In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one bond and no other functional groups form a homologous series with the general chemical formula CnH2n−2. Alkynes are traditionally known as acetylenes, although the name also refers specifically to C2H2. Like other hydrocarbons, alkynes are generally hydrophobic but tend to be more reactive, alkynes are characteristically more unsaturated than alkenes. Thus they add two equivalents of bromine whereas an alkene adds only one equivalent in the reaction, in some reactions, alkynes are less reactive than alkenes. For example, in a molecule with an -ene and an -yne group, possible explanations involve the two π-bonds in the alkyne delocalising, which would reduce the energy of the π-system or the stability of the intermediates during the reaction. They show greater tendency to polymerize or oligomerize than alkenes do, the resulting polymers, called polyacetylenes are conjugated and can exhibit semiconducting properties. In acetylene, the H–C≡C bond angles are 180°, by virtue of this bond angle, alkynes are rod-like. The C≡C bond distance of 121 picometers is much shorter than the C=C distance in alkenes or the C–C bond in alkanes, the triple bond is very strong with a bond strength of 839 kJ/mol. The sigma bond contributes 369 kJ/mol, the first pi bond contributes 268 kJ/mol, bonding usually discussed in the context of molecular orbital theory, which recognizes the triple bond as arising from overlap of s and p orbitals. In the language of valence bond theory, the atoms in an alkyne bond are sp hybridized. Overlap of an sp orbital from each atom forms one sp–sp sigma bond, each p orbital on one atom overlaps one on the other atom, forming two pi bonds, giving a total of three bonds. The remaining sp orbital on each atom can form a bond to another atom. The two sp orbitals project on opposite sides of the carbon atom, internal alkynes feature carbon substituents on each acetylenic carbon. Symmetrical examples include diphenylacetylene and 3-hexyne, terminal alkynes have the formula RC2H. Terminal alkynes, like itself, are mildly acidic, with pKa values of around 25. They are far more acidic than alkenes and alkanes, which have pKa values of around 40 and 50, the acidic hydrogen on terminal alkynes can be replaced by a variety of groups resulting in halo-, silyl-, and alkoxoalkynes. The carbanions generated by deprotonation of terminal alkynes are called acetylides, in systematic chemical nomenclature, alkynes are named with the Greek prefix system without any additional letters
A 3D model of ethyne (acetylene), the simplest alkyne
Illustrative alkynes: a, acetylene, b, two depictions of propyne, c, 1-butyne, d, 2-butyne, e, the naturally-occurring 1-phenylhepta-1,3,5-triyne, and f, the strained cycloheptyne. Triple bonds are highlighted blue.