Sartori The War Gases Chemistry and analysis

174 HALOGENATED NITROCOMPOUNDS

ligroin (not over 60° C.) and the tetrachlorodinitroethane crystallised out as plates.

The nitrogen peroxide is prepared by drying the product obtained by the action of nitric acid on arsenious oxide by means of calcium nitrate and saturating it with oxygen.

PHYSICAL AND CHEMICAL PROPERTIES

It occurs as crystals which are decomposed by heating to 130° C. with evolution of nitrogen peroxide. In a closed tube it melts at 142° to 143° C.

It is volatile in steam and insoluble in water, but easily soluble in benzene and ether. It dissolves also in alcohol and in acetic acid. From these two solvents it may be reprecipitated by addition of water.

It is not decomposed, even at the boiling point, by aqueous alkali solutions. With alcoholic potash it is converted into a crystalline substance of the formula x :

CC12N02

CC1(OK)NO2

Potassium cyanide causes complete breakdown of the molecule, and potassium carbonate, cyanogen chloride and carbon are formed.

Heat is developed on adding an alcoholic solution of tetrachlorodinitroethane to an aqueous solution of potassium iodide, and iodine separates as well as a crystalline substance : potassium tetranitroethane.2

Tetrachlorodinitroethane has powerful irritant3 properties, and besides being about six times4 as toxic as chloropicrin, has about eight times the lachrymatory power of the latter, according to Nekrassov.

acid. It was never employed as a war gas in the war of 1914-18.

PREPARATION

This compound may be obtained in various ways :

(a) By the action of picric acid on " bromide of lime." 6

5STENHOUSE, Ann., 1854, 91, 307.

•GROVES and BOLAS, Ber., 1870, 3, 370.

(b)By the action of bromine and potash on nitromethane. x

(c)By the action of bromine on nitranilic acid (i.e., ^-dinitro dihydroxy quinone).2

The most practical method of preparing bromopicrin in the laboratory is the following, according to Bolas and Groves3 :

Four parts of calcium oxide and 50 parts of water are mixed in a flask. Six parts of bromine are then added in small portions while the flask is shaken and externally cooled to prevent an excessive rise in temperature. One part of picric acid is then added and the mixture distilled under reduced pressure. The bromopicrin passes over in the first fractions of the distillate. It is separated from the water and dried over calcium chloride.

It is only slightly soluble in water, but dissolves easily in benzene, carbon tetrachloride, chloroform, alcohol and ether. Bromopicrin is precipitated from its alcoholic solution by the addition of water.

It dissolves small quantities of iodine to form a violet solution. When heated rapidly at the ordinary pressure it decomposes with explosion. On heating to 130° C. it forms carbonyl bromide

and nitrosyl bromide according to the equation : CBr3NO2 -> COBr2 + NOBr.

In general, bromopicrin behaves chemically like chloropicrin, but is less stable to chemical reagents. By the action of brominating agents it is converted into carbon tetrabromide (Bolas and Groves).

It reacts with potassium iodide variously according to the conditions of the reaction :

176 HALOGENATED NITROCOMPOUNDS

bromopicrin in the cold to give, as final products of reaction, the potassium salt of symmetrical tetranitroethane and cyanogen bromide.x

Alcoholic solutions of bromopicrin precipitate silver bromide by the action of silver nitrate, slowly in the cold and rapidly on warming (Stenhouse).

When bromopicrin is treated with a solution of potassium hydroxide (i part KOH and 1-5 parts H2O) it first dissolves slowly and then decomposes as follows 2 :

CBr3N02 + 6KOH = KNO2 + K2CO3 + sKBr + 3H2O.

or else,

2CBr3NO2 + zoKOH = 2K2CO3 + N2 + sKBr + 5H2O + KBrO3.

It is not decomposed by cold sulphuric acid.

Like chloropicrin, it reacts with hydrazine to evolve nitrogen.3 It also reacts with sodium ethylate, but very slowly.4 Sodium or potassium sulphide react differently with bromopicrin,

according to the conditions of the reaction.5

Bromopicrin vapour irritates the eyes : the minimum concentration which causes irritation is 30 mgm. per cu. m. of air, according to Lindemann.6 According to Mayer's researches,7 bromopicrin has a toxic power only one-eighth to one-tenth of that of chloropicrin.

Analysis of the Halogenated Nitro-compounds

DETECTION OF CHLOROPICRIN

The detection of chloropicrin is most simply carried out by direct sensory perception.

Various methods have been suggested for the detection of chloropicrin by chemical means. None of these is as sensitive as perception by odour, according to Deckert.8

Method of Pyrogenic Decomposition.9 One of the most widely used methods for the detection of chloropicrin in air consists in decomposing it by heat and then testing for chlorine in the products.

1 SCROLL and BRENNEISEN, Ber., 1898, 31, 642.

• W. DECKERT, Z. Hyg. Infektionskrankh., 1929, 109, 485.

• A. C. FIELDNER, OBERFELL, etc., /. Ind. Eng. Chem., 1919, 11, 519.

The gas mixture is passed through a tube of quartz or porcelain heated almost to redness and the products of the decomposition are then bubbled through a solution containing potassium iodide and starch paste. If chloropicrin is present in the mixture under examination, chlorine will be liberated and this will liberate iodine from the potassium iodide and so colour the starch blue.1

Engel's "Indicator Apparatus " is based on a similar principle.2 It consists of a glass tube (see Fig. 12) through which passes a

FIG. 12.

silica rod which can be electrically heated to redness. The tube is connected with a special glass receiver inside which the stem of a tap-funnel projects so as to hold in suspension a drop of starchiodide solution. The detection of chloropicrin in air is carried out by passing the gas to be examined through the apparatus. If chloropicrin is present, the hanging drop of starch-iodide solution will be coloured blue.

The sensitivity of this reaction depends very much on the age of the starch solution and it is advisable to use a recently prepared solution.3

The Flame Test. Another method of detection, also based on the pyrogenic decomposition of chloropicrin, consists in passing the gas mixture to be examined into a gas jet, the flame of which maintains a copper spiral at red heat.4

This flame, in presence of even 0-25 mgm. chloropicrin per litre of air, is coloured green (Krczil).

Ray and Das's Method.5 This method of detection is based on

1 This method of detecting chloropicrin was employed to determine the duration of the protection afforded by anti-gas niters against mixturesof air and chloropicrin. DUBININ, /. Prikl. Khim. 1931, 1109.

178 HALOGENATED NITROCOMPOUNDS

a reaction recently discovered by Ray and Das, according to which chloropicrin, in reacting with the potassium salts of mercaptans, forms insoluble condensation products.

According to Nekrassov,1 in this method the gas to be tested is passed through an alcoholic solution of the potassium salt of dithioethylene glycol. In the presence of chloropicrin a yellow precipitate, m.p. 123° C., separates.

This method may also be conveniently applied to the quantitative determination of chloropicrin, since the chlorine in the chloropicrin molecule is quantitatively converted into potassium chloride.

Sodium Ethylate Method. This method consists in decomposing chloropicrin with sodium ethylate (see p. 171) and then proceeding to the detection of the sodium nitrite or chloride formed in the reaction :

CC13NO2 + 4C2H6ONa = C(OC2H6)4 + 3NaCl + NaNO2.

By means of this reaction chloropicrin may be detected at a concentration of about 6 mgm. per cu. m. of air, according to Ireland.2

Dimethylaniline Paper. The detection of chloropicrin by means of this paper depends on the change in colour from white to yellow or maroon which takes place in presence of this gas.

The dimethylaniline papers are prepared by soaking strips of filter paper in a 10% solution of dimethylaniline in benzene.3

If the concentration of chloropicrin is low, the colour change may be seen when the paper is moved about in the gaseous

mixture to be tested through an alcoholic solution of thiophenol. In the presence of chloropicrin a white precipitate or opalescence

chloropicrin by metallic calcium to form nitrous acid, which is detectable by the Griess reaction.7

1 NEKRASSOV, Voina i Tecnica, 1926, 275, 32.

2IRELAND, op. cit., 298.

3DECKERT, Z. Hyg, Infektionskrankh., 1929, 109, 485 ; Z. anal. Chem., 1938,

113, 183.

180 HALOGENATED NITROCOMPOUNDS

This method, according to Aksenov,1 may be applied to the determination of chloropicrin vapour in air. A measured volume of the air is passed through an aqueous alcoholic solution of potassium sulphite, the latter then boiled and the amount of chloride determined by Volhard's method.

SODIUM PEROXIDE METHOD. This method consists in decomposing the chloropicrin with sodium peroxide and then volumetrically determining the chlorine liberated in the reaction.2 A measured volume of the gas mixture to be examined is passed through a wash-bottle containing 50 ml. of a i% solution of sodium peroxide in 50% ethyl alcohol prepared in the following manner: 2 gm. sodium peroxide is dissolved in 100 ml. iced water and just before using 25 ml. of this solution is diluted

with 25 ml. 95% alcohol.

The chloropicrin, on coming into contact with the alcoholic solution of sodium peroxide, decomposes, forming sodium chloride, which may be determined volumetrically with a N/ioo solution of silver nitrate (using potassium chromate as indicator), after neutralising the alcoholic solution with sulphuric acid (to the phenolphthalein end-point).

The number of ml. of silver nitrate solution used multiplied by 0-546 gives the quantity of chloropicrin in mgm. present in the volume of air passed through the sodium peroxide solution.

This method of analysis was also used by Dubinin 3 for the determination of chloropicrin in the gas mixtures prepared for the testing of activated carbons.

1 AKSENOV, Metodica Toksikologii boevikh otravliajuscikh Vescestv, Moscow, I93i. 95-

1FIELDNER and coll., /. Ind. Eng. Chem., 1919, 11, 519.

*DUBININ, J. Prikl. Khim., 1931, noo.

182 CYANOGEN DERIVATIVES

during the war of 1914-18 because of its high vapour tension and its rapid diffusion.

Only the French used it, and not more than 4,000 tons were employed during the whole war period.

Hydrocyanic acid may be prepared in various ways: by passing electric sparks through a mixture of acetylene and nitrogen,

C2H2 + N2 = 2HCN

or by heating chloroform with ammonia,

/Cl H\

H-Cf Cl + H-)N = 3 HC1 + HCN

\C1 H/

The method most commonly used, especially if it is desired to prepare anhydrous hydrocyanic acid, consists in decomposing a cyanide with an acid (hydrochloric, sulphuric, hydrosulphuric, carbonic, etc.).

KCN + HC1= KC1 + HCN

Hg(CN),.+ H2S = HgS + 2 HCN

LABORATORY PREPARATION 1

100 gm. potassium cyanide, as free as possible from carbonate and in pellet form, are placed in a flask fitted with a tap-funnel and a delivery tube. The delivery tube leads to two U tubes in series which are filled with a mixture of fused and granular calcium chloride, and immersed in a water-bath at 35° C. The second of these tubes may be connected to two more U tubes, the first of which is maintained at about — 10° C.2 in a freezing mixture and the second at + 20° C. Each of these tubes, which are designed to receive the condensate of hydrocyanic acid formed in the reaction, is fitted at the bottom with a glass lead-off tube. This is connected through a glass cock to a cooled flask in which the liquid hydrocyanic acid collects.

Before the preparation is commenced it is advisable to pass a current of dry air through the whole of the apparatus. Cooled aqueous sulphuric acid (i: i) is allowed to drop slowly through

first receiver every second minute. Towards the end of the