Alkyne

Alkynes are a class of unsaturated hydrocarbons defined by the presence of at least one carbon–carbon triple bond. The simplest straight-chain alkynes with a single triple bond and no other functional groups belong to a homologous series with the formula CnH2n−2. While often called acetylenes as a group, the term "acetylene" specifically refers to the compound C2H2, which is formally named ethyne under IUPAC rules. Like other hydrocarbons, alkynes are generally hydrophobic.

In terms of structure, the H–C≡C bond angles in acetylene are 180°, giving alkynes a rod-like shape. Because of this linear geometry, cyclic alkynes are uncommon due to bond strain; for instance, benzyne cannot be isolated. The carbon–carbon triple bond distance in acetylene is 118 picometers, notably shorter than the double bond in alkenes (132 pm in ethene) and the single bond in alkanes (153 pm). The triple bond is strong, with a total bond strength of 839 kJ/mol: the sigma bond contributes 369 kJ/mol, while the two pi bonds contribute 268 kJ/mol and 202 kJ/mol. Bonding is typically explained using molecular orbital theory, where the triple bond forms from the overlap of s and p orbitals. In valence bond terms, each carbon in an alkyne bond is sp hybridized, possessing two unhybridized p orbitals and two sp hybrid orbitals. Overlap of one sp orbital from each carbon creates an sp–sp sigma bond, while each p orbital on one carbon overlaps with one on the other to form two pi bonds, totaling three bonds. The remaining sp orbital on each carbon can form a sigma bond with another atom, such as hydrogen in acetylene. These two sp orbitals project from opposite sides of the carbon atom.

Alkynes are classified as internal or terminal. Internal alkynes have carbon substituents on both acetylenic carbons; symmetrical examples include diphenylacetylene and 3-hexyne, while asymmetrical ones include 2-pentyne. Terminal alkynes have the formula RC≡CH, with at least one end being a hydrogen atom, as in methylacetylene (propyne). They are often prepared by alkylating monosodium acetylide. Terminal alkynes, including acetylene itself, are mildly acidic, with pKa values around 25—far more acidic than alkenes (pKa ~40) or alkanes (pKa ~50). The acidic hydrogen on terminal alkynes can be replaced by various groups to form halo-, silyl-, or alkoxoalkynes. The carbanions from deprotonating terminal alkynes are called acetylides. Internal alkynes are also more acidic than alkenes and alkanes, though less so than terminal alkynes. The C–H bonds at the α position (propargylic C–H bonds) can be deprotonated with strong bases, with an estimated pKa of 35, and this acidity allows the alkyne zipper reaction to isomerize internal alkynes into terminal ones.

Alkynes with four or more carbons can form structural isomers by placing the triple bond in different positions or by having carbon atoms as substituents rather than part of the main chain. Other non-alkyne structural isomers are also possible. For example: C2H2 has only acetylene; C3H4 has only propyne; C4H6 has two isomers (1-butyne and 2-butyne); C5H8 has three (1-pentyne, 2-pentyne, and 3-methyl-1-butyne); C6H10 has seven (1-hexyne, 2-hexyne, 3-hexyne, 4-methyl-1-pentyne, 4-methyl-2-pentyne, 3-methyl-1-pentyne, and 3,3-dimethyl-1-butyne).

In systematic naming, alkynes use the Greek prefix system without extra letters, such as ethyne or octyne. For chains with four or more carbons, the triple bond’s position must be indicated, using the lowest possible number—for example, 3-octyne or oct-3-yne. When no higher-priority functional groups are present, the parent chain must include the triple bond even if it is not the longest carbon chain. Ethyne is commonly called by its trivial name, acetylene. The suffix "-yne" denotes a triple bond in organic chemistry, following IUPAC nomenclature. Inorganic compounds with triple bonds may use similar substitutive naming, replacing "-ane" with "-yne." For two triple bonds, "-diyne" is used, and so on. For multiple triple bonds, a numerical locant before the "-yne" suffix indicates the position of unsaturation, chosen to keep numbers as low as possible. The suffix "-yne" also names substituent groups triply bound to the parent compound. Sometimes a number between hyphens is inserted to specify which atoms the triple bond connects. This suffix comes from a collapsed form of "acetylene," and the final "-e" is dropped if followed by another suffix starting with a vowel.

Classically, acetylene is prepared by hydrolyzing calcium carbide (Ca²⁺[:C≡C:]²⁻) with water, yielding HC≡CH and calcium ions.