 250^ acres of land near Radford are being purchased as the site for the plant, which will cost approximately $25,000,000. Construction, which will begin immediately, will require about 10 months. Production capacity will be 200,000 pounds a day. Signing of the contract concluded negotiations which the War Department initiated. The National Defense Advisory Committee cleared the contract. The construction work will give employment to .5000 men. The plant, when completed, will require a force of about 2500 men.
0«A
