Problem 34 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The number and type of bonds between two carbon atoms in \(\mathrm{CaC}_{2}\) are [1996-1 Mark] (a) one sigma \((\sigma)\) and one pi \((\pi)\) bonds (b) one sigma ( \(\sigma\) ) and two pi \((\pi)\) bonds (c) one sigma \((\sigma)\) and one and a half pi \((\pi)\) bonds (d) one sigma ( \(\sigma\) ) bond.

Step-by-Step Solution

Answer

(b) one sigma and two pi bonds.

Step 1: Identify the Compound

The compound given is calcium carbide, \( \text{CaC}_2 \). This compound features carbon atoms bonded to each other, and calcium as an ionic partner.

Step 2: Understand the Carbon Bonding

In \( \text{CaC}_2 \), the two carbon atoms form an acetylide ion, denoted as \( \text{C}_2^{2-} \). The bonding between these carbon atoms needs to be understood.

Step 3: Acetylide Structure Analysis

The acetylide ion \( \text{C}_2^{2-} \) consists of a triple bond between the two carbon atoms. This implies it contains one sigma bond and two pi bonds.

Step 4: Select the Correct Option

Given the triple bond nature in acetylide \( \text{C}_2^{2-} \), the correct option that describes the bonding is one sigma (\( \sigma \)) and two pi (\( \pi \)) bonds. This corresponds to option (b).

Key Concepts

Sigma and Pi Bonds

Chemical bonds are the forces that hold atoms together, forming molecules and compounds. Understanding the types of bonds between atoms is essential when studying molecular structures. Two primary types of covalent bonds are *sigma* (\(\sigma\)) and *pi* (\(\pi\)) bonds. In a sigma bond, the electron density is concentrated directly between the nuclei of the bonding atoms. This "head-on" overlap of orbitals makes sigma bonds strong, allowing them to form the backbone of molecular structures.
On the other hand, pi bonds are formed by the "side-by-side" overlap of p-orbitals. Pi bonds occur alongside sigma bonds in double or triple bonds, contributing additional strength but allowing for less flexibility in the bond structure. In the case of calcium carbide, the acetylide ion contains two pi bonds, each formed by this lateral overlap. Hence, a complete triple bond is composed of one sigma bond and two pi bonds, stabilizing the carbon atoms significantly.

Acetylide Ion Structure

The acetylide ion, denoted as \( \text{C}_2^{2-} \), is a fascinating ion due to its unique and highly stable bonding. It is found in calcium carbide \( \text{CaC}_2 \), where the acetylide ion is formed by two carbon atoms tied together by a total of three covalent bonds. These bonds consist of one sigma bond and two pi bonds, creating a strong and linear triple-bonded structure.
The acetylide ion carries a -2 charge, indicating the loss of electrons that contribute to its stability and strength. This charge is delocalized over the bonding electrons, allowing them to form the triple bond effectively.
The formation of the acetylide ion is significant in many chemical reactions, given that it serves as a precursor or reactant in organic synthesis. Understanding the structural aspects of this ion helps one appreciate its role in industrial applications and chemical transformations.

Triple Bond Characteristics

Triple bonds are one of the strongest and shortest types of covalent bonds found in chemistry, formed by the combination of one sigma bond and two pi bonds. This strength comes from the linear overlap of orbitals in a sigma bond, coupled with the lateral overlap of p-orbitals in pi bonds.
Due to the presence of multiple bonds, a triple bond has a few distinct characteristics:

• **Short Bond Length**: The presence of three overlapping orbitals makes the bond shorter compared to single or double bonds.
• **High Bond Energy**: More energy is required to break a triple bond due to its multiple bonding interactions.
• **Restricted Rotation**: The pi bonds prevent free rotation around the bond axis, giving compounds with triple bonds distinct geometric structures.

These characteristics are essential in determining the reactivity and the physical properties of compounds such as calcium carbide, making them crucial in understanding their role in various chemical processes and applications.