Problem 22 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Molecule in which the distance between the two adjacent carbon atoms is largest is (a) Ethane (b) Ethene (c) Ethyne (d) Benzene

Step-by-Step Solution

Answer

Ethane has the largest distance between adjacent carbon atoms.

Step 1: Understand Bond Types

Carbon atoms can be bonded by single, double, or triple bonds. A single bond is the longest, double bond is of intermediate length, and a triple bond is the shortest due to stronger attraction between atoms.

Step 2: Analyze Each Molecule

Ethane ( ext{C}_2 ext{H}_6) has carbon-carbon single bonds. Ethene ( ext{C}_2 ext{H}_4) has a carbon-carbon double bond. Ethyne ( ext{C}_2 ext{H}_2) has a carbon-carbon triple bond. Benzene (C_6H_6) has alternating single and double bonds between carbon atoms (due to resonance, bond lengths are equal and between single and double bond lengths).

Step 3: Compare Bond Lengths

Single bonds, such as in ethane, are the longest because they involve only shared electron pairs without additional pi bonds shortening them, unlike the bonds in ethene, ethyne, or benzene.

Step 4: Determine the Molecule with Longest Bond

Since ethane has a single bond between carbon atoms, its bond length is the largest compared to the double bonds in ethene, the triple bond in ethyne, and the resonance bond length in benzene.

Key Concepts

Carbon-Carbon Bonds

Carbon-carbon bonds are fundamental in the structure of organic molecules. These bonds can be categorized into three main types based on the number of shared electrons between two carbon atoms: single, double, and triple bonds.

• Single bonds involve one shared pair of electrons and are represented as \(-\text{C}-\text{C}-\), providing a strong and stable connection with flexibility allowing rotation.
• Double bonds, denoted as \(\text{C} = \text{C}\), involve two shared pairs of electrons. They are shorter and stronger than single bonds, introducing rigidity to the molecular structure preventing rotation.
• Triple bonds, shown as \(\text{C} \equiv \text{C}\), involve three shared pairs of electrons. These are the shortest and strongest of the carbon-carbon bonds, making the molecule very rigid.

The type of bond affects the molecule's properties, including its stability, reactivity, and the overall shape of the molecule. Understanding these differences is crucial for molecular structure analysis and predicting chemical behavior.

Bond Length

Bond length refers to the distance between the nuclei of two bonded atoms. In organic chemistry, the length of carbon-carbon bonds varies significantly with the type of bond.

• Single bonds have the longest bond length, as there is less electron density pulling the nuclei together.
• Double bonds are shorter due to increased electron density between the nuclei from the additional shared pair of electrons.
• Triple bonds are the shortest because of three pairs of electrons creating even more electron density between the nuclei.

For example, in ethane (single bond), the bond length is longer compared to ethene (double bond) and ethyne (triple bond). This is a key factor in determining how a molecule interacts with other substances and influences physical properties such as boiling and melting points.

Molecular Structure Analysis

Molecular structure analysis helps in understanding how atoms are arranged in a molecule and the implications this has on its physical and chemical properties. This analysis often involves examining bond types, lengths, and the molecular geometry.

• In ethane, single carbon-carbon bonds allow rotation around the bond axis, giving ethane flexibility.
• Ethene's double bond restricts rotation, resulting in a planar structure around the bond, affecting the molecule's reactivity and interactions.
• Ethyne with its triple bond maintains a linear geometry due to its rigid structure.
• Benzene features a unique structure where carbon-carbon bonds are equal due to resonance, which distributes electron density evenly across the ring.

This analysis provides insights into molecular behavior in different conditions, predicts reactivity patterns, and assists in designing synthetic pathways. A thorough understanding is essential for fields such as materials science, pharmaceuticals, and any chemistry-related industry.