The alkynes are the third homologous series of organic compounds of hydrogen and carbon, where there is at least one triple-bond between the atoms in the molecules. The alkenes are said to be unsaturated because of the existence of a multiple bond in the molecule. The general structure of the alkene series of hydrocarbons is CnH2n-2. Rules for the systematic naming of alkynes are similar to those for alkenes. In the case of higher members of the alkene series, the triple bond may be between the terminal carbon atoms of the chain, or may be between internal carbon atoms in the chain.
Acetylene is a linear molecule, all four atoms lying along a straight line.
This linear structure can only be explained by the existence of sp hybridisation of the orbitals of the carbon atoms of ethyne. The C-C distance is 1.2 A, as compared with 1.34 A in ethylene and 1.54 A in ethane and is a more electronegative grouping than that formed by carbon atoms joined by either a double or a single bond. A significant result of this bonding is that ethyne can unite with metals and so be distinguished from alkenes by chemical means. The linear structure does not permit geometric isomerism of ethyne. The ethynes are highly dangerously explosives when mixed with air or oxygen.