Structure of Triple Bond in Alkynes

Quantum mechanics helps us a great deal to study the structure of different molecules that are found in nature. The concept of chemical bonding in combination with quantum mechanics has revealed numerous information about various organic and inorganic compounds that are essential for life on this planet. In this article, we will learn about a special class of organic compounds known as alkynes. Acetylene which is also known as ethyne is the simplest member of the alkyne family. Alkynes are unsaturated hydrocarbons in which there is a triple bond between the two carbon atoms in a compound.

What is a Triple Bond?

When there are three pairs of electrons shared between two carbon atoms, it is known as a triple bond between the two carbon atoms. The only distinguishing features of alkynes from the other hydrocarbons are the triple bond that exists between the carbon atoms in alkynes.

The triple bonded carbon atoms of acetylene are sp hybridised. Sigma bond between the carbon atoms is obtained by an overlapping of the two hybridised sp orbitals. The leftover orbitals of the carbon atoms overlap with each other along the internuclear axis in the 1s orbital of each hydrogen atom which results in the formation of one C-H sigma bond and 2 weaker pi-bonds in the molecule. These bonds are formed due to orbital overlapping in the molecule. Each carbon atom uses only one of its three p-orbitals for hybridization.

Hence, the two remaining p orbitals are occupied by a single electron in the carbon atom. As it is the same as the other carbon atom, it permits the pairing of electrons which results in the formation of 2 pi bonds in the molecule. The carbon-carbon triple bond is made stronger by the presence of a one sigma bond with it.

The Acetylene molecule is a linear molecule having a  C-C distance of 1.21Ao. The distance in acetylene is 1.08 shorter than that of alkenes due to the fact that sp hybridised carbon has more s character than an sp2 hybridised carbon.

Structure of Triple Bond-Ethyne

The chemical compound acetylene also known as ethyne has the formula C2H2, which is also the simplest alkyne. This colourless gas is widely used as a fuel and chemical building material in our day-to-day life. It is usually treated and experimented with as a solution because it is unstable in its pure state. Although acetylene in a pure state is odourless, contaminants such as divinyl sulphide and phosphine give commercial grades a distinct odour to it.

How is ethyne prepared?

Ethyne can be made by partially combustible methane. Calcium carbide hydrolysis can also be used to generate this compound (a chemical compound with the formula CaC2, also known as calcium acetylide). The chemical equation for the reaction of calcium carbide with water is given below.

C2H2 + Ca(OH)2 → H2O + CaC2

As a result of the hydrolysis of calcium carbide, calcium hydroxide and ethyne are formed as products.

Bonding (H – C ≡ C – H)

According to the valence bond theory, the 2s orbital in each carbon atom is hybridised with one 2p orbital, resulting in an sp hybrid. The remaining two 2p orbitals of the carbon atom are not hybridised. The two ends of the two sp hybrid orbitals overlap each other to produce a strong valence link between the carbon atoms, while hydrogen atoms attach via bonds on the other two ends. The two unmodified 2p orbitals from two weaker bonds in the molecule.

The D∞h point group is present in acetylene since it is a linear symmetrical molecule.

Triple bond structure of ethyne 

When three pairs of electrons are shared between two carbon atoms, it is known as a triple bond between the two carbon atoms. With a bond order of three, triple bonds are more strong than corresponding single or double bonds. Alkynes have a triple bond between two carbon atoms. Cyanides and isocyanides are two more functional groups with a triple bond between them. Some diatomic molecules are also triple bonds, such as nitrogen and carbon monoxide. The triple bond is represented in skeletal equations by three parallel lines between the two linked atoms.

Unhybridized p orbitals in ethyne 

Since each carbon atom is sp hybridised, it means that each carbon atom has two unhybridized p atomic orbitals. The two C−H sigma bonds are formed by the overlapping of carbon sp hybrid orbitals with the hydrogen 1s atomic orbitals.

Conclusion

As we have seen in this article, we have talked about the definition of a triple bond, a brief introduction to alkynes, and then came up with the topic of triple bond in alkynes. We have even talked about the preparation of ethyne, and an unhybridized p orbital in ethyne. The concept of chemical bonding in combination with quantum mechanics has revealed numerous information about various organic and inorganic compounds that are essential for life on this planet.