2936 REILLY AND MADDEN THE VELOCITY OF DECOMPOSITION CCCCVI.-The Velocity of Decomposition of Heterocyclic Diamnium Salts. Part I . Diaxonium Xalts of the Pyraxole and Pyrazolone Series. By JOSEPH REILLY and DENIS MADDEN. HITHERTO so far as the authors are aware no quantitative measure-ments of the stability 01 heterocyclic diazonium salts have been made. Qualitative examination has shown that aminopyrazole and amino-pyrazolone compounds form a very distinct group and therefore, an investigation has been made in the first instance of the com-parative stability of these substances under certain conditions. The FIG. 1. physicochemical method of Hausser and Miiller (Bull. Soc. chim., 1892 7 721) or the modification of it devised by Cain and Nicoll (J. 1902,81,1412) wm found unsuitable for this particular problem, owing fo the extraordinary comparative stability of some of the compounds examined and the consequent exceptionally long period of time during which the reaction had to be followed.The process adopted by the authors mas to heat a definite volume of a solution containing a known weight of the amine (sufficient to give 56 C.C. of gas at S.T.P. normal diazotisation being assumed) in a specially constructed quartz vessel A (Fig. l) fitted with a ground stopper into which was sealed a tube passing to the bottom of the vessel. The vessel was containecl in a thermostat B at 101-5" which temperature was found to keep the solution of the diazonium salt a t lOO" and was connected through a condenser C to a water-jacketed nitrometer D.A current of air-free carbon dioxide wa OF IEETEROCYCLIC DIAZONrtJM SALTS. PART I. 2937 periodically passed through the solution and the liberated nitrogen waa collected over concentrated aqueous potassium hydroxide. By this method it is possible to overcome the diEculties met with by earlier investigators. The air is completely expelled from the apparatus the vessel containing the solution is placed in the thermo-stat and a current of carbon dioxide is passed through it until the temperature becomes constant (the necesmy time having been determined in a control experiment); the nitrogen evolved during this time is collected and measured. It is thus possible to have an exact starting point for a solution of definite concentration. Further, no allowances such as had to be made by former investigators are necessary for the expansion of the solution and of the air in the apparatus.(The appstratus is so arranged that the air space between the surface of the solution and the mercury in the nitrometer is as small as conveniently possible.) 1-Phenyl-2 3-dimethylpyrazolone-4-diazonium ChEoride (Anti-pyrine-Pdiamium C h i d e ) .-A solution of 0.5075 g. of 4-amino-antipyrine in air-free water and 7.5 C.C. of N-hydrochloric acid (3 equivs.) was treated with 20 C.C. of aqueous sodium nitrite (1.73 g. per 200 c.c.) and the resulting solution made up to 70 C.C. The solution of the diazonium salt originally pale yellow developed a reddish-brown colour on warming. This evidence of a secondary reaction is supported by the fact that after 90% of the substance had de-composed the rate of gas evolution declined rapidly.The decom-position however was mainly a unimolecular reaction. In the table x = the volume of nitrogen evolved after t hours (measured a t 22" and 766 mm.) and k = l / t . log (a/(a - z)) where a = the theoretical volume of " diazo "-nitrogen under the same conditions (= 60 c.c.). t. 2. k x lo3. 0 0.2 -1 8.0 62.15 2 13.4 64-80 3 18.2 52.32 4 22.7 61-61 5 27.0 51.93 6 30-8 52-13 t. x. k x lo5. t* x. k x lo3. 7 344 52-84 14; 49.6 52.50 8 37.5 53-20 15Q 50-5 51.60 9 40.1 53.20 51.2 50.60 10 42.4 53.31 i;i 51-8 50.00 11 44.3 53-00 53.0 46-00 12 46.1 62-93 zi 53-4 -13 47-7 53-00 Neither the addition of excess of mid nor the introduction of a small amount of colloidal gold had any marked effect on the rate of decomposition.Antipyrine-4-diazonium A'itrute.-The rate of decomposition was of the same order as that of the chloride. The amount of secondary reaction was however slightly greater the rate of gas evolution falling off more rapidly towards the end. Antipyriue-4diamizcm Su1phate.-The rate of decomposition w~b 2938 REILLY AND MADDEN THE VELOCITY OF DECOMPOSITION much more rapid a t the beginning and did not follow the uni-molecdar law the value of k falling from 0.1305 (8 C.C. in the h t 8 hour) to 0.0544 (38.8 C.C. in 9 hours) ; a behg 57.35 C.C. That the amount of secondary reaction was considerably greater was shown by the more marked deepening of colour and the formation of a dark reddish-brown precipitate.The rate of decomposition then is in this case considerably influenced by the nature of the anion unlike the results obtained by Cain (Ber. 1905 38 2511) with benzenoid diazonium salts. ChEoride.4-Amino-3 ; 5-di-methylpyrazole (0.2775 g.) was dissolved in 15 C.C. of N-hydrochloric acid (6 equivs.) and treated as in the foregoing cases. The amount of decomposition in 3 hours was almost inappreciable less than 2% of the total " diazo "-nitrogen being evolved. In a second experiment, 3 equivs . of acid being used the decomposition followed the peculiar course indicated by the following figures : Temp. = 18". 3 5-Dimethylpyrazole-4-diazonium Pressure = 756 mrn. Q = 60 C.C. t ............... 0 1 2 3 4 5 6 7 z ...............0.0 1-0 2-4 4-2 6.0 7.8 9.7 11-6 t ............... 8 9 10 11 12 14 16 1s x.... ........... 13.6 15.4 17-4 19.4 21.4 25.0 28.7 32.1 t ............... 20 24 28 32 36 40 44 48 x ............... 35.0 40-6 45-4 49-3 52.1 54.6 56.2 57-2 These figures show that even after 48 h o ~ ' heating the decom-position of the diazonium salt is not complete. It is evidently irregular a t the beginning less nitrogen being evolved during the first hour than during any of the subsequent 20 hours. This be-haviour is probably due to the existence of two isomeric forms of the diazonium chloride and this view is supported by the fact that when a small quantity of the base is diazotised a t the ordinary temperature and added to alkaline p-naphthol no coupling takes place for some time whereas a colour is immediately developed if the diazonium solution has previously been boiled for a few minutes.After the fourth hour the decomposition proceeds with some degree of regularity but not in accordance with the unimoleculax law. 3 5-Dimethylpyrazole-4-diazonium Su1phate.-As in the case of the chloride the reaction was irregular a t the beginning. From the end of the fourth hour to the end of the twelfth hour the rate of decomposition followed the unimolecular law the mean value of k being 0-0720. A solution of the diazonium salt containing an excess of sulphuric acid (6 equivs.) was remarkably stable only 2% of the total " diazo "-nitrogen being evolved in 3 hours. Pyrazole-4-diazonizcm Chloride.-3 5-Dimethylpyrazole was con-verted by Knorr's method (Annulen 1894,279 218) into the corre OF HETESOCYCLIC DIAZONIUM SALTS.PABT I. 2939 sponcling 3 5-dicarboxylic acid the latter however being isolatd from the oxidation product by the addition of hydrochloric acid without the intermediate formation of the acid potassium mlt. Pyrazole obtained by heating the dry dicarboxylic acid at 270-275* was converted by Buchner and Fritsch’s method (An&, 1893 273 265) into the 4-nitro-compound. This was isolated by adding ice to the nitration mixture and was reduced as follows : A solution of 3 g. of the nitropyrazole in 100 C.C. of moist ether waa treated with cooling with a large excess of an aluminium-mercury couple during a few hours complete reduction being indicated by FIG. 2. 4 8 12 16 20 24 28 32 36 Time of heating (hour8).I o-Nitrobenzenediaumium chloride. 11 Pyrazole-4-diawnium chlopide. III 1 -Ph.elenyl-2 3-dillaethylw~zololae-4-diazonium cJdoride. IIIa l-Phenyl-2 3-dimethybpymzolone-4-diazm~um 8 u l p W . IV 2 3-Dimethylp~azoie-4-dkzoniwn c h l u d e . N o 2 3 - D i m e t h y l p ~ a ~ e - 4 - d i a ~ ~ i ~ ~ d p h a t e . the disappearance of the purple colour initially developed. The ether was distilled off the residue extracted a few times with boiling alcohol air being excluded and the extract was immediately treated with hydrochloric acid. The product was evaporated, and the residue recrystallised from alcohol ; 4-aminopyrazole dihydrochloride then separated. As the quantity obtained was small the somewhat dark product was not further purified before diazo tisation.The rate of decomposition of the diazonium chloride from 0.39 g. of the dihydrochloride approximately obeyed the unimolecular law. In 5 hour^ 83% of the total “diazo”-nitrogen was evolved but thereafter the rate of decomposition declined very rapidly. The much more rapid decomposition of this substance seems to indicate VOL. cxxm. 5 2940 U D L E S THE SWELLING AND DISPERSION OF that the methyl groups in diazotised 3 5-dimethylaminopyrazole have a slabiliaing influence on that diazonium salt. The comparative stability of the diazonium chlorides from 4-aminoantipyrine 4-amino-3 5-dimethylpyrazole and 4-amho-pyrazole may be judged from the curves in Fig. 2 showing the rate of gas evolution at various intervals. Of the large number of benzenoid diazonium salts investigated by Cain and Nicoll (Zoc.cit.) the most stable (and the only two apparently sufficiently stable for examination a t 100") were those from 0- and m-nitroaniline. The conditions under which the experiments described above were carried out were different from those employed by earlier investigators but results closely analogous to those of Cain and Nicoll were obtained in a control experiment in which o-nitroaniline was used. A comparison of the results obtained from the diazonium chlorides in the pyrazole and pyrazol-one series with those from 0- and m-nitroaniline is therefore justifi-able (for 4-amino-3 5-dimethylpyrazole the results are the mean of the experiments in which only 3 equivalents of hydrochloric acid were used in the diazotisation). Time (mins.) k (t measured yo Decomp. for half de-Substance. in mins.). in first hr. composition. m . Ni trotmiline ........................ 0-03 3 2 100 10 o-Nitroaniline ........................... 0.00564 54 53 4-Aminopyrazole ..................... 0.003 17 40 80 4-Aminoantipyrine ..................... 0.0008 8 13 360 4-Amino-3 5-dimethylpyrazole ... 0.00030 1.7-2 1020 In the case of 4-amino-3 5-dimethylpyrazole the rate of decom-position of the diazonium chloride was considerably slower (3 to 4 times) when excess of acid (6 equivs.) was employed. It cannot be said as of benzenoid salts that the rate of decomposition in the case of heterocyclic diazonium salts is uninfluenced by the nature of the anion for the diazonium sulphates of both 4-aminoantipyrine and 4-amino-3 5-dimethylpyrazole decomposed much more rapidly than the corresponding chlorides. UNIVEIWITY COLIXGE CORE. [Received September 7th 1925.