Sartori The War Gases Chemistry and analysis

184 CYANOGEN DERIVATIVES

through a coil immersed in water at 15° C. and then through a coil in brine at o° C.

Several other methods are now employed industrially for the preparation of hydrocyanic acid. Synthesis from the elements is widely used. In this a mixture of hydrogen, carbon monoxide and nitrogen is passed through an electric arc, mixtures of nitrogen and hydrocarbons being sometimes employed, e.g., 20% methane, 10% hydrogen und 70% nitrogen.

Also the formation of hydrocyanic acid from spent fermentation wash may be mentioned. This material is obtained from molasses, which, after fermenting and distilling, leaves a residue which still contains about 4% nitrogen as betaine. On further distilling this spent wash to about 40° Be. the evolved gas, after separating from the tar by cooling, is passed through a superheater consisting of a quartz tube heated to about 1,000° C. In this way the nitrogenous compounds are converted to ammonia and hydrocyanic acid. The gas issuing from the superheater is washed with sulphuric acid to remove ammonia, while the hydrocyanic acid is taken out in an alkaline absorbent. By this means about 50% of the nitrogen of the spent wash is converted into ammonia and hydrocyanic acid, while the other half is lost as elementary nitrogen.

PHYSICAL AND CHEMICAL PROPERTIES

Pure anhydrous hydrocyanic acid is a clear colourless liquid with a peculiar odour which is usually compared with that of bitter almonds. It has been observed, however, that this substance has an indefinite odour which varies with the degree of its dilution with air and the period of exposure to it.1

The vapour from the pure liquid or concentrated aqueous solution causes an irritation at the back of the throat and a bitter taste in a short time. Diluted with air it is not very irritant, but has a less disagreeable odour which is somewhat aromatic. It is a peculiar property of this substance that even in minute quantity it paralyses the nerves of odour and taste, and after a few seconds the first sensitivity to the odour is lost.

If liquid hydrocyanic acid is coloured yellow or brown, it may be considered as being less dangerous, because it is in process of alteration. The commercial product should be colourless and contain 96-98% of hydrocyanic acid, the balance being water.

In the gaseous state it is colourless with a vapour density of 1 ANON., Die Gasmaske, 1935, Nos.4-5.

0-948 ; that is, i litre of gaseous hydrocyanic acid weighs 1-21 gm. at o° C. and 760 mm. The specific gravity of liquid hydrocyanic acid is 07058 at 7° C. and 0-6969 at 18° C. The coefficient of thermal expansion between o° C. and 15° C.. is 0-0019. The critical temperature is 138-5° C. and the critical pressure 53-3 atmospheres. The heat of vaporisation is 210-7 cals. per gm. By spontaneous evaporation the current of air produced passes over the surface of the hydrocyanic acid, loweringthe temperature to that of freezing (— 13-4° C.).

The vapour tension at various temperatures is as follows J :

Its volatility at 20° C. is 873,000 mgm. per cu. m.

Because of its high vapour tension as well as its low density it is difficult to maintain a high concentration of hydrocyanic acid in an open place. As the toxic action is considerably reduced by dilution, numerous artifices were used during the war to render this gas more persistent. Lebeau, in France, suggested mixing it with the smoke-producing chlorides, as stannic, titanium or arsenic chloride. The result was that the already low stability of the hydrocyanic acid was still further reduced. Then it was proposed to add a proportion of chloroform to the mixture, this forming the mixture termed " Vincennite " by the French. It consisted of 50% hydrocyanic acid, 30% arsenic trichloride, 15% stannic chloride and 5% chloroform.

Hydrocyanic acid is miscible in all proportions with alcohol, ether, glycerol, chloroform, benzene, tricresyl phosphate, etc. It does not dissolve nitro-cellulose, but cellulose acetate and the other cellulose esters are soluble. Gums, rubber and gelatin are not dissolved. Carbon dioxide and hydrogen sulphide are slightly soluble and sulphur dioxide is soluble in all proportions.2

It dissolves in water to form a solution which has a weak acid

Hydrocyanic acid reacts with benzoyl chloride in presence of pyridine, benzoyl cyanide being formed x :

C6H5COC1 + HCN = C6H5COCN + HC1.

Benzoyl cyanide forms crystals melting at 33° C. and boiling at 206° to 208° C. It is decomposed by water.

Hydrocyanic acid forms additive compounds with several inorganic salts, such as the following :

Stannic chloride forms SnCl4.2HCN.

It can be transported in the liquid condition in metal containers, for example, in tinned-iron cans, which are cooled with ice in summer, or in iron cylinders,like the compressed gases (Buchanan).

The lower limit of sensitivity to the odour is about i mgm. per cu. m. of air.2

It is a powerful poison, but the human body is capable of neutralising the effects of the gas within certain limits. Thus in a concentration of about 30 mgm. per cu. m. hydrocyanic acid is eliminated from the human organism as rapidly as it is absorbed and so no dangerous consequences follow. At higher concentrations, however, poisoning is rapid.

The lethal concentration, according to Flury, is 120-150 mgm. per cu. m. of air for |-i hour's exposure, while, according to Prentiss, it is 200 mgm. per cu. m. for 10 minutes' exposure and 150 mgm. per cu. m. for 30 minutes' exposure.

2. Cyanogen Fluoride. CNF (M.Wt. 45)

Moissan attempted to prepare this substance by acting on cyanogen with fluorine.3 A very violent reaction took place without any deposition of carbon.

Recently cyanogen fluoride has been prepared by Cosslett 4 by the action of silver fluoride on cyanogen iodide :

AgF + CNI = Agl + CNF.

It may also be prepared in lower yield by the action of silver fluoride on cyanogen bromide, but in this case it is always impure with brominated products.

1CLAISEN, Ber., 1898, 31, 1024.

2SMOLCZYK, Die Gas-Maske, 1930, 32.

3H. MOISSAN, Le Fluor et ses composes, 1900, 138.

4V. COSSLETT, Z. anorg. Chem., 1931. 201, 75.

188 CYANOGEN DERIVATIVES

PREPARATION

1-3 gm. silver fluoride is powdered in a mortar with 1-5 gm. cyanogen iodide and transferred to a Jena glass tube about 50 cm. long and 2-5 cm. diameter. After evacuating the tube it is sealed off in the blowpipe and then heated in a furnace to 220° C. for i\ hours. After cooling, the tube is immersed in liquid air and the products allowed to condense. The tube is then opened and the contents distilled at —70° C. in a freezing mixture of acetone and carbon dioxide. Yield 20-25% (Cosslett).

PROPERTIES

Cyanogen fluoride is a colourless gas at normal temperature, and on cooling to a low temperature it forms a white pulverent mass which sublimes at —72° C. at atmospheric pressure.

It is insoluble in water. In glass vessels it is stable and does not attack mercury. However, after storing for about a week in glass containers it attacks the surface ; this is attributed to the action of light.

Cyanogen was discovered by Wiirtz and prepared for the first time by Berthollet.1 It was used during the war by the French (October, 1916) both alone and in admixture with arsenic trichloride (" Vivrite ").

LABORATORY PREPARATION z

It is prepared by the action of chlorine on potassium cyanide. About 100 ml. of a solution of chlorine saturated at o° C. are placed in a 300-ml. flask and cooled while a solution of potassium cyanide is allowed to flow in slowly from a tap-funnel until the yellow colour of the chlorine disappears. The liquid is again saturated with chlorine and further potassium cyanide added still slowly, taking care not to add an excess, which causes decomposition of the cyanogen chloride. The latter is then liberated from the aqueous solution by warming on the water-bath

to 60° to 70° C.

According to Held,3 in order to prevent the formation of paracyanogen, a secondary product in the reaction between chlorine and potassium cyanide, it is advisable to add to the latter I molecule of zinc sulphate per 4 molecules of the cyanide.

1 BERTHOLLET, Ann. chim. phys., 1802, [i] 35, 1789.

2 HANTZSCH and MAI,Ber., 1895, 28, 2-171 ; ZAPPI, Bull, soc. chim.. 1930, [4] 47, 453 ; JONESCU, Antigaz, 1932, 6, Nos. 1-2, 17.

8 HELD, Bull. soc.chim., 1897, [3] 17, 287.

INDUSTRIAL MANUFACTURE

Cyanogen chloride is also prepared industrially by the reaction of chlorine on sodium cyanide.1

Two solutions are added to a large iron vessel fitted with cooling coils, one of sulphuric acid and the other of sodium cyanide. After cooling, chlorine is bubbled in. At the end of the reaction, the cyanogen chloride is distilled off and purified from the gaseous reaction products by passing it through a series of purification towers, two containing calcium chloride to remove traces of moisture and one containing pumice and arsenic to remove the chlorine still present. Yield 80%.

specific gravity is 1-2 and its vapour density 2-1. It is very soluble in water (i volume water at 20° C. dissolves 25 volumes cyanogen chloride) and in the organic solvents such as alcohol, ether, etc.2 The alcoholic solution easily decomposes.

The vapour tension of cyanogen chloride at various temperatures is given in the following table 3 :

mixed cyanogen chloride with arsenic chloride. Water, chlorine and hydrochloric acid tend to favour the polymerisation.5

Chem., 1938, 235, 429.

4NEF,Ann., 1895, 287, 358.

•WURTZ, Ann., 1851, 79, 284 ; NAUMANN, Ann., 1870, 155, 175.

igo CYANOGEN DERIVATIVES

Water slowly hydrolyses cyanogen chloride, forming cyanic acid and hydrochloric acid :

CNC1 + H20 -> HCNO + HC1.

This reaction is accelerated by the alkali hydroxides.1 Cyanogen chloride reacts quantitatively with an alcoholic solution of ammonia to form cyanamide 2 :

CNC1 + 2NH3 = NH2CN + NH4C1.

By the action of alkaline reducing agents, like sodium sulphite with sodium hydroxide, sodium cyanide, sodium chloride and sodium sulphate are formed.

When hydriodic acid is added to an aqueous solution of cyanogen chloride at ordinary temperatures, iodine is liberated. The amount set free increases slightly on standing, but more rapidly on heating to about 100° C., reaching a maximum at 80% of that required by the equation (Chattaway) :

CNC1 + 2HI = HCN + HC1 + I2.

On heating an aqueous solution of hydrogen sulphide with a solution of cyanogen chloride to about 100° C., sulphur separates, while the hydrocyanic acid which is formed is partly hydrolysed and partly combines with the sulphur, forming thiocyanic acid.

Aqueous solutions of cyanogen chloride do not react with silver nitrate,3 but aqueous-alcoholic solutions form silver chloride.4 Unlike cyanogen bromide, the Prussian blue reaction does not take place with cyanogen chloride.5

It has practically no corrosive action on iron, lead, aluminium, tin or silver. It does attack copper and brass slightly, however, a protective coating being formed which prevents further corrosion.

Commercial cyanogen chloride contains 2-5% of hydrocyanic acid.

It may be mixed with chloropicrin and with dichloroethyl sulphide without change.

It is added to "Zyklon B " (see p. 104) both to prevent the polymerisation of the hydrocyanic acid present and also to act as a warning substance.6

Concentrations of 2-5 mgm. per cu. m. of air produce abundant

1 CHATTAWAY and WADMORE, J. Chem. Soc., 1902, 81, 191. 2 CLOEZ and CANNIZZARO, Ann., 1851, 78, 229.

3SERULLAS, Berzelius Jahresber, 8, 90.

4E. ZAPPI, Rev. de Ciencias Quimicas. Uyiiv. La Plata, 1930, 7.

6E. ZAPPI, Bull. soc. chim., 1930, [4] 47, 453.

•RIMARSKY and coll., Jahresber. Chem. Techn. Reichsanstalt,, 1930, 8, 71.

lachrymation in a few minutes. The maximum concentration which a normal man can support without damage for i minute is 50 mgm. per cu. m. of air (Flury).

The lethal concentration for 10 minutes' exposure is 400 mgm. per cu. m. (Prentiss).

Cyanogen bromide was first employed by the Austrians in September, 1917, both in benzol solution and in admixture with bromoacetone and benzol.

It was first prepared by Serullas in 1827.1 It is obtained by the action of bromine on potassium cyanide, similarly to cyanogen chloride.

LABORATORY PREPARATION 2

150 gm. bromine and 50 ml. water are placed in a flask fitted by means of a three-holed stopper with a tap-funnel, a condenser and a thermometer. A solution of 65 gm. potassium cyanide in

as the red colour of the bromine commences to pale, the cyanide solution remaining in the tap-funnel is diluted to about double its volume and added carefully, avoiding an excess, which would cause the decomposition of the cyanogen bromide with formation of azulmic products, darkening the colour of the mass.

At the end of the reaction the contents of the flask are transferred to a retort, which is partly filled with calcium chloride and stoppered, and then warmed on the water-bath at 65-70° C. The cyanogen bromide distils over and condenses in white needles, which may be purified further by redistilling over calcium chloride. The yield is about 90%.3

INDUSTRIAL MANUFACTURE

In the preparation of cyanogen bromide from bromine and sodium cyanide there is a loss of half of the bromine as sodium

bromide4:

NaCN + Br2 = NaBr + CNBr.

In order to avoid this loss, sodium chlorate is generally added,

* V. GRIGNARD and P. CROUZIER, Bull. soc. chim., 1921, 29, 214.

192 CYANOGEN DERIVATIVES

as in the preparation of the halogenated ketones (see p. 147). The following reaction takes place :

NaClO3 + sNaBr + sNaCN + 6H,S04 = aCNBr + NaCl + 6NaHS04 + 3H20.

The manufacture of cyanogen bromide is carried out in large metal vessels in which a solution of the three salts is first prepared. 30% sulphuric acid at a temperature of 3o°C. is then slowly run in. At the end of the reaction the product is separated by distillation. Yield 75%.1

PHYSICAL AND CHEMICAL PROPERTIES.

Cyanogen bromide forms transparent crystals which are either acicular or prismatic. It has a penetrating odour and melts at 52° C. Its boiling point at 750mm. is 61-3° C. Its specificgravity is 1-92 and its vapour density 3-6. The vapour tension at any temperature may be calculated by means of the following formula (see p. 5) :

273 + t

Baxter and Wilson 2 give the following experimental values :

The volatility at 16° C. is about 155,000 mgm. per cu. m. and at 20° C. about 200,000 mgm. per cu. m.

It is only sparingly soluble in water, but more readily in alcohol, ether, carbon disulphide, acetone, benzene and carbon tetrachloride.

The chemical behaviour of cyanogen bromide is very similar to that of cyanogen chloride. For instance, like the chloride it polymerises in time to the trimer, (CNBr)3.3 This polymer is reconverted to cyanogen bromide by heat, so that in order to purify polymerised cyanogen bromide it is merely necessary to distil it.

It is decomposed by water, slowly at ordinary temperatures,

1 For more detailed descriptions of the.industrial manufacture of cyanogen

i94 CYANOGEN DERIVATIVES

By the action of cyanogen bromide on pyridine in alcoholic solution in the cold, a reddish-brown colouration is formed and white crystals separate.1

Cyanogen bromide has a powerfully corrosive effect on metals, attacking copper, iron, zinc, aluminium, and in time even lead and brass.2

Unlike cyanogen chloride, the bromide is not miscible with dichloroethyl sulphide, but reacts with it. It is, however, miscible with chloropicrin.

During the war of 1914-18 it was employed in solution in benzene and bromoacetone in the following proportions : cyanogen bromide 25%,bromoacetone 25%, benzene 50%. This mixture was known as " Campiellite."

Concentrations of 6 mgm. per cu. m. cause strong irritation of the conjunctiva and of the mucous membranes of the respiratory system. The limit of insupportability, that is, the maximum concentration which a normal man can support for a period of not over i minute, is 85 mgm. per cu. m., according to Flury, and 40-45 mgm. per cu. m., according to Ferrarolo.3 The mortality-

Cyanogen iodide has a powerful lachrymatory action. It was prepared by Serullas in 1824.* It was not used as a war gas in the war of 1914-18 however.

PREPARATION

It is prepared by the action of iodine on mercuric cyanide 5 or sodium cyanide.6

12 gm. iodine and 20 gm. water are placed in a flask of about 200 ml. capacity fitted with (i) a tap-funnel containing 100 ml. of a 5% aqueous solution of sodium cyanide, (2) an inlet tube for gas, and (3) an outlet tube. The contents of the flask are stirred and 50 ml. of the sodium cyanide solution are run in little by little with continued agitation. The sodium iodide formed in the reaction dissolves the iodine and the liquid in the flask is continuously decolourised. When the 50 ml. sodium cyanide solution have been added, a slow current of chlorine is passed into