Question Video: Identifying the Number of 𝜎 and 𝜋 Bonds in the Triple Bond of Ethyne Chemistry

Video Transcript

In a molecule of ethene, the double covalent bond is composed of one 𝜎 bond and one 𝜋 bond. Furthermore, three sp2 hybrid orbitals exist around each of the carbon atoms of ethene. A second two-carbon molecule, ethyne C2H2, has a triple covalent bond between the two central carbon atoms with 2sp hybrid orbitals around each of its carbon atoms. Given your knowledge of ethene and how 𝜎 bonds and 𝜋 bonds are formed, predict the composition of the triple bond in ethyne. (A) The triple bond in ethyne is composed of one 𝜎 bond and two 𝜋 bonds. (B) The triple bond in ethyne is composed of three 𝜎 bonds. (C) The triple bond in ethyne is composed of two 𝜎 bonds and one 𝜋 bond. (D) The triple bond in ethyne is composed of three 𝜋 bonds. Or (E) the triple bond in ethyne is composed of one 𝜎 bond, one 𝜋 bond, and one coordinate bond.

In this question, we want to know what the correct numbers of 𝜎 and 𝜋 bonds in the triple bond of ethyne are and whether there are any other types of bonds present. Let’s first look at the information we are given. We will need to clear some space to do this.

We are told that in ethene, there is a double covalent bond made from one 𝜎 and one 𝜋 bond and that there are three sp2 hybrid orbitals around each carbon atom. So we could label the double bond of ethene like this: one bond is a 𝜎 bond and one is a 𝜋 bond. Each of the carbon atoms has one, two, three sp2 hybrid orbitals available for bonding, as well as one p orbital available for bonding. These two orbitals overlap to form a 𝜎 bond. And the p orbitals overlap to form one 𝜋 bond, which looks like a broken donut. Four hydrogen atoms can bond with the remaining sp2 orbitals.

Now let’s look at the molecule in question. We are told ethyne has a molecular formula C2H2. It has a triple bond, and each carbon atom has two sp hybrid orbitals. Now let’s draw ethyne. It has a triple carbon bond. And each carbon atom has one, two sp hybrid orbitals, as well as two p orbitals in different planes available for bonding. The sp orbitals can overlap to form a 𝜎 bond. P orbitals in the same plane can overlap to form a 𝜋 bond. And p orbitals in another plane can overlap to form a second 𝜋 bond. Hydrogen atoms can 𝜎 bond with the remaining sp hybrid orbitals. So the bonds in the ethyne triple bond are a 𝜎 bond and two 𝜋 bonds.

The question asked us to predict the composition of the triple bond in ethyne. The answer is (A). The triple bond in ethyne is composed of one 𝜎 bond and two 𝜋 bonds.