PREPARATION OF CHLOROPICRIN FROM PICRIC ACID ETC. 20 I.-Prepalration of Chloropickn from Picric Acid * and Trinitrotoluenes. By KENNEDY JOSEPH PREVITB ORTON and PHYLLIS VIOLET MCKIE. CHLOROPICRIN was first prepared by Shenstone (Annalen 1847 66, 241) from picric acid by the action of bleaching powder chlorine, potassium chlorate and hydrochloric acid or aqua regia. Hofmann (ibid. 1866 139 111) described in some detail the procedure when bleaching powder is used. He obtained a yield of 114 per cent., that is 53 per cent. of the maximum (215 per cent.) if all the nitro-grcups of the picric acid appear in the chloropicrin: C6H,(N0,),*OH + 3CC13*N0,. * Orton and Pope English Patent No. 1428.78 (1918) 30 ORION AND MCKIE PREPARATION OF CHLOROPICRIN Hcfmann’s yield is undoubtedly low; there is no difficulty in obtain-ing a yield of 150-160 per cent.a figure which is somewhat above the yield of 143.5 per cent. which corresponds with the appearance of two nitro-groups as chloropicrin : C,H,(NO,),*OH -+ 2CC13-N02.* A consistently better yield reaching 200 per cent. can be obtained by passing chlorine into a cooled suspension of picric acid (sodium picrake) in aqueous sodium carbonate. The products are chloropicrin and some nitric acid together with chloride and some hypochlorite and chlorate which has arisen from transf ormation of hypochlorite. No other compounds are present in appreciable quantity. From determinations of the hypochlorite chlorate and chloride in the aqueous product it is found that the proportion of the last accords well with the opinion that chloropicrin is formed in a reaction between hypochlorite (hypochlorous acid) and picric acid thus: that is, when the maximum yield would be 215 per cent.; or, C,H,(NO2),*OH + llHCl0 = 3CCl,*NO + 2HC1+ 3C0 + 6Hz0, C6H2(NO2),.OH + 11C1 + 5H,O = 3CC1,*N02 + 13HC1+ 3C02 (a) C,Hg(N0,)3*OH + 12C12 + SH,O = 2CCl,*NO + 18HC1+ 4C0 + HNO, (U) when the maximum yield would be 143.5 per cent.When the yield of chloropicrin is low however other substances appear among the products. The quantity of nitric acid in the product (delermined by boiling with excess of ferrous chloride and measuring the nitric oxide evolved) is a measure of the exieiit t o which the reaction has followed a course other than that represented by equation A .Nearly the whole amount of nitro-group of the picric acid-up to 96-97 per cent.-is accounted for by the chloropicrin and the nitric acid; probably most of the deficiency is due to want of preci-sion in measurement and to the loss of chloropicrin by evaporation during the preparation. Eflect of Degree of 3 llcatiizity O ~ L the Reaction.-According to equation A 23 equivalents of alkali are required by the left-hand side of the equation. The right-hand side however only needs 19 equivalents. Hence during the reaction the alkalinity would increase if 23 equivalents were used initially. Moreover, * Since the work described in this paper was completed Gardner and Fox (T. 1919,115 1188) have recorded that the yield by the Shenstone-Hofmmn method can be raised to 180-190per cent.when the proportion of the reagents and the quality of the bleaching powder are right FROM PICRIC ACID AND TRINITROTOLUENES. 31 6 of these 19 equivalents are required by carbon dioxide regenerat-ing carbonate which is again available for reaction with chlorine. Hence the minimum proportion of alkali for the reaction repre-sented by equation A is 13 equivalents. In equation B 25 equiva-lents of alkali are required on the left-hand side and 27 on the right-hand side; of these again 8 are in the form of carbonate and therefore available; hence the minimum number is 19. I n practice since under the best conditions nitric acid is always formed and therefore the reaction in part follows equation B it is best to use 15-17 equivalents of alkali.I n strongly alkaline media such as sodium hydroxide in place of sodium carbonate the yield is greatly diminished. When the sodium hydroxide is concentrated the react’ion is very slow for the solubility of the picrate is depressed and moreover only hypo-chlorite and not hypochlorous acid is present at least in the initial stages. At the same time a red solid appears as a product. I f the concentration of the sodium hydroxide is kept below 0.lX and the hydroxide (13 equivalents in all) added gradually during the reaction the yield of chloropicrin is as good as when sodium carbonate is used. When bleaching powder is used calcium hydroxide would be formed i f the reaction follows that course (equation A ) in which tlhe maximum yield is obtained thus : 2C,H,(N0,)3*OII -t- llCa(OCl),= GCCI,*NO + 6CaC03 + 2CaC1 + 3Ca(OH),.(Using bleaching powder containing 30 per cent. of “active chlorine,” 1 part of picric acid would require 3-4 parts of bleaching powder.) Owing to the low solubility of calcium hydroxide the aqueous medium in which probably the reaction only takes place, will not have an alkalinity above 0*04-Om02N which would not be an unfavourahle degree of alkalinity. TABLE I. , Yield of chloropicrin. Alkali. Per cent. NaOH 23 equivalents. Concentration N . . ........ 110 Na,C03 9 equivalents; NaOH 8 equivalents. ... 182 Na,CO 3 17 equivalents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 Soda ash (Na,CO, 90 per cent. ; NaOH 5 per cent.) 194 Na,CO, 12 equivalents ; NsHCO, 5 equivalents ...196 T”he temperature was in all experiments 2-5O. The Effect of Temperature.-A low temperature is favourable to the yield of chloropicrin that is t o the reaction represented b 32 ORTON AND McKlE PREPARAT'ION OF CHLOROPICRIN equation A . In table 11 17 equivalents of sodium carbonate were used throughout. TABLE 11, Temperature. Yield. 16-17O 168 per cent, 13-14*5" 173 1 oo 182 2-5O 192-1 94 O0 198-199 Only a small amount of heat is set free in the reaction when sodium carbonate is used. Effect of the Presence of Sodium C'h1oride.-The presence of sodium chloride is deleterious to the reaction in which chloropicrin is formed thus in 18 per cent. aqueous sodium chloride the yield was 149 per cent. and in 30 per cent. sodium chloride 92 per cent.The '' red solid " is formed in considerable proportion especially in the more concentrated salt solution. Owing to the depression of solubility in water of chlorine by sodium chloride the gas is absorbed poorly. Further the high concentration of chloridion keeps the concentration of free hypochlorous low (Jakowkin Zeitsch. physilcal. Chem. 1899 29 613) and the solubility of the sodium picrate is depressed by the sodium chloride; hence the reaction will be retarded. Again the concentration of chlorine will be greater, and any reaction with picric acid not leading t o chloropicrin there-fore favoured. Use of the filtrate (which contains about 8 per cent. of sodium chloride) from one operation is therefore inadvis-able; a second re-use causes a serious diminution of yield.It is not improbable that this effect of chloride accounts for the great in5uence of " quality " of the bleaching powder on the yield of chloropicrin when this agent is used. E X P E R I M E N T A L . Picric acid is dissolved in a hot solution of 4 parts of sodium carbonate (17 equivalents) in 50 parts of water. On rapidly cool-ing sodium picrate separates in small crystals. Large crystals should be avoided. 'The thin paste is cooled to below 5O and stirred slowly while a current of washed chlorine is introduced. The treat-ment with chlorine should either be intermittent or fairly slow. Too rapid a current of chlorine not only leads to loss of gas but also t o the production of chlorate. The speed of the reaction is mainly determined by the size of the crystals of the sodium picrate.The scdium picrat'e gradually dissolves and ultimately a colourless oil and a colourless solution are obtained. When industrial chlorin RROM PlUBlC ACID AND TRINlTROTOLUlGNES. 33 has been used the oil is yellow. The oil can easily be directly sepa-rated from the aqueous layer or distilled in a current of steam. It amounts to 1-17 parts by volume and is approximately pure chloropicrin. Solubility of G'kloropicrin in Water.-At loo the solubility is about 0.058 gram per 100 C.C. It rises somewhat with the tem-perature. By vigorous distillation 75 per cent. of the chloropicrin can be recovered from the solution. A smaller proportion can be salted out. 'The aqueous solution is stable for a long period and remains neutral after boiling for some time.Action of Hypochlorite Solution o n o- and p-Mononitro- and 2 4-Binitro-phenol.-2 4-Dinitrophenol behaves much as picric acid. There is a greater development of heat and ultimately a colourless oil and solution are obtained. The yield of chloropicrin is only 50 per cent. of the maximum. p-Nitrophenol gives a t first a heavy precipitate which ultimately dissolves yielding a colourless oil and solution. The yield oi chloropicrin is 33-34 per cent. of the maximum. o-Nitrophenol gives a deeply coloured precipitate and liquid. The solid resists t I eatment wilh hypochlorite. The yield of chloropicrin is only 10 per cent. of the maximum. Action of Nypochlorite O I Z Trinitrotoluene and on Trinitrotoluene Residues.-As these materials are insoluble in alkali contact is most easily effected by " sludging " with water and bleaching powder. As a result of many trials the proportion of 1 part of nitro-compound to 15 parts of bleaching powder (" active chlorine," 29 per cent.) and 25 parts of water were found best. The mixture is gradually heated and then a current of steam passed through. If the mixing has been thorough the colour of the paste is completely discharged save from trinitrotoluene residues. TABLE 111. Yidd ~ ~ 1 3 a. percentage of weight of Material used. material used. T.N.T. Grade I. ..................... 82 T.N.T. Grade 111. .................. 84 T.N.T. '' residues " .................. 70 Dinitrotoluene ........................ 4 s-Trinitrobenzene .................. 53 UXIVERSITY COLLEGE OF N. WALES, BANGOR. [Received NovembeT 22nd 1920.1 VOL. CXIY.