Problem 91 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following has the least hindered rotation about carbon-carbon bond? (a) ethane (b) ethylene (c) acetylene (d) hexachloroethane

Step-by-Step Solution

Answer

Ethane has the least hindered rotation around the carbon-carbon bond.

Step 1: Define Hindered Rotation

Hindered rotation in organic molecules refers to the restricted ability of two atoms or groups of atoms within a molecule to rotate freely around a specific bond, often due to steric or electronic factors.

Step 2: Analyze Ethane

Ethane (a) consists of two carbon atoms single-bonded to each other with hydrogen atoms bonded to each carbon. The single bond allows for free rotation, hence it has the least hindered rotation.

Step 3: Analyze Ethylene

Ethylene (b) has a double bond between the two carbon atoms. Double bonds restrict rotation due to the pi-bond, which locks the molecular geometry, preventing rotation without breaking the bond.

Step 4: Analyze Acetylene

Acetylene (c) features a triple bond between the two carbon atoms. Triple bonds further restrict rotation more than double bonds because there are two pi-bonds that prevent rotation.

Step 5: Analyze Hexachloroethane

Hexachloroethane (d) contains a single bond between carbon atoms, like ethane, however, all carbons are bonded to chlorine atoms. The presence of bulky chlorine atoms could sterically hinder rotation around the single bond more than in ethane.

Key Concepts

Ethane

Ethane is a simple hydrocarbon comprising two carbon atoms single-bonded to each other. Each carbon atom is also single-bonded to three hydrogen atoms. Its chemical formula is C\(_2\)H\(_6\). The single bond between the carbon atoms is a sigma bond, which allows free rotation about the bond axis.

Due to this free rotation, ethane exhibits the least hindered rotation among the options presented. Since there are no additional bonds preventing movement, the molecule can freely twist around the carbon-carbon bond.

• Simple structure with a sigma bond.
• Allows full rotation due to absence of additional bonding constraints.

This characteristic makes ethane an ideal candidate when considering unhindered or easily rotated molecular structures.

Ethylene

Ethylene is a hydrocarbon that has a double bond between its two carbon atoms, represented by C\(_2\)H\(_4\). Unlike ethane, ethylene's carbon-carbon double bond comprises one sigma bond and one pi bond.

The presence of the pi bond introduces restrictions. These bonds are not aligned linearly as sigma bonds are, but rather they exist above and below the plane of the molecule, creating a planar geometry. This planar nature locks the molecules in place, making rotation impossible without breaking the bond.

• Contains a double bond with a sigma and pi bond.
• Restricted rotation due to the pi bond lock.
• Planar structure preventing bond rotation.

As a result of this structure, ethylene cannot rotate around its carbon-carbon bond without breaking the double bond.

Acetylene

Acetylene is a simple molecule composed of two carbon atoms triple-bonded to one another, with each carbon also bonded to a hydrogen atom, giving a chemical formula of C\(_2\)H\(_2\). This triple bond consists of one sigma bond and two pi bonds.

The combination of the sigma and two pi bonds in the triple bond greatly restricts any rotational movement. The pi bonds are tightly held within an electron cloud that completely stabilizes and secures the linear structure of the molecule.

• Triple-bonded structure forming a linear configuration.
• Two pi bonds further restricting rotation.
• Acetylene showcases the most restricted rotation among simple hydrocarbons.

The triple bond makes acetylene especially resistant to rotation, more so than ethylene, emphasizing its linear design and high-energy bond requirements.