Post 354295 (https://www.thevespiary.org/talk/index.php?topic=9285.msg35429500#msg35429500)
(Bandil: "trans-4-MAR synth w/o cyanogenbromide writeup!", Methods Discourse) and if one really wants to, they can even be reduced further to the significantly more toxic cyanidesPost 385632 (missing)
(Rhodium: "Yes. Alkali metal cyanide plus bromine gives ...", Stimulants)Post 378904 (https://www.thevespiary.org/talk/index.php?topic=5270.msg37890400#msg37890400)
(Polverone: "I can top that", Chemicals & Equipment).Post 383277 (https://www.thevespiary.org/talk/index.php?topic=7430.msg38327700#msg38327700)
(Aurelius: "Urea", Chemistry Discourse). In addition, the alkali cyanates can also be prepared by heating alkali carbonates in a mixture with powdered cyanuric acidPost 340082 (https://www.thevespiary.org/talk/index.php?topic=7620.msg34008200#msg34008200)
(Polverone: "Encouraging results", Chemistry Discourse). Perhaps an even more OTC method for the production of cyanates was explored by Polverone whereby sodium cyanate was produced from sodium bicarbonate and urea. Sodium carbonate was formed by direct heating of the bicarbonate and then combined with urea to generate sodium cyanatePost 378904 (https://www.thevespiary.org/talk/index.php?topic=5270.msg37890400#msg37890400)
(Polverone: "I can top that", Chemicals & Equipment). Experimentally, it was reported that direct heating of urea and sodium carbonate at 150°C produced an effervescing mixture that evolved large quantities of ammonia. Sodium cyanate was thereafter reduced to sodium cyanide when heated further in the presence of charcoal.Post 379016 (https://www.thevespiary.org/talk/index.php?topic=5270.msg37901600#msg37901600)
(Wraith: "Just an alternative", Chemicals & Equipment). However, others indicated that this should be avoided as alkali cyanates hydrolyze easily and the conversion of the carbonate to the cyanate can be ensured through the addition of excess ureaPost 379434 (https://www.thevespiary.org/talk/index.php?topic=5270.msg37943400#msg37943400)
(Polverone: "First-hand experience", Chemicals & Equipment).Post 214057 (https://www.thevespiary.org/talk/index.php?topic=11418.msg21405700#msg21405700)
(foxy2: "NaOCN and KOCN Production", Novel Discourse). In addition, these abstracts contained vital information about the starting reagent molar ratios, reaction conditions, yields, and purity of product. A strong similiarity among these abstracts was evident. All the authors seem to agree that the production of high quality NaOCN proceeds from the addition of urea to the carbonate at molar ratio of (2.0-2.2):1 with stirring and gradually in 3 equal portions, giving each portion to time to react. Furthermore, Schunk et al. (1) noted that the reaction progresses through 3 stages: Na allophanate and Na cyanate are formed in the 1st stage at 100-120°C, water is evaporated in the 2nd stage at 130-140°C, and Na allophanate is converted into Na cyanate in the 3rd stage at 140-180°C. During the entire course, the reaction gases are simultaneously being removed in all stages. Dragalov et al.(2)recommended, that after the addition of the entire amount of urea, it is important calcine the resulting NaOCN is at 180-300°C until the gasification of impurities ceases. To calcine means to heat (a substance) to a high temperature but below the melting or fusing point, causing loss of moisture, reduction or oxidation, and the decomposition of carbonates and other compounds.Cyanates: Properties
In aqueous solution, sodium and potassium cyanate undergo hydrolysis according to Equations (1) and (2) [8].
OCN– + 2 H2O --> CO32– + NH4+ (1)
OCN– + NH4+ --> CO(NH2)2 (2)
The rate of hydrolysis depends strongly on temperature, pH and concentration. In dilute strong acids, the reaction proceeds according to Equation (1). The reaction is rapid and complete within a short time. It is therefore used in the chemical analysis of cyanates. In concentrated hydrochloric acid, the trimerization product cyanuric acid can be observed as a byproduct. In alkaline solution, the rate of hydrolysis is very slow. Some cyanate decomposes with the formation of urea according to Equation (2).
The formation of cyanate ions is the first step in the detoxification of cyanide ions in water using hydrogen peroxide [9], followed by the further decomposition of the cyanate according to Equation (1) or (2). Industrially, this is the most important reaction involving cyanate ions in aqueous solution.
In the dry state, cyanate salts decompose above 450 °C to form cyanides. The decomposition is influenced by certain metals. Both salts are hygroscopic, potassium cyanate much more so than sodium cyanate. When cyanate salts are heated to around 300 °C in moist air, they undergo hydrolysis to the corresponding carbonate with liberation of ammonia.
Sodium and potassium cyanate dissolve only in water. Solubility in organic solvents is very low.
Sodium cyanate solubility, g/100 g solvent:
water: 10.68 at 16 degrees C
ethanol: 0.5 at boiling point
benzene: 0.13 at boiling point
liquid ammonia: 1.72 at -19.8 degrees C, 0.72 at 45 degrees C
Potassium cyanate solubility, g/100 g solvent:
water: 75 at 25 degrees C
ethanol: 0.53 at boiling point
benzene: 0.18 at boiling point.
liquid ammonia: 1.70 at 25 degrees C
[8] J. A. Kemp, G. Kohnstam, J. Chem. Soc. 1956 no. 11, 900.
[9] Degussa, DE-OS 2 352 856, 1976 (J. Fischer, H. Knorre). Degussa, DRP 742 074, 1943 (H. Beier). H. Knorre, Galvanotechnik 66 (1975) no. 5, 374 – 383.
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Cyanates: Production
Sodium Cyanate, Technical-Grade. In production, sodium carbonate is used as sodium source [4]. Sodium hydroxide [5] is only useful for laboratory synthesis. Under production conditions, safety considerations make sodium hydroxide unpractical. Urea is the preferred source for the cyanate part of the molecule, giving rise to ammonium carbonate as byproduct. Sodium allophanate has been postulated as a possible reaction intermediate [6]. The appearance and disappearance of such an intermediate can be observed by HPLC analysis. The reaction equation is therefore postulated as follows (Eqs. 3 – 5).
H2NCONH2 --> NH4+OCN– (3)
Na2CO3 + NH4+OCN– + H2NCONH2 --> NaOCN + NH2CONHCOONa+ + NH3 + H2O (4)
NH2CONHCOONa --> NaOCN + NH3 + CO2 (5)
The overall reaction is the formation of two moles of sodium cyanate and one mole of ammonium carbonate from one mole of sodium carbonate and two moles of urea (Eq. 6).
Na2CO3 + 2 H2NCONH2 --> 2 NaOCN + (NH4)2CO3 (6)
The reaction is endothermic and requires strong heating above 200 °C for a prolonged period of time to reach completion. The reaction takes place in a urea melt with a high content of dispersed solids.
The product from this process has a purity of 89 – 92 %. The main impurity is excess sodium carbonate with small amounts of sodium allophanate, cyanuric acid, biuret, and urea.
Sodium Cyanate, High-Grade. Sodium cyanate with at least 95 % purity can be obtained by using technical-grade sodium cyanate and urea as starting materials. The process proceeds in the molten state, as described for potassium cyanate [7]. Because of the high melting point of the sodium cyanate, the reaction temperature must be raised to 550 – 600 °C. At these high temperatures, some cyanate decomposes to sodium cyanide, which can lead to formation of undesired byproducts and requires treatment of the wastewater. Also, the byproduct ammonia may ignite (flash point 600 °C). For these reasons, production is not without risk.
Potassium Cyanate, High-Grade. The production of potassium cyanate from potassium carbonate and urea requires reaction temperatures of 400 °C or above. The product is obtained from the reactor as a liquid, which must be solidified by cooling and ground after solidification. Since the crystalline structure of the potassium carbonate is destroyed in the melting process, the urea can reach nearly all potassium ions and convert them to potassium cyanate at a much higher rate than in the case of the technical-grade sodium salt. It is therefore easy to reach high purities above 95 %. While possible impurities like urea, biuret, cyanuric acid, and potassium allophanate are unstable at the reaction temperature and therefore are easily eliminated, the temperature is still too low to allow for significant formation of potassium cyanide.
[4] DuPont, GB 1 145 777, 1969. Sakae Food Suff Industry Co., DE 1 205 065, 1966 (I. Tomiro). Degussa, GB 339 220, 1930. DuPont/Degussa, US 1 915 425, 1933 (H. Kloepfer). FMC, EP 0 122 031 B1, 1987 (W. B. Dodge, M. Halfon).
[5] Asahi Chemical Industry Co. Ltd., JP 49 042 800, 1966 (Y. Nakayama et al.). Stamicarbon B. V. Neth., DE-OS 2 431 205, 1975 (J. Verstegen).
[6] VEB Agrochemie Piesteritz, DD 221 449, 1985 (W. Schunk et al.).
[7] US 2 690 957, 1954 (W. P. Horst).
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Cyanates: Uses:
A review of the chemical reactions involving cyanate salts has been published [10].
In chemical synthesis, sodium cyanate is often the salt of choice. In most reactions, it can be regarded as a safe alternative to phosgene chemistry. The following reactions are or have been of some commercial value.
Reactions with primary or secondary amines afford monosubstituted ureas [11] (Eq. 7).
RNH2 · HCl + MOCN --> RNHCONH2 + MCl (7)
where, e.g., R = ethyl, tert-butyl, cyclohexyl
Sodium and potassium cyanate behave alike. Potassium cyanate is sometimes preferred to sodium cyanate because of its higher purity and its much higher solubility in water, a popular solvent for this type of reaction. Another solvent used for this reaction is ethyl acetate. Well-known pharmaceuticals produced by this type of reaction are the antiepileptic carbamazepine and the dermatotherapeutic agent hydroxyurea. The fungicides lenacil and terbacil are also produced by this type of reaction, and the herbicides isoproturon and diuron can also be produced.
In alkylation reactions, only sodium cyanate gives satisfactory yields (Eq. 8); potassium cyanate cannot be used.
RX + NaOCN --> RNCO + NaX (8)
Suitable alkylating agents are alkyl halides or sulfates. For example, allyl isocyanate is obtained from the reaction of sodium cyanate with allyl chloride. The product trimerizes readily to form triallyl isocyanurate [12]. The reaction of dimethyl sulfate with sodium cyanate results in the formation of methyl isocyanate, an intermediate for a number of plant-protecting agents [13].
Aromatic acid chlorides can react with sodium cyanate to yield benzoylurea derivatives, a novel class of herbizides [14]. Similarly, some sulfonylureas are antidiabetic agents, and sodium cyanate can be used for their synthesis. The reaction of sodium cyanate with sulfonyl chloride to give chlorosulfonylisocyanate in situ, which can be treated further in a one-pot synthesis (Eq. 9) [15]. To obtain high yields, a sodium cyanate with a low sodium carbonate content must be used.
NaOCN + SO2Cl2 --> ClSO2NCO + NaCl (9)
Sodium cyanate is also used as a building block in the multistep synthesis of the herbizides carfentrazone and sulfentrazone [16].
Unique applications for potassium cyanate are rare. The major application of potassium cyanate is as an ingredient in certain steel-hardening salts for the Tufftride/Tenifer process [17].
The reaction of amidosulfuric acid with potassium cyanate yields potassium hydroxyureasulfonate, which has found a minor application as a polymerization catalyst for acrylonitrile [18].
[10] Houben-Weyl, 4th ed., vol. VIII, pp. 89, 125; Ethyl Corp., US 2 866 801, 1958 (C. H. Himel, L. Richards). H. Böhme, W. Pasche, Arch. Pharm. 302 (1969) 335. K. A. Jensen, M. Due, A. Holm, Acta Chem. Synd. 19 (1965) 438.
[11] DuPont, US 3 235 357, 1966 (H. Loux). DuPont, US 3 235 360, 1966 (E. Soboczenski). DuPont, US 3 352 862, 1965 (E. Soboczenski).
[12] Nippon Kasei Chemical Co., DE-OS 2 839 084, 1979 (T. Nakamuro et al.).
[13] Degussa, US 4 206 136, 1980 (G. Gieselmann et al.).Degussa, DE 2 828 259 C2, 1980 (G. Gieselmann, K. Günther).
[14] Ciba-Geigy Res. Discl. 351, 442-7; Chem Abstr. 119 : 271113.
[15] Hoechst Agrevo, EP 560 178, 1996 (G. Schlegel); Hoechst Agrevo, EP 507 093, 1996 (G. Schlegel et al.).
[16] FMC, US 5 256 793, 1993 (A. R. Bailey, M. Halfon, E. W. Sortore).
[17] Daimler-Benz, DE-OS 2 602 754, 1976 (C. Lovasz et al.), Degussa, DE 1 191 655 OT, 1965 (J. Müller). Degussa, DE 1 149 035 OT, 1963 (J. Müller, C. Albrecht). Degussa, DE 1 234 873 OT, 1967 (J. Müller). Degussa, DE 1 280 018 OT, 1968 (J. Müller).
[18] Asahi Kasei Kogyo, DE 1 720 202, 1973 (T. Ohfuka, K. Shirode, Y. Ichikawa).
Patent GB710143 (http://l2.espacenet.com/dips/viewer?PN=GB710143&CY=gb&LG=en&DB=EPD)
inspired me to make cyanates (and then cyanides) from cyanuric acid before I realized I could use urea. Where I'm located, if you want only moderate quantities, it's actually easier to buy cyanuric acid.