Bonding in alkynes

Acetylene is linear- and alkynes have a linear- geometry of their X—C=C—Y units. The carbon-carbon triple bond in alkynes is composed of a a and two tt components. The triply bonded carbons are sp-hybridized. The a component of the triple bond contains two electrons in an orbital generated by the overlap of 5/r-hybridized orbitals on adjacent car bons. Each of these car bons also has two 2p orbitals, which overlap in pairs so as to give two tt orbitals, each of which contains two electrons. 

Pepe, C. Dif, K. The Use of Ethanethiol to Locate the Triple Bond in Alkynes and the Double Bond in Substituted Alkenes by Gas Chromatography/Mass Spectrometry. Rapid Commun. Mass Spectrom. 2001, 75,97-103. 

A simple-minded picture suggests that the CC jt bonds in alkynes and alkenes ought to be similar. Are they Consider the thermodynamics of reduction of phenylacetylene to first give styrene and then phenylethane. (The energy for H2 is given at right.)... 

Oxidative cleavages can help to determine the positions of the triple bonds in alkynes. 

As you might expect, the presence of a triple bond in alkynes makes their physical and chemical properties different from those of alkanes and alkenes. A structure with a triple bond must be linear around the bond. (See Figure 13.24.) This means that the shapes of alkynes are different from the shapes of alkanes and alkenes. As well, the triple bond makes the molecule much more reactive—even more so than the double bond. 

A more comprehensive molecular orbital treatment of the bonding in alkyne-substituted trimetallic clusters has recently appeared. See Halet et al Inorg. Chem 24, p. 218 (1985). 

The C NMR spectra of the ionic carbides CaC2 and BaC2 have been determined by Wrackmeyer et al. (1990) who report 8iso values of 206.2 ppm for CaC2 and 232.1 ppm for BaC2, both shifts with respect to TMS. The NMR results indicate that the environment of the carbon atoms in these compounds is not axially symmetric, and that the carbide unit in these ionic carbides is not comparable with the carbon-carbon triple bond in alkynes. 

TABLE 13. Bond lengths of C-H bonds adjacent to the CsC bond in alkynes. 

Hydroalumination of the terminal and internal C=C triple bonds in alkynes with R2A1H, which usually occurs quite smoothly, gives a ds product. One therefore obtains the following alkenyl alanes (253) ... 

Theoretical and computational studies of the reactivities and molecular interactions of acetylene have exploded during recent years. Most studies are aimed at gaining theoretical understanding of the difference in the reactivities between the triple bond in alkynes and the double bond in alkenes. In the following section, we will summarize the reactivities of acetylene involved in pericyclic reactions, electrophilic reactions, and nucleophilic additions. Then, we will give a brief review of the studies probing molecular interactions of acetylene. 

The position of the triple bond in alkynes and alkynecarboxylie esters can be established by ozonolysis in methanol, as in reaction 1. Ozonolysis yields are in the 90-99% range and no degradation of the corresponding carboxylic acid is observed. The resulting carboxylic... 

Acetylene is linear and alkynes have a linear geometry of their X—C=C—Y units. The carbon-carbon triple bond in alkynes is composed of a a and two tt components. 

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