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Chemical Properties of Ethylene

Decomposition of Ethylene

It was shown by Dalton and Henry that carbon and hydrogen result from passing electric sparks through the gas. Marchand believed that ethylene was decomposed into carbon and methane (C2H4 = C + CH4) at a bright red heat, and Buff and Hofmann observed this result when a platinum wire was heated electrically to dull redness in the gas. No account, however, was taken by these observers of intermediate products which are undoubtedly formed. Berthelot represented the thermal decomposition of ethylene thus:

C2H4 = C2H2 + H2; 2C2H4 = C2H6 + C2H2

acetylene being the ultimate decomposition product - whether CH4, C2H6, or C2H4 is decomposed - which subsequently polymerises to benzene, etc. Lewes represented the decomposition thus: 3C2H4 = 2C2H2 + 2CH4; but the views of Bone and Coward differ somewhat from any of the foregoing.

According to these observers the primary effect of high temperature upon ethylene is to eliminate hydrogen and simultaneously loosen the "double bond" between the carbon atoms, so that the residues CH2•• and CH••• have a momentary existence. These residues may then either (a) form ethylene or acetylene, (b) break down into carbon and hydrogen, or (c) be hydrogenised to methane, thus:

Bone and Coward observed the generation of brown vapours which condensed to a viscous tar when ethylene at a pressure of 365 mm. circulated through a tube at 570°-580° C. These products were complex, but consisted of aromatic hydrocarbons formed by the polymerisation of acetylene or: CH residues.

Additive Reactions of Ethylene

As an unsaturated hydrocarbon ethylene is capable of numerous additive reactions. It combines with hydrogen to form ethane (C2H6) under the catalytic influence of platinum-black at atmospheric temperature; and finely divided nickel produces an almost quantitative yield of ethane at 130°-150° C. Ethylene unites directly with chlorine, bromine, and iodine to form the corresponding dihalides (C2H4X2), and also with NO2, S2Cl2, and SO2. It combines with HBr and HI at 100° C., but not with HCl, forming C2H5Br and C2H5I respectively. It unites with concentrated sulphuric acid, slowly at atmospheric temperature, quickly at 160°-174°C., forming ethyl-sulphuric or sulphovinic acid, C2H5HSO4; with fuming sulphuric acid it forms ethionic acid, SO3H-CH2-CH2-OSO3H or its anhydride, carbyl-sulphate, . Fuming sulphuric acid is used for the absorption of ethylene in gas analysis.

Chlorosulphonic acid (Cl-SO3H) produces isethionyl chloride, which yields isethionic acid with water.

Hypochlorous acid forms glycol monochlorhydrin, and fuming nitric acid yields ethylene nitrate, C2H4(N2O5). Although water does not combine directly with ethylene to produce ethyl alcohol, the latter may be obtained by hydrolysing the additive compound with sulphuric or nitric acid.

There is evidence of the existence of an oxonium compound with methyl ether, .

Ethylene forms the following double compounds with salts: C2H4.FeBr2, C2H4.PtBr2, C2H4.2KCl; and the following compounds with mercuric salts:

  1. Ethene mercury salts, CH2:CH.HgX.
  2. Ethanol mercury salts, HO.CH2.CH2.HgX.
  3. Ethylether mercury salts, O(CH2.CH2.HgX)2.
  4. Polymerised ethene mercury salts (CH2:CH.HgX)n.
It is supposed that the saturated compound XCH2-CH2HgX is first formed, and then that it loses HX (1), or undergoes hydrolysis (2), which may be followed by condensation (3).

Ethylene also forms an unstable compound with cuprous chloride: C2H4.CuCl.

Oxidation of Ethylene

Through the careful oxidation of ethylene by cold dilute permanganate solution ethylene glycol is produced:

CH2=CH2 + H2O + O = (CH2OH)2

More vigorous oxidation by permanganic, chromic, or nitric acid produces formic, acetic, and oxalic acids, and carbon dioxide.

Combustion of Ethylene and of Hydrocarbons in general

Ethylene burns with a flame of considerable luminosity. This is equal, according to Frankland, to 68.5 candles when the gas is burnt at the rate of 5 cubic feet per hour, that of methane being equal to only about 5 candleb. Ethylene is the chief cause of the luminosity of coal-gas, in which it is present to the extent of 2-5 per cent.

There are three possible views of the manner of combustion of a hydrocarbon: (i) the preferential combustion of hydrogen with the consequent separation of carbon; (ii) the preferential combustion of carbon with formation of (CO + H2); (iii) the combustion of the hydrocarbon as a whole without previous dissociation or preferential combustion.

The first view, although it survived till 1892, has been discredited. To disprove it, it is sufficient to show that the outer zone of a hydrocarbon flame contains burning hydrogen, and that the interconal part of a Bunsen flame consists chiefly of hydrogen and carbon monoxide.

The second view was held by Dalton, was put forward again by Kersten in 1861, was reinforced by Smithells and Ingle in 1892, and was revived by Misteli in 1905.

The third view, which is the outcome of the researches of Bone and his co-workers, and to which reference is made under methane, may now be noticed.

Much of this work has been concerned with the slow combustion of hydrocarbons below their temperatures of ignition, but in a paper on the "Explosive Combustion of Hydrocarbons" Bone and Drugman contend that the initial changes are the same whether they take place below or above the temperature of ignition of the gas.

Bone and Wheeler conclude that the combustion of a hydrocarbon is a process of hydroxylation, and that "oxygen initially enters the hydrocarbon and is distributed between the carbon and hydrogen, giving rise to unstable hydroxylated molecules which, sooner or later, according to the rapidity of the process, undergo thermal decompositions into simpler products."

The combustion of ethylene is accordingly represented by the following scheme.

So the stages are: (1) vinyl alcohol, (2) formaldehyde, (3) formic acid, (4) carbonic acid - the three latter substances yielding as secondary products carbon monoxide, hydrogen, carbon dioxide, and water. Formaldehyde is regarded as the most prominent intermediate oxidation product.

No separation of carbon, such as takes place in ordinary hydrocarbon flames, is provided for in the above scheme. This separation takes place only when the supply of oxygen is insufficient to burn the ethylene to formaldehyde; then there result from thermal decomposition carbon, hydrogen, methane, and traces of acetylene. Thus the luminosity of a flame is still to be accounted for by the liberation of carbon consequent upon the thermal decomposition of its hydrocarbon gases; but side by side with this decomposition the reactions established by Bone and his co-workers take place.

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