Chemical Properties of Acetylene

In common with many endothermic compounds acetylene is liable to explosive decomposition. The explosibility of this gas has been studied by Berthelot in conjunction with Vieille and Le Chatelier. Under ordinary atmospheric pressure acetylene is not exploded by a red-hot wire, or by mercury fulminate, but towards two atmospheres it becomes explosive, and carbon separates from it in the form of fine soot. It is consequently forbidden in this and some other countries to store acetylene under a pressure of more than 100 inches of water over atmospheric pressure; it may, however, be safely compressed in contact with acetone.

At ordinary temperature acetylene undergoes very slow spontaneous decomposition. Rapid decomposition sets in at 780° C., and in the presence of copper powder, as a catalyst, at 400°-500° C. The thermal decomposition of acetylene has been studied by Bone and Coward, who find that the primary effect of heat upon this gas may be one of polymerisation or dissolution according to temperature. Polymerisation produces tarry matter containing benzene, etc.; dissolution separates the residues (≡CH), which may split into their elements or become hydrogenised, yielding methane. At the same time ethylene and ethane may be formed at low temperatures by the union of liberated hydrogen with acetylene.

The following scheme represents the thermal decomposition of acetylene:



The tendency of acetylene to polymerise reaches a maximum between 600° and 700° C.,and rapidly diminishes towards 1000° C., above which temperature the gaseous product consists of hydrogen with a little methane. The products of polymerisation are very complex. Meyer and Tanzen have identified no fewer than seventeen coal-tar constituents amongst the pyrogenetic condensation-products of acetylene.

Sunlight, the silent electric discharge, platinum-black, and finely divided iron, nickel, and cobalt promote polymerisation, which may take place even at atmospheric temperature.

When acetylene and ammonia are passed over over alumina, ferric oxide, pyridine bases result thus:

pyrole: 2C₂H₂ + NH₃ = C₄H₅N + H₂
picoline = methyl-pyridine: 3C₂H₂ + NH₃ = C₆H₇N + H₂
methyl-ethyl-pyridine: 4C₂H₂ + NH₃ = C₈H₁₁N

Sulphur may be incorporated within the molecule by the use of hydrogen sulphide or sulphur vapour; thus thiophene, C₄H₄S, or its homologues may be produced.

Such reactions will account for the presence of heterocyclic compounds containing nitrogen and sulphur among the products of the dry distillation of natural petroleum if this is derived from a mineral source.

One molecule of acetylene can combine with 2 and with 4 hydrogen atoms or their equivalent, forming respectively ethylene and ethane or their derivatives.

Nascent hydrogen, or hydrogen gas through the catalytic agency of platinum-black or finely divided nickel, converts acetylene into ethylene and ethane:

C₂H₂ + H₂ → C₂H₄ + H₂ → C₂H₆

Platinum-black induces these reactions at atmospheric temperature, ethane resulting when there is excess of hydrogen, ethylene when excess of acetylene. When finely divided nickel is employed at 180° C. or above, polymerisation accompanies addition of hydrogen, and the products resemble natural petroleum.

The halogens combine with acetylene with decreasing readiness from chlorine to iodine, giving rise to halogen derivatives of ethylene and ethane.

Acetylene combines with chlorine, sometimes with explosive violence, forming acetylene dichloride or dichlorethylene, CHCl:CHCl, and acetylene tetrachloride or tetrachlorethane, CHCl₂-CHCl₂. According to Mouneyrat a mixture of chlorine and acetylene, when exposed to diffused daylight, forms acetylene tetrachloride without explosion, but the presence of oxygen or of a gas capable of liberating oxygen causes an explosion. Acetylene forms with SbCl₅ the crystalline compound C₂H₂.SbCl₅, which on heating decomposes into C₂H₂Cl₂ and SbCl₃. Chavanne states that acetylene dichloride consists of two isomerides separable by fractional distillation, and boiling at 49° C. and 60.2° C.

With bromine acetylene forms C₂H₂Br₂ and C₂H₂Br₄; with iodine C₂H₂I₂, known in two stereoisomeric forms. Acetylene slowly absorbs halogen hydracid, HX, producing CH₂:CHX and CH₃-CHX₂; and when sparked with nitrogen forms HCN.

Acetylene combines with cuprous chloride to form the white solids C₂H₂.CuCl and C₂H₂.2CuCl. The former substance is soluble in water, but is obtained from absolute alcoholic solution at 0° C.

Water combines with acetylene under the influence of catalysts to form acetaldehyde:

CH≡CH + H₂O = CH₃-CHO.

Concentrated sulphuric acid slowly absorbs acetylene, forming the sulphonic acid C₂H₃-SO₃H, which by hydrolysis yields acetaldehyde, and also its condensation product crotonaldehyde:

2CH≡CH → 2C₂H₃.SO₃H → 2CH₃.CHO → CH₃.CH=CH.CHO + H₂O.

Mercuric oxide, bromide, and other salts are also efficient catalysts, producing aldehyde in the cold.

Substituted, as distinct from additive, halogen derivatives of acetylene are known. Monochloracetylene, CCl≡CH, is an explosive gas obtained by the action of baryta on dichloracrylic acid, CCl₂=CH-COOH. Monobromacetylene, CBr≡CH, obtained by the action of alcoholic potash on acetylene dibromide, CHBr=CHBr, is a spontaneously inflammable gas. Mono-iodoacetylene, CI≡CH, is obtained by boiling the potassium salt of iodopropiolic acid, CI≡CH-COOK, with water. All three halogen derivatives can also be obtained by the decomposition of the corresponding halogen propiolic acids.

The mercuric derivatives Hg(C≡CCl)₂ and Hg(C≡CBr)₂ have been prepared.

Symmetrical di-iodoacetylene, CI≡CI, formed when iodine acts on silver acetylide, decomposes at 78° C. Asymmetrical di-iodo-acetylene, CI₂=C, containing bivalent carbon, is said to be formed by the action of sodium hypoiodite on acetylene. According to Biltz, however, there is no ground for assuming the existence of asymmetrical "acetylidene" derivatives, there being thus only one iodine derivative, viz. CI≡CI.

Pure, moist acetylene attacks nickel and copper, and the crude, moist gas attacks zinc, lead, brass, nickel, phosphor-bronze, and copper. Copper is attacked rapidly, showing an increase in weight of 80 to 90 per cent, in six months. The product is black and non-explosive, and does not give acetylene with acids. In consequence of this action acetylene should not be brought in contact with copper tubes, but tubes conveying it should be tinned.

The proportion of carbon to hydrogen in acetylene is so large that this hydrocarbon behaves as a feeble acid. It is not, however, sufficiently ionised in aqueous solution to show an acid reaction, its strength being only about 1/4000th that of carbonic acid, and its ionisation l/10th that of water. In consequence of this property salts of acetylene can be prepared in presence of water only when they are insoluble and may thus be precipitated; otherwise they will be hydrolysed by water, and must be prepared in the dry way. Thus calcium carbide, prepared in the dry way, is hydrolysed into base and acid in the familiar preparation of acetylene: CaC₂ + 2H₂O = Ca(OH)₂ + C₂H₂. Nevertheless the increase in the solubility of acetylene in water brought about by the addition of alkali may indicate the formation of small quantities of salts in solution. Alkali and alkaline earth carbides are formed when acetylene is passed into solutions of the metals in liquid ammonia, i.e. into solutions of metal-ammoniums (e.g. NaNH₃). Hydrogen carbides are first formed, e.g. C₂HNa, or more probably C₂Na₂.C₂H₂, the liberated hydrogen converting some of the acetylene into ethylene, thus:

3C₂H₂ + 2NaNH₃ = C₂Na₂.C₂H₂ + 2NH₃ + C₂H₄.

The following compounds have thus been obtained:

C₂Na₂.C₂H₂; C₂K₂.C₂H₂; C₂Li₂.C₂H₂.2NH₃; C₂Ca.C₂H₂.4NH₃.

All these compounds decompose when heated, leaving the carbides C₂Na₂, C₂K₂, C₂Li₂, C₂Ca. The molecular conductivity of NaHC₂ in liquid ammonia is of the same order as that of sodium acetate.

Insoluble silver arid cuprous acetylides (or carbides) - C₂Ag₂ and C₂Cu₂ - are precipitated when acetylene is led into ammoniacal solutions of silver and cuprous salts. These compounds give off acetylene on treatment with acids, but are explosive when dry. The formation of cuprous acetylide, which is dark red, serves as a test for acetylene gas. The most sensitive cuprous solution for this test is made by saturating copper sulphate solution with sodium chloride, warming, and adding sodium hydrogen sulphite until the green colour disappears. A few drops of ammonia increase the sensitiveness of the reagent.

Mercuric acetylide, C₂Hg, is formed when acetylene is passed into an alkaline solution of mercuric oxide, but acetylene reacts with an aqueous solution of mercuric chloride, thus:

CH≡CH + H₂O + 3HgCl₂ = + 3HCl.

The trichlormercuric acetaldehyde is a precipitate which is decomposed by concentrated hydrochloric -acid into aldehyde and mercuric chloride. This reaction explains the catalytic action of mercuric salts in bringing about the conversion of acetylene into aldehyde. A mixture of magnesium acetylide and allylide is formed when magnesium powder reacts with acetylene at 450° C.

Acetylene is oxidised to acetic acid when it is passed through solutions of hydrogen peroxide, persulphuric acid, permonosulphuric acid, or a salt of one of these acids in presence of mercury or a mercury compound. It may also be oxidised to acetic acid electrolytically.

Acetylene ignites in contact with air at 480° C., and may beset fire to by means of red-hot carbonaceous matter. It burns with a flame which when suitably regulated is white and intense. The luminosity of acetylene when burnt at the rate of 5 cubic feet per hour is equal to 240 candle-power, while that of coal-gas equals from 14 to 18 candle-power. In order to burn acetylene without separation of carbon special burners have been constructed, in which the gas issues from two nozzles so made that air mixes with it as in the Bunsen burner. The two jets of gas then impinge upon each other obliquely, and produce a small, intense flame which does not smoke. The temperature of the acetylene flame is higher than that of coal-gas, and the temperature of the oxy-acetylene blowpipe flame approximates to that of the electric arc (3500° C.). The oxy-acetylene flame is used for the autogenous welding of steel and the cutting of steel plates.

Acetylene requires about twelve times its volume of air for complete combustion, and shows a wider range of explosibility when mixed with air than any other gas, since mixtures containing between 3 and 82 per cent, of aqetylene explode.

The slow combustion of acetylene has been studied by Bone and Andrew. These observers find that interaction between acetylene and oxygen, sealed up together in a glass vessel at atmospheric temperature, begins at 250° C., proceeds rapidly at 300° C., and becomes explosive from 350° to 375° C. The process of combustion of acetylene is essentially similar to that of methane, ethane, and ethylene, and consists primarily in the incorporation of oxygen within the molecule rather than the preferential oxidation of either carbon or hydrogen. The whole process is represented by the following scheme:



The separation of carbon, which occurs in an acetylene flame, is not a part of the process of combustion of this hydrocarbon, but is due to thermal decomposition of an excess over that required by the equation C₂H₂ + O₂ = 2CO + H₂.

Hot porcelain promotes the catalytic formation of acetaldehyde, thus:



which may then produce CH₄ + CO.

There is no polymerisation to benzene in the presence of oxygen below the ignition-point.

Acetylene is much less poisonous than carbon monoxide, and even less poisonous than coal-gas. This is because the compound it forms with haemoglobin is unstable and easily decomposed by aeration.

Acetylene may be estimated gravimetrically by causing it to react with ammoniacal cuprous solution, collecting the precipitated cuprous acetylide, C₂Cu₂, and converting this into the sulphide, Cu₂S, which is weighed. It may also be estimated volumetrically by titrating the nitric acid produced in the reaction:

C₂H₂ + 3AgNO₃ = C₂Ag₂.AgNO₃ + 2HNO₃;

and gasometrically by absorbing it in a solution of ammoniacal silver chloride, in which ethylene is almost insoluble.