Alkynes are hydrocarbons that have a triple bond between two carbon atoms, with the formula CnH2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature. 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. Other reactions are listed below. Alkynes are usually more reactive than alkenes. They show greater tendency to polymerize or oligomerize than alkenes do. The resulting polymers, called polyacetylenes (which do not contain alkyne units) are conjugated and can exhibit semiconducting properties.
Structure and bonding
In acetylene, the H-C≡C bond angles are 180°. By virtue of this bond angle, alkynes tend to be rod-like. Correspondingly, cyclic alkynes are rare. Benzyne is highly unstable. The C≡C bond distance of 121 picometers is much shorter than the C=C distance in alkenes (134 pm) or the C-C bond in alkanes (153 pm).
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 and the second pi bond of 202 kJ/mol bond strength. 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 carbon atoms in an alkyne bond are sp hybridized: they each have two unhybridized p orbitals and two sp hybrid orbitals. 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 sigma bond to another atom, for example to hydrogen atoms in the parent acetylene]. The two sp orbitals project on opposite sides of the carbon atom.
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