Problem 41 - A Complete Resource Book in Chemistry for JEE Main

Question

Acetylene has ......... bonds (a) \(2 \sigma, 2 \pi\) (b) \(2 \sigma, 3 \pi\) (c) \(3 \sigma, 2 \pi\) (d) \(3 \sigma, 3 \pi\)

Step-by-Step Solution

Answer

Acetylene has 3 sigma and 2 pi bonds, so answer (c) is correct.

Step 1: Understanding the Structure of Acetylene

Acetylene, also known as ethyne, has the chemical formula \(C_2H_2\). Its structure consists of two carbon atoms connected by a triple bond and each carbon is also bonded to one hydrogen atom.

Step 2: Analyzing the Bonds within Acetylene

A triple bond consists of one sigma (\(\sigma\)) bond and two pi (\(\pi\)) bonds. In acetylene, each triple bond between the carbon atoms contributes 1 \(\sigma\) and 2 \(\pi\) bonds.

Step 3: Determining the Total Sigma and Pi Bonds

Acetylene has 2 carbon-hydrogen single bonds, each is a \(\sigma\) bond. Additionally, the carbon-carbon triple bond provides 1 \(\sigma\) bond and 2 \(\pi\) bonds. In total, acetylene has 3 \(\sigma\) bonds (1 from the triple bond and 2 from the carbon-hydrogen bonds) and 2 \(\pi\) bonds.

Step 4: Choosing the Correct Answer

Comparing the total number of \(\sigma\) and \(\pi\) bonds with the given options, option (c) \(3 \sigma, 2 \pi\) fits the structure of acetylene the best.

Key Concepts

Acetylene Structure

In studying the structure of acetylene, it's important to recognize its unique composition. Acetylene, often referred to by its systematic name ethyne, has the simple formula \( C_2H_2 \). This means it's made up of two carbon atoms and two hydrogen atoms.

When you picture the structure, envision two carbon atoms linked by a triple bond in the center. Each carbon atom is also singly bonded to a hydrogen atom. These carbon-hydrogen bonds are straightforward single bonds, contributing to the molecule's overall framework. This distinct linear arrangement is characteristic of acetylene, resulting from its specific bonding pattern.

Sigma and Pi Bonds

Bonds within acetylene are fundamental in understanding how chemical interactions occur. A single bond, known as a sigma bond (\( \sigma \)), is formed by the direct overlap of orbitals between two atoms. In acetylene, each of the carbon-hydrogen bonds is a sigma bond.

Now, regarding the triple bond between the carbon atoms, the situation becomes more nuanced. Each triple bond comprises one sigma bond and two pi bonds (\( \pi \)). The sigma bond arises from head-on overlap between orbitals, establishing the primary link.

• Sigma bonds are strong and form the foundation of molecular structures.
• Pi bonds occur when there is side-to-side overlap between p orbitals.

So, in acetylene, the triple bond accounts for one sigma bond plus the two pi bonds.

Triple Bond

The triple bond in acetylene is a hallmark of its chemistry, providing strength and unique reactivity. A triple bond is significantly stronger than a single or double bond due to its threefold electron pair sharing between atoms.

The combination of one sigma and two pi bonds contributes to this strength. While the sigma bond lays down the framework with its head-on orbital overlap, the pi bonds offer additional stability due to their side-to-side orbital interaction.

• Triple bonds are linear, which gives acetylene its straight-line structure.
• The presence of triple bonds results in relatively high bond energy and shorter bond length.

This bond strength and linear structure make acetylene fascinating in both organic chemistry and industrial applications, like welding, where its energy release is put to use.