Structure of Triple Bond (Ethyne): Hybridisation, Bond Length

We know that the building block of structural organic chemistry is the tetravalent carbon atom. With some exceptions, carbon compounds can be formulated with four covalent bonds with carbon or some other element. The two-electron bond, which is shown by the carbon-hydrogen bonds in methane or  ethane and the carbon-carbon bond in ethane, is called a single bond. In these and many similar substances, each carbon is attached to four other atoms as:

There are compounds such as ethene (ethylene), C2H4, in which two electrons from each of the carbon atoms are mutually shared, producing two two-electron bonds, an arrangement which is called a double bond. Each carbon in ethene is attached to three other atoms as:

Further, in ethyne (acetylene), C2H2, three electrons from each carbon atom are mutually shared, producing three two-electron bonds, called a triple bond, wherein each carbon atom is attached to two other atoms:

Triple bonds  are stronger than the equivalent single bonds or double bonds, having a bond order of three. The most common triple bond, that between two carbon atoms, can be found is in alkynes. Other functional groups containing a triple bonds are cyanides and isocyanides. Some diatomic molecules, such as di nitrogen and carbon monoxide, are also triple bonded. In skeletal structure, the triple bond is drawn as three parallel lines (≡) between the two connected atoms.

Alkynes are hydrocarbons which contain carbon-carbon triple bonds.  Their general formula is CnH2n-2 for molecules with one triple bond.

Triple-bonded carbons are sp-hybridized, and have linear shapes, with the bonded atoms at angles of 180° to each other.  It is interesting to note that because of this linear shape, geometric isomerism does not occur in alkynes.

Alkynes are nonpolar, since they contain nothing other than carbon and hydrogen, and so, like the alkanes and alkenes, they are insoluble in water, and are usually less dense than water.

Ethyne is more commonly known under the name acetylene.  It is the simplest of the alkynes, consisting of two carbon atoms connected by a triple bond, leaving each carbon to bond to one hydrogen atom.  Acetylene is a colourless, unpleasant-smelling gas of bp -28.1°C, that burns in air to produce a sooty flame.  In the presence of pure oxygen, it burns at very high temperatures (up to 2800°C), and hence is used in welding and cutting torches.  

Hybridization

Double and triple bonds can be explained by orbital hybridization, or the ‘mixing’ of atomic orbitals to form new hybrid orbitals. Hybridization is a theoretical concept that describes the bonding situation from a specific atom’s point of view. A combination of s and p orbitals results in the formation of  hybrid orbitals. The newly formed hybrid orbitals have the same energy and have a particular geometrical arrangement that agrees with the observed bonding geometry in molecules. Hybrid orbitals are written as spx, where s and p tell the orbitals used for the mixing process, and the value of the superscript x ranges from 1 to 3, depending on how many p orbitals are required in the observed bonding.

Pi, or π, bonds occur when there is overlap between unhybridized p orbitals of two adjacent atoms. The overlap does not occur between the nuclei of the atoms, which is the main difference between sigma and pi bonds. For the bond to form well, there has to be a proper geometrical relationship between the unhybridized p orbitals: they must be in the same plane.

Pi bond formation: Overlap between adjacent unhybridized p orbitals produces a pi bond. Here the electron density corresponding to the shared electrons is not concentrated along the internuclear, unlike in sigma bonds.

So it is clear that multiple bonds between atoms always consist of a sigma bond, and any additional bonds are of the π type.

Ethyne molecule is linear, that is, all four atoms lie in a straight line. Ethyneeis built from hydrogen atoms (1s1) and carbon atoms (1s22s22px12py1). The carbon atom does not have enough unpaired electrons to form four bonds (1 to the hydrogen and three to the other carbon), so it has to promote one of the 2s2 pair into the vacant 2pz orbital. The carbon-carbon triple bond is 1.20Å long. In the hybrid orbital picture of ethyne, both carbons are sp-hybridized. In an sp-hybridized carbon, the 2s orbital combines with the 2px orbital to form two sp hybrid orbitals that are oriented at an angle of 180° with respect to each other (let’s say along the x axis). The 2py and 2pz orbitals remain non -hybridized and are oriented perpendicularly along the y and z axes respectively.

The C-C sigma bond is formed by the overlap of one sp orbital from each of the carbons, while the two C-H sigma bonds are formed by the overlap of the second sp orbital on each carbon with a 1s orbital of a hydrogen atom. Each carbon atom still has two half-filled 2py and 2pz orbitals, which are perpendicular both to each other and to the sigma bonds. These two perpendicular pairs of p orbitals form two pi bonds between the carbons, giving in a triple bond overall (one sigma bond and two pi bonds).

Ethyne, hence, has three sigma bonds and two pi bonds. The carbon-carbon triple bond in ethyne is the shortest (120 pm) and the strongest (965 kJ/mol) of all the carbon-carbon bond types. Because each carbon in acetylene has two electronggroups, VSEPR predicts a linear geometry and H-C-C bond angle of 180o.

Bond Length

Another consequence of  the presence of multiple bonds between atoms is the difference in the distance between the nuclei of the bonded atoms.Double bonds have shorter bond length than single bonds, and triple bonds are shorter than double bonds.

Uses of Alkynes

• The most common use of Ethyne is for making organic compounds such as ethanol, ethanoic acid, acrylic acid, etc.
• It is also used for making polymers and its initiating materials. For example, vinyl chloride is used as the monomer for PVC and chloroprene is used for synthetic rubber neoprene.
• Ethyne is used for preparing many organic solvents as well.
• Alkynes are usually used to artificially ripe fruits.