Question

For each of the questions, four choices have been provided. Select the correct alternative. The number of \(\sigma\) and \(\pi\) bonds in \(\mathrm{C}_{2} \mathrm{H}_{2}\) is (a) 0 and 4 (b) 2 and 2 (c) 3 and 2 (d) 4 and 2

Short answer

Answer: In ethyne, there are 3 σ bonds and 2 π bonds.

Step-by-step solution

Draw the Lewis structure of \(\mathrm{C}_{2} \mathrm{H}_{2}\) (ethyne).

First, we need to write down the Lewis structure of ethyne. Ethyne is an unsaturated hydrocarbon containing a triple bond between two carbon atoms. So, its Lewis structure can be represented as: H-C≡C-H

Identify all the bonds

Now, analyze the structure and identify each bond. Notice that there are 3 bonds between the two carbon atoms (one \(\sigma\) bond and two \(\pi\) bonds) and 1 \(\sigma\) bond between each carbon and hydrogen atom.

Count the total number of \(\sigma\) and \(\pi\) bonds

As we can see from the analysis above, there are 3 \(\sigma\) bonds (1 C-C and 2 C-H) and 2 \(\pi\) bonds (in the C≡C triple bond) in the molecule. Thus, the correct alternative is: (c) 3 and 2

Key concepts

Lewis Structure

Lewis structures are essential tools in understanding chemical bonding, specifically in organic molecules. They are simple diagrams that display the arrangement of atoms and the bonds between them in a molecule. To draw a Lewis structure, you must know the total number of valence electrons available. Then, distribute these electrons among the atoms, so that each atom (other than hydrogen) aims to follow the octet rule—having eight electrons in its valence shell.
In the case of ethyne, or acetylene \((\mathrm{C}_2 \mathrm{H}_2)\), the Lewis structure represents the connectivity between atoms. Ethyne is unsaturated because it contains multiple bonds—specifically a triple bond between the carbon atoms. The Lewis structure for ethyne is linear: \(\mathrm{H}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}\). Each carbon atom in the triple bond shares three pairs of electrons, forming what we observe as three lines between the carbon atoms.
It's crucial for students to practice drawing these structures as they lay the groundwork for understanding more complex molecular shapes and interactions.

Sigma and Pi Bonds

Understanding sigma \((\sigma)\) and pi \((\pi)\) bonds is key to grasping molecular structure and bonding. These terms describe the types of covalent bonds formed when atomic orbitals overlap.

• A sigma bond is the first bond formed between two atoms. It is created by the head-on overlap of atomic orbitals, like \(s\), \(p\), or hybrid orbitals such as \(\text{sp}^3\). This head-to-head overlap allows for free rotation around the bond axis, which provides flexibility to molecular structure.
• Pi bonds, on the other hand, form when the side-to-side overlap of \(p\) orbitals occurs above and below the plane of the atoms involved. Pi bonds are more rigid compared to sigma bonds and do not allow for rotation around the bond axis. Typically, pi bonds are found in double and triple bonds as additional bonds after the sigma bond is formed.

In ethyne, two carbon atoms form one sigma bond and two pi bonds. The single bond between each carbon and hydrogen atom is a sigma bond. These distinctions are important because they affect the geometry and reactivity of organic molecules.

Triple Bond in Ethyne

The triple bond in ethyne \((\mathrm{C}_2 \mathrm{H}_2)\) is a fascinating feature that showcases how molecules can achieve stability through complex bonding.
In a triple bond, like that between the two carbon atoms in ethyne, one sigma bond and two pi bonds are present. Here’s how it works:

• The single sigma bond is formed by the overlap of the \(\text{sp}^\text{hybridized}\) orbitals from each carbon, aligning the molecule in a linear shape.
• The two pi bonds are formed from the side-to-side overlap of the \(\text{p}\) orbitals. One orbital pair overlaps above the plane of the sigma bond, while the other pair overlaps below.

These pi bonds are more vulnerable than sigma bonds due to their orientation, rendering ethyne more chemically reactive—a crucial insight for organic chemical reactions.
The presence of the triple bond, therefore, provides ethyne with unique chemical attributes, such as its potential to participate in addition reactions, which are central to organic syntheses. Understanding this bonding concept is fundamental for anyone studying organic chemistry.