An organic compound with the formula C₂H₂ or HC≡CH. The first member of the alkynes, acetylene is a gas with a narrow liquid range; the triple point is -81 °C (-114°F). The heat of formation (ΔHf) is +227 kilojoules/mole, and acetylene is the most endothermic compound per carbon of any hydrocarbon. The compound is thus extremely energy-rich and can decompose with explosive force. At one time acetylene was a basic compound for much chemical manufacturing. It is highly reactive and is a versatile source of other reactive compounds.
The availability of acetylene does not depend on petroleum liquids, since it can be prepared by hydrolysis of calcium carbide (CaC₂), obtained from lime (CaO), and charcoal or coke (C). In modern practice, methane (CH₂) is passed through a zone heated to 1500°C (2732°F) for a few milliseconds, and acetylene is then separated from the hydrogen in the effluent gas.
The main use of acetylene is in the manufacture of compounds derived from butyne-1,4-diol. The latter is obtained by condensation of acetylene with two moles of formaldehyde and is converted to butyrolactone, tetrahydrofuran, and pyrrolidone. Two additional products, vinyl fluoride and vinyl ether, are also based on acetylene.
Because of the very high heat of formation and combustion, an acetylene-oxygen mixture provides a very high temperature flame for welding and metal cutting. For this purpose acetylene is shipped as a solution in acetone, loaded under pressure in cylinders that contain a noncombustible spongy packing.