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Analysis of the Ethylene used in these experiments.
Table 11.-The Action of Heat upon Ethylene flowing at the rate of
4:2 c.c. per minute through 6 in. of heated tube.
I. Analysis of heated gas made with paraffin absorption and explosion.
gas in tube .... 800° C. 960° C. 1000° C. 1200° C.
18:31 Saturated hydro
28:57 .. 0
23.405 24.01 Hydrogen
49:51 Carbon monoxide.
1:10 Carbon dioxide...
II. Analysis of heated gas made without paraffin absorption. Unsaturated hydrocarbons... 92.0
34.11 Carbon nionoxide.
These experiments show conclusively that when only flowing slowly through the tube a higher member of the paraffin group, probably ethane, is formed up to 900° C., whilst at 1000° C. the quantity has
rapidly diminished, and at 1200° C. methane is the only member of the series present.
In the same way a small trace of some unsaturated hydrocarbon, probably benzene, is present at the lower temperatures, but disappears when 1200° C. is reached.
The next step was to see if the action of heat upon ethane under the same conditions as those existing in the previous set of experiments bore out the results arrived at.
Ethane was prepared by acting on ethyl iodide with a copper-zinc conple in presence of water, and passing the evolved gas through fuming sulphuric acid to purify it.
Table III.-The Action of Heat upon flowing Ethane.
These results show that, even below 900° C., ethane decomposes with liberation of hydrogen and formation of unsaturated hydrocarbons, which on examination prove to consist of ethylene with small quantities of acetylene, rise of temperature completing this decomposition, and also causing the ethylene to decompose as before.
It is evident that, if ethane had been a primary product in the decomposition of ethylene, it would in turn have decomposed with liberation of hydrogen at or below 900° C., and hydrogen would have been found at that temperature as a product of the decomposition of the ethylene instead of its appearance being coincident with the deposition of carbon at 1200° C.
The fact that with a rapid flow I was unable to detect a trace of free hydrogen until carbon has begun to deposit, or vice versa, shows the fallacy of the text-book equation
CH = CH+C, and on examining the evidence upon which the statement is based we find that Marchand,* who originated it, passed ethylene through glass and earthenware tubes heated to redness, and analysed the resulting
* 'Journal für prakt. Chemie,' vol. 26, p. 478.
products by passing them over heated copper oxide, and estimating the carbon dioxide and water vapour formed. At first the proportion of carbon to hydrogen was 100 to 17-236, which corresponds to nearly pure ethylene, and on heating to a higher temperature the ratio altered to 100 carbon to 30:771 hydrogen, which nearly corresponds to methane, carbon being at the same time deposited; but this is manifestly no proof of the gas being methane, as a mixture of undecomposed ethylene and hydrogen, or mixtures of ethylene, methane, and hydrogen, such as those formed at 1200° C., would give the same result.
His results were to a certain extent confirmed by Buff and Hofmann, who noticed that when the platinum spiral was heated in pure ethylene there was at once a deposition of carbon, whilst the gas scarcely expands, from which they concluded that methane had been formed at the same time. When, however, the experiments tabulated in Table I are examined, it will be seen that as soon as 1200° C. is reached and carbon is deposited expansion takes place; but that at all temperatures short of that there is contraction due to some of the gaseous products undergoing polymerisation and yielding liquids. In Buff and Hofmann's experiment the gas
in contact with the incandescent wire was decomposed with liberation of hydrogen and separation of carbon ; but the expansion caused by this action happened to be nearly equalised by the contraction due to polymerisation in the less heated portions of the gas.
The simultaneous appearance of carbon and hydrogen indicates clearly the liberation being due to the splitting up of a hydrocarbon, and the proportion in which these elements are liberated point to acetylene as being the body concerned.
In a paper read before the Chemical Society* I showed that in the interior of a luminous flame the olefines are to a great extent converted into acetylene, which decomposes at about 1200° C. with liberation of carbon, which, being heated partly by its own combustion and partly by the combustion of methane and hydrogen, becomes incandescent, and gives luminosity to the flame, and in the experiments which I have described I fully expected to find a higher percentage of acetylene; but the great tendency towards polymerisation which that body exhibits seems to at once determine its conversion into benzene, which can readily be distinguished among the liquid products, whilst a number of other more complex hydrocarbons are produced, among which crystals of naphthalene are conspicuous.
In order to ascertain if the behaviour of acetylene when passed through the heated tube under the conditions of these experiments gave results which support this view, acetylene was prepared by the action of dilute hydrochloric acid on acetylide of copper.
* Chem. Soc. Jour.,' vol. 61, p. 322.
Analysis of Original Gas.
4.60 The gas was passed through the platinum tube, 25 mm. of which was heated to a temperature of 1000° C.
showing that even under these conditions nearly three quarters of the acetylene had undergone polymerisation, so that it is probable that in the case of nascent acetylene, liberated from ethylene by the action of heat, the condensation of the acetylene molecules to form benzene would be practically instantaneous, unless the temperature were sufficiently high to cause dissociation to carbon and hydrogen at the moment of liberation.
The unsaturated hydrocarbons consisted chiefly of ethylene with some benzene vapour, the ethylene probably having been formed by the direct combination of acetylene and hydrogen, an interaction first noticed by Berthelot,
C,H+H, = C,H. This also accounts for the small quantity of free hydrogen found on analysis, which, having regard to the amount of carbon deposited, should have been considerably higher.
It will be noticed that with the rate of flow employed in the experiments shown in Table I, the largest amount of acetylene found in the gas after heating was 3.60, which occurs just at the temperature when carbon begins to deposit freely, and is therefore sufficiently
high to check the polymerisation of the acetylene, and many attempts were made to find conditions under which the acetylene could be liberated and prevented from polymerising, and it was found that this could apparently be, to a certain extent, effected either by diluting the ethylene with a considerable volume of an inert gas, or else increasing the rate of flow through the heated tube.
On passing a mixture of 75 per cent. hydrogen and 25 per cent. ethylene through the tube, heated as before, 3:43 per cent. of acetylene was produced, which would be equivalent to 13.72 on the ethylene present, whilst the following results show the effect of increasing the rate of flow of the gas through the tube. The original gas taken was a bad sample containing 87:49 per cent. of ethylene and 12:51 of nitrogen, and the rate of flow was increased to 15 c.c. per minute, the tube being heated to 1250° C. Unsaturated hydrocarbons.
containing acetylene. 4:49 Saturated hydrocarbons..
100.00 Increase in volume .....
Large Carbon and oil deposited.... 0·006 gram per 100 c.c. showing a very marked increase in the amount of acetylene formed.
Before it was possible to trace the primary action taking place during the heating of ethylene, it was necessary to find how the temperatures and methods I was employing affected pnre methane, which plays so important a part amongst the products of decomposition.
Methane was prepared by acting on methyl iodide by means of the copper
zinc couple in the presence of alcohol and water.
Table IV.–The action of Heat upon flowing Methane.
Percentage of methane in the ori
ginal gas .. Temperature of gas in the decompos.
1200°C. | 1300°C. \ 1500° C.
ing tube.... Analysis of gas after heatingUnsaturated hydrocarbons
Containing acetylene Saturated hydrocarbons..
0.07 0.07 90.00 8:53
0:39 0:39 88:52 10:37
1.20 0.963 19.22 78.66