T H E THERMAL DECOMPOSITION OF DIAZOMETHAKE BY E. W. R. STEACIE
According to v. Pechmann' diazomethane decomposes thermally to give ethylene and nitrogen as indicated by the equation CH&z = Nz
+ W CzH,;
and if the vapour is heated above ~ 0 0 ° Cit may explode violently. Staudinger and Kupfer,z however, claim to have used the vapour of diazomethane at 400-j00°C in the synthesis of ketene. An attempt was made by the writer to investigate the slow decomposition of the substance at temperatures below that at which it explodes. This attempt was abandoned after a number of serious explosions, but a few of the results which were obtained may be of interest and are given in the present communication. Preparation of Diazomethane Diazomethane was prepared by treating an amyl ether solution of nitroso methyl urethane with a 7 0 per cent aqueous solution of potassium hydroxide, and refluxing the m i ~ t u r e . The ~ gas evolved was condensed in a bulb immersed in a carbon dioxide-acetone mixture. The liquid thus obtained was fractionally distilled and was stored as a liquid in a bulb at -80°C. The Products of the Thermal Decomposition Experiments on the thermal decomposition were carried out by letting small quantities of the vapour into a bulb contained in an electric furnace at temperatures from 140' to zzo°C. The progress of the decomposition was followed by pressure measurements made with a McLeod gauge. In the slow decomposition the products were ethylene and nitrogen as indicated by the equation CHPKz = K2 $5C2Hi.
+
This would correspond to an increase in pressure of so per cent when the reaction reached completion. Some of the observed increases in pressure under various conditions of temperature and initial pressure were so, 54, 5 1 , 48, and 5 4 per cent. The products of the reaction were analysed in a number of cases, and two typical analyses follow. 217°C. Initial pressure ii,% cms. K?= 6 8 . 8 9 , C2H4 = 28.8% saturated hydrocarbons (as CH,) = 2 . 4 $ . Ber., 31, 2643 (1898). 508 (1912). v. Pechmann: Ber., 28, 8 j j (1895).
* Ber., 45, 8
I494
E. W . R. STEACIE
18Y"C. Initial pressure 10.9 cms. S2 = 67.2%, CzHl = 32.0(,7,, CH4 = 0.87;. These are in satisfactory agreement with the calculated composition froni the above equation, viz. NS = 66.65,C2H4 = 3 3 . 3 5 . I n the explosive decomposition a large amount of carbon and tarry material was deposited, the gaseous products being mainly nitrogen, together with various hydrocarbons and some hydrogen. This is to be expected since, a t the high temperature of the explosion wave, ethylene would be largely decomposed.' The Explosive Decomposition The explosion limits of the reaction were determined approximately. At z 1 7 T an explosion occurred if the pressure of the diazomethane was above I O ems. At 187'C the limiting pressure was about 18 ems. Any accurate determination of these limits was impossible since, as will be discussed later, the explosive nature of the substance is very susceptible to traces of organic matter. The explosion limit was definitely lowered, however, by the presence of an inert gas as is usual with an explosion of the chain type. Thus a mixture having partial pressures of 8 ems. of diazomethane and 30 ems. of nitrogen exploded violently a t 18i'C. It was found that explosions also occurred a t low temperatures due t o traces of organic matter such as carbon or tarry material deposited in a previous explosion. I n one case a bulb containing about one gram of liquid diazomethane was connected to a small evacuated bulb which was a t room temperature. When the pressure of the vapour in the small bulb reached about 5 m.m. an explosion occurred which detonated the supply of liquid a t -80°C. A number of serious explosions occurred in this way and, as it was not found possible to control the behaviour of the substance even a t - Xo'i-', the work was abandoned. The Slow Decomposition Only a few experiments were performed on the slow decomposition. The results of these are listed below: Slow Decomposition in Quartz Bulb Initial Pressure, mms.
Time in seconds for a 1 . 0 5 ~ increase in pressure. 1.e. for 20'; conipletion.
217
52
I1
217
45
I2
187
'09
120
Temperaturp "C.
187
231
162
358 ;60
162
I335 9400
162
I3 5 I35 I3 5
* Hurd: "Pyrolysis
20600
45 of Carbon Compounds," j 6 [ 1929).
3 8000
THE TKERJI.%L DECOMPOSITIOS OF DIIZOMETHBNE
'495
The reaction is apparently bimolecular since the time required for a given fraction of the reactant to decompose is approximately proportional t o the reciprocal of the initial concentration. The addition of powdered quartz to the reaction vessel had no appreciable effect on the rate of decomposition. The reaction therefore is homogeneous.
Fro.
I
The Temperature Coefficient In Fig. I the logarithm of the tinie to 2 0 5 completion is plotted against the reciprocal of the absolute temperature, for an initial pressure of 45 mni. The points fall satisfactorily on a straight line as required by the Arrhenius equation. The heat of activation calculated from the slope of the line is 36000 calories per gram molecule. I t should be emphasized that owing to the small number of experiments reported these results are to be regarded as merely semi-quantitative.
