The alkynes are a functional group characterized by the presence of a carbon-carbon triple bond. Originally the alkynes were called the acetylenes. This is still the common name given to the simplest member of the group, although the IUPAC name is ethyne (Figure 1) . The alkynes are all distinguished by the ending -yne.
Non-cyclic alkyne hydrocarbons, such as butyne (CH3C=CCH3), are characterized by an empirical formula of CnH(2n-2). The bonding relationship of the triple alkynes is most commonly described as bonding between two sp- p2 hybridized carbon atoms. The alkyne carbon atoms bond together through one sigma bond, comprised of one of the two sp orbitals, and two pi bonds. The sigma bond occupies the internuclear space, and the two pi bonds are at 90° angles to each other surrounding the carbon-carbon internuclear space. This results in a linear relationship of the four carbons of butyne (i.e., all four carbon nuclei of butyne are constrained by the bonding to the same line), and a substantial pi electrondensity surrounding all sides of the carbon-carbon triple bond. Due to this density of the pi electrons, and reflecting the lower thermodynamic stability of the pi bond compared to the sigma bond, the dominant reactivity of the alkyne is electrophile addition to the triple bonds. For example, molecular hydrogen (H2) in the presence of metal catalysts will add to (hydrogenate) alkynes in an energetically favorable (i.e., exothermic) reaction. The addition of one mole of H2 to an alkyne gives the alkene, whereas the addition of a second mole of H2 gives the alkane.
Halogens, such as Cl2 and Br2, add as electrophiles to the alkyne to give the dihalo-substituted alkene, that continues to react with a second mole of the halogen to give the tetrahaloalkane. Hydrohalogenation, exemplified by the reaction of an alkyne with HCl, also occurs in the same manner. Acetylene (ethyne, HC=CH), for example, reacts with HCl at high pressure and in the presence of a metal catalyst to give chloroethene (vinyl chloride, H2C=CHCl) as the product. Polymerization of this product gives polyvinyl chloride (PVC) which is extensively used in the plastics industry.Oxidation also is used to break down the triple bond and this can be an explosive process. Ethyne burned in oxygen produces carbon dioxide and water (with complete combustion) and a very hot flame. This is the principle behind oxy- acetylene welding.
Alkynes contain a carbon-carbon triple bond, and the reactivity of this triple bond dominates their chemical behavior.
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