It is a tetrabasic acid, of markedly acid character, and readily
decomposes carbonates and acetates. It dissolves unchanged in
concentrated sulphuric acid, and oxidizes readily in moist air,
forming Prussian blue.
Prussian blue, Fe7(NC)13 or Fe4[Fe(NC)6]3, ferric ferrocyanide, was discovered in 1710 by a German manufacturer named Diesbach, who obtained it by the action of fused alkali and iron salts on nitrogenous organic matter (e.g. blood). It is now prepared from the calcium ferrocyanide formed in gas purifiers (see above) by decomposition with ferrous sulphate. J. Bueb (Congress of German Gas industries, March 1900) brings gas (free from tar) into intimate contact with a saturated solution of ferrous sulphate, when a “cyanogen mud” is obtained. This is heated to boiling, and the residue after filtration contains about 30 % of Prussian blue. On the small scale it ma be pre ared by adding an acid solution of a ferrous salt to a soiiition ofp potassium ferrocyanide. The grey precipitate first formed is allowed to stand for some hours, well washed, and then oxidised by a warm solution of ferric chloride: 6K2Fe[Fe(NC)6] + 3O＝Fe7(NC)18+3K4Fe(NC)6 + Fe2O3. It is a dark blue powder with a marked coppery lustre. It is insoluble in water and is not decomposed by acids.
Soluble Prussian blue, K2Fe2[Fe(NC)6]2, potassium ferric ferrocyanide, is formed when a solution of potassium ferrocyanide is added to an insufficiency of a solution of a ferric salt (1), or when potassium ferricyanide is added to a ferrous salt (2):
- (1) 2K4Fe(NC)6 + 2FeCl3＝6KCl + K2Fe2[Fe(NC)6]2
- (2) 2K3Fe(NC)6 + 2FeCl2＝4KCl + K2Fe2[Fe(NC)6]2.
It is soluble in water, but is insoluble in salt solutions.
Potassium ferricyanide, K3Fe(NC)5, red prussiate of potash, is obtained by oxidizing potassium ferrocyanide with chlorine, bromine, &c., 2K4Fe(NC)6 + Cl2＝2K3Fe(NC)6 + 2KCl. G. Kassner (Chem. Zeit., 1889, 13, p. 1701; 17, p. 1712) adds calcium plumbate to a solution of potassium ferrocyanide and passes carbon dioxide through the mixture: 2K4Fe(NC)6 +Ca2PbO4+4CO2＝2K3Fe(NC)6+ KZCO3-l-PbCO3+2CaCO3. The mixture o calcium and lead carbonates is filtered off and roasted at a low red heat in order to regenerate the calcium plumbate. It crystallizes in dark red mono clinic prisms which are readily soluble in water. The solution decomposes on standing, and in the presence of an alkali acts as an oxidizing agent: 2K3Fe(NC)5+2KHO＝2K4Fe(NC)6+H2O+O. With silver nitrate it gives an orange red precipitate of silver ferricyanide, Ag;, Fe(NC)6, . With a pure ferric salt it only gives a brown coloration. It can be estimated quantitatively by mixing a dilute solution with potassium iodide and hydrochloric acid in excess, adding excess of zinc sulphate, neutralizing the excess of free acid with sodium bicarbonate, and determining the amount of free iodine by a standard solution of sodium thiosul hate. The zinc sulphate is added in order to remove the ferrocyanid)e formed as an insoluble zinc salt: 2K3Fe(NC)5+2KI＝2K6Fe(NC)6+I2. As an alternative method it may be decomposed by hydrogen peroxide in alkaline solution and the amount of evolved oxygen measured: 2K3F€(NC)5 + + H202＝2K4F€(NC)5 + 2H, O + Oz.
Turnbull’s blue, Fe5(NC)12 or Fe3[Fe(NC)5]2, ferrous ferricyanide, is best obtained by adding a hot solution of potassium ferricyanide to a ferrous salt, and allowing the mixture to stand some time in the presence of an iron salt: 2K3Fe(NC), +3FeSO6＝Fe3[Fe(NC)6]2+ 3K6SO4. It is insoluble in dilute acids.
Hydroferrioyanie acid, H3Fe(NC)6, obtained by adding concentrated hydrochloric acid to a cold saturated solution of potassium ferricyanide, crystallizes in brown needles, and is easily decomposed.
Nitroprussides.—The nitroprussides are salts of the type M2Fe(NC)5·NO. The free acid forms dark red deliquescent crystals and is obtained by decomposing the silver salt with hydrochloric acid, or the barium salt with dilute sulphuric acid.
Sodium nitroprusside, Na2Fe(NC)5NO2H2O, is the commonest salt. It is prepared by oxidizing potassium ferrocyanide with a diluted nitric acid. The solution is evaporated, separated from potassium nitrate, the free acid neutralized with soda, and the solution concentrate ed. It crystallizes in dark red prisms which are readily soluble in water; it is a valuable reagent for the detection of sulphur, this element when in the form of an alkaline sulphide giving a characteristic purple blue coloration with the nitroprusside. The potassium salt may be prepared by adding potassium cyanide to ferrous sulphate solution, the brown precipitate so formed being then heated with potassium nitrite:
5 KNC + 2 FeSO.＝2 K2SO6 + KFE2(NC)5,
2 KFe2(NC)5 -l-° 2 ＝2 F60 + 2 K2F€(NC)5'NO.
Other complex cyanides are known which may be regarded as derived from the acids H2X(CN)4, X＝-Ni, Pd, Pt; H4X(CN)6, X＝ Fe, Co, Ru; I-l3X(CN)e, X＝Fe, Co, Rh; and H2R(CN)5 (see Abegg, Anorganischen Chemie).
Organic Cyanides or Nitriles.—Hydrocyanic acid forms two series of derivatives by the exchange of its hydrogen atom for alkyl or aryl groups; namely the nitriles, of type R·CN, and the isonitriles, of type R~NC. The latter compounds may be considered as derivatives of the as yet unknown isohydrocyanic acid HNC.
Nitriles.—These substances were first isolated in 1834 by J. Pélouze (Ann., 1834, 10, p. 249). They may be prepared by heating the alkyl iodides with potassium cyanide; by heating sulphuric acid esters with potassium cyanide; by distilling the acid-amides with phosphorus pent oxide; and by distilling amines (containing more than five atoms of carbon) with bromine and potash (A. W. Hofmann), for example
C7H15CH2NH2→C7H15CH2NBr2→C7H35CN.
In addition to these methods, the nitriles of the aromatic series may be prepared by distilling the aromatic acids with potassium sulphocyanide: C6H5CO2H + KCNS＝HCNS + CeH5CO2K, C6H5CO2H + HCNS＝C6H5CN + H25 + CO2; from the primary aromatic amines by converting them into diazonium Salts, which are then decomposed by boiling with potassium cyanide and copper sulphate; by fusing the potassium salts of the sulphonic acids with potassium cyanide; by leading cyanogen gas into a boiling hydrocarbon in the presence of aluminium chloride (A. Desgrex, Bull. soc. chim., 1895, (3) 13, p. 735); and from the syn-aldoximes by the action of acetyl chloride or acetic anhydride.
They are mostly colourless liquids which boil without decomposition, or solids of low melting point. The lower members of the series are somewhat soluble in water. They behave in most respects as unsaturated compounds; they combine with hydrogen to form amines; with water to form acid amides; with sulphuretted hydrogen to form thio-amides; with alcohols, in the presence of acids, to form imido-ethers R-C(:NH)-OR'; with ammonia and primary amines to form amidines R-C(:NH)-NH2; and with hydroxylamine to form amidoximes, R-C(:NOH)-NH2. When heated with sodium they frequently polymerize. Heated with acids or alkalis they hydrolyse to acids: RCN + HCl + 2H2O＝R-COOH + NH4C . This reaction shows that the alkyl or aryl group is attached to the carbon atom in the nitrile.
Acetonitrile boils at 81·6° C., and is readily miscible with water. Propionitrile boils at 97° C.; it is somewhat easily soluble in water, but is thrown out of solution by calcium chloride. It was obtained by E. Frankl and C. C. Graham (Journ. Chem. Soc., 1880, 37, p. 740) by the action of cyanogen gas on zinc ethyl. Allyl cyanide boils at 119° C. Benzonitrile boils at 190·6° C. When solidified it melts at- 17° C. It is easily soluble in alcohol and ether.
The Isonitriles (isocyanides or carbylamines) were first prepared in 1866 by A. Gautier (Ann., 1869, 151, p. 239) by the action of alkyl iodides on silver cyanide, and the distillation of the resulting compound with potassium cyanide in concentrated aqueous solution: RI→R-Ag(NC)2→R·NC+KAg(NC)2. They may also be obtained by distilling a primary amine with alcoholic potash and chloroform: R·NH2 + CHCI3 + 3KHO＝3KCl + 3H20 + R·NC (A. W. Hofmann, Ann., 1868, 146, p. 107). They are colourless liquids, readily soluble in alcohol and in ether, but insoluble in water. They possess an exceedingly unpleasant smell and are poisonous. They boil at temperatures somewhat lower than those of the corresponding nitriles; and, are stable towards alkalis, but in the presence of mineral acids they readily hydrolyse, forming primary amines and formic acid: RNC +2H20＝RN H2+H2CO6. This reaction shows that the alkyl or aryl group is linked to the nitrogen atom. The carbon atom in the isonitri es is assumed by J. U. Nef to be divalent, since these substances readily form addition compounds, such addition taking place on the carbon atom, as is shown by the products of hydrolysis; for example with ethyl carbylamine:—
C2H5NC + CH3COCl→C2H5NC(COCHs)Cl→HCl + C2H5NH; + CH3CO-COQH.
This view was confirmed by J. Wade (Journ. Chem. Soc., 1902, 81, p. 1396) who showed that the products obtained by the action of alky iodides on the isonitriles in alcoholic solution at 100° C. yield amine hydroidides and formic acid when hydrolysed. Such a reaction can only take place if the addition of the alkyl group takes place on the nitrogen atom of the isonitrile, from which it follows that the nitrogen atom must be trivalent and consequently the carbon atom divalent. The reactions may probably be represented as follows:—
C2H5NC+C2H5I+4C2H5OH＝C2H5NH2'HI+HCO2C2H5+2 (C2H5)zO, C6H5NC(+C2H5l)9C2H5N(C2H5-I)C(+3C2Hf, OH)
6 (C2H5)2NH'HI + H'CO2C2H5+(C2H5)2O.
The isonitriles dissolve silver cyanide readily, forming a soluble silve;' salt (cf. KNC). At 200° C. the isonitriles are converted into mtri es.
Constitution of Metallic Cyanides.—Considerable discussion has taken place as to the structure of the metallic cyanides, since potassium cyanide and silver cyanide react with alkyl iodides to orm nitriles and isonitriles respectively, thus apparently pointing to the fact that these two compounds possess the formulae KCN and AgNC. The metallic cyanides are analogous to the alkyl isocyanides, since they form soluble double silver salts, and the fact that ethyl ferrocyanide on distillation yields ethyl isocyanide also
points to their isocyanide structure. J. Wade (loc. cit.) explains
