Properties and Bonding in the Alkynes

The nature of the triple bond helps explain the physical and chemical properties of the alkynes. In molecular-orbital terms, we shall see that the carbons are sp hybridized, and the four singly filled p orbitals form two perpendicular tt bonds. 

Alkynes have boiling points very similar to those of the corresponding alkenes and alkanes. Ethyne is nnnsnal in that it has no boiling point at atmospheric pressnre rather, it sublimes at 84°C. Propyne (b.p. -23.2°C) and 1-butyne (b.p. 8.1°C) are gases, whereas 2-butyne is barely a Uquid (b.p. 27°C) at room temperature. The medium-sized alkynes are distillable liquids. 

The alkyne triple bond is characterized by a concentration of four tt electrons in a relatively small volume of space. The resulting electron-electron repulsion contributes to the relative weakness of the two tt bonds and to a very high energy content of the alkyne molecule itself. Because of this property, alkynes often react with the release of considerable amounts of energy and must be handled with care They polymerize very easily and are prone to explosive decomposition. Ethyne can be shipped in pressurized cylinders that contain acetone and porous fillers such as pumice as stabilizers. 

The high energy content of ethyne is reflected in its heat of combustion of 311 kcal moF. As shown in the equation for ethyne combustion, this energy is distributed among only three 

The high temperatutes required for weiding are attained by combustion of ethyne (acetyiene). 

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