Preparation of Acetylene

Acetylene is formed when a Bunsen burner "strikes back" and burns at the base. At the same time a peculiar and penetrating odour is smelt which, although not due to acetylene itself, serves to draw attention to the improper burning of the coal-gas. It is usual to attribute this production of acetylene to the imperfect combustion of the hydrocarbons present in the coal-gas, but it is probable that this gas is produced by the thermal dissociation of ethylene, and then escapes into the air owing to imperfect combustion.

It was first observed by Wohler in 1862 that acetylene is formed by the action of water on calcium carbide:

CaC₂ + 2H₂O = Ca(OH)₂ + C₂H₂.

The carbide was prepared on a large scale by Willson in 1892, and independently by Moissan slightly later, by heating together lime and charcoal in the electric furnace. "Carbide" is now an important article of commerce owing to the employment of acetylene as an illuminant. It is prepared by heating limestone with coke or small coal in an electric furnace, and contains as impurities calcium sulphide, phosphide, and silicide, and magnesium nitride, as well as the silicides of carbon and iron and also graphite. The acetylene generated from this carbide consequently contains hydrogen sulphide, phosphine, ammonia, and other impurities; and these are got rid of by absorption in water and oxidation by bleaching powder and other reagents.

Acetylene is formed by removing HBr from ethylene dibromide by boiling it with alcohol potash, just as ethylene is obtained from ethyl bromide.

HBr is removed from the dibromide in two stages, thus:

C₂H₄Br₂ + KOH = C₂H₃Br + KBr + H₂O
C₂H₃Br + KOH = C₂H₂ + KBr + H₂O.

Sodium abstracts chlorine from chloroform yielding acetylene:

2CHCl₃ + 6Na = C₂H₂ + 6NaCl.

Electrolysis of solutions of the alkali salts of maleic and fumaric acids produces acetylene, just as electrolysis of alkali succinate yields ethylene:

HOOC-CH=CH-COOH → CH≡CH + 2CO₂ + H₂

The silver salt of acetylene dicarboxylic acid is decomposed by hot water, yielding silver acetylide and carbon dioxide:

AgOOC-CH≡C-COOH → AgC≡CAg + 2CO₂

and from silver acetylide pure acetylene is obtained by the action of acid.