9.1: Structure and Physical Properties of Alkynes
In nature, compounds containing both carbon and hydrogen are known as "hydrocarbons". Aliphatic hydrocarbons are compounds whose molecules contain saturated single bonds (i.e., alkanes) or unsaturated double or triple bonds. Alkenes contain carbon–carbon double bonds and have a structural formula CnH2n. Unsaturated hydrocarbons containing carbon–carbon triple bonds are called "alkynes" and are structurally represented by the formula CnH2n-2.
The simplest alkyne is ethyne, or acetylene, a colorless gas, which undergoes combustion at high temperatures and is used as a fuel for welding. Alkynes are found in a variety of substances of both natural and synthetic origin. For example, natural alkynes can be found in the poison obtained from South American tree frogs. On the other hand, synthetic alkyne-containing compounds play an important role in drugs like oral contraceptives such as ethynylestradiol. Other alkyne-based drugs, such as selegiline, are used in combination with synthetic dopamine or L-dopa to treat Parkinson's disease.
Hybridization of Alkynes:
The triple bond in alkynes occurs due to the overlap of the carbon–carbon (C–C) bond's sp orbitals forming a sigma (σ) bond, and the lateral overlapping of the 2py and 2pz orbitals forming the two pi (π) bonds, respectively. The overlap of the carbon sp orbital with the 1s orbital of the hydrogen atom results in the carbon–hydrogen (C–H) sp–1s sigma bond of the alkyne. Alkynes show an sp hybridization with a 50% s character. As electrons in the s orbital show lower energies than the p orbitals, an increase in the atom's s character increases its electronegativity.
Molecular Geometry: Bond length, Bond angles, and Bond Strengths
Owing to the sp hybridization involving the lateral overlap of the orbitals, alkynes have a linear geometry with the bond angle being 180°. The C–C triple bond's length in acetylene is measured to be 121 pm or 1.21 Å, which is lesser than that of alkenes (134 pm or 1.34 Å) and alkanes (153 pm or 1.53 Å) due to the increased number of bonds holding the two carbons together.
The C–H bond in alkynes (1.06 Å in acetylene) is also shorter compared to that in alkenes and alkanes due to the increased s-character of the sp hybridized carbon orbital forming the sp-1s σ bond with the hydrogen. In addition to having lower energies, electrons in the s-orbital are closer to the atomic nucleus and are bound more tightly than those in the p orbitals. As a result, a greater s-character increases the strength of the bond. Hence, C-C triple bonds and C-H bonds in alkynes are shorter and stronger than those of alkenes and alkanes, mainly due to their increased s character. The bond dissociation energy of alkynes is 966 kJ or 231 kcal/mol, which is higher than that of alkanes and alkenes due to the presence of these shorter and stronger bonds.
Physical Properties of Alkynes:
Alkynes show similar physical properties as their parent alkane or alkene. They are nonpolar compounds having a lower density than water and are insoluble in water and polar solvents. However, they show good solubility in nonpolar organic solvents. The lower-molecular weight alkynes such as ethyne and propyne exist as gases at room temperature, while higher molecular weight alkynes such as 1-octyne and 1-decyne are liquids.