9.1 : アルキンの構造と物性

Unsaturated hydrocarbons containing carbon–carbon triple bonds are called "alkynes" and are structurally represented as C_nH_2n-2.

Ethyne or acetylene is the simplest alkyne. It is a colorless gas that undergoes combustion at high temperatures and is used for welding.

The alkyne group is of significance as a bioactive moiety in drugs used to treat Parkinson's disease, such as selegiline, and in oral contraceptives like ethynylestradiol.

The carbon–carbon triple bond of acetylene is composed of one σ and two π bonds. The carbon's sp orbitals overlap to create a σ bond, while the 2p_y and 2p_z orbitals' lateral overlap forms two π bonds. The second sp orbital of each carbon participates in a σ bond with the 1s orbital of the hydrogen, generating the carbon–hydrogen bonds.

Thus, alkynes are sp hybridized with a linear geometry and a bond angle of 180°, resulting in the molecules' structural rigidity.

Alkynes show a fifty percent s character given their sp hybridization. As electrons in the s orbital are closer to the nucleus than p orbitals, their energies are lower, and they are more tightly bound to the nucleus. Hence, an increased s character results in stronger and shorter bonds. 

This is reflected in acetylene's carbon–carbon triple bond length of 1.21 Å and the carbon–hydrogen bond of 1.06 Å. Both bonds are shorter compared to their corresponding alkenes and alkanes.

Aside from the higher s character, the increased number of bonds between the two carbons contributes to the shorter and stronger bonds as well. 

Hence, the bond dissociation energy of alkynes is significantly higher than that of alkenes or alkanes.

Alkynes are nonpolar compounds that are insoluble in water but are soluble in nonpolar organic solvents like benzene. 

Lower molecular weight alkynes such as acetylene are gases; those with medium molecular weights, such as 1-octyne are liquids. In contrast, higher molecular weight alkynes exist as solids at room temperature.