Question

Which of the following has the least hindered rotation about carbon-carbon bond? (a) Ethane (b) Ethylene (c) Acetylene (d) Hexachloroethane

Short answer

Ethane (a) has the least hindered rotation.

Step-by-step solution

Understanding the Problem

We are asked to determine which molecule has the least hindered rotation about a carbon-carbon bond. To do this, we need to assess the types of carbon bonds present in each molecule and how these bonds affect rotational freedom.

Examining Each Option

Let's analyze each compound: (a) Ethane: Contains a single C-C sigma bond, allowing free rotation. (b) Ethylene: Contains a C=C double bond, which restricts rotation due to the pi bond. (c) Acetylene: Contains a C≡C triple bond, which also restricts rotation due to the overlapping of two pi bonds. (d) Hexachloroethane: Although it has single bonds, the presence of bulky chloride groups around each carbon lowers flexibility and hinders rotation due to steric hindrance.

Determining Least Hindered Rotation

Between these options, ethane (a) has only a single carbon-carbon sigma bond. Unlike double and triple bonds, a single bond does not prevent rotation, allowing ethane to freely rotate, unlike the other compounds which have restricted or hindered rotation due to double bonds, triple bonds, or steric hindrance.

Key concepts

Carbon-Carbon Bond Rotation

In organic chemistry, the rotation about carbon-carbon bonds is an important concept that affects the structure and behavior of molecules. When we talk about rotation, we usually refer to the movement around single bonds, specifically sigma ( bonds. Single bonds allow atoms connected by them to freely rotate. Ethane is a classic example of such a molecule, where each of its carbon atoms is joined by a single sigma bond. This allows the atoms in ethane to freely rotate relative to each other. However, when a molecule involves double or triple bonds, rotation is restricted or even impossible. For example, in ethylene, the presence of a carbon-carbon double bond limits rotation because of the pi bonding, contributing to a rigid structure. Acetylene, with a triple bond, faces an even greater restriction due to the pair of pi bonds overlapping directly with the sigma bond. Thus, ethane presents the least hindered rotation capability due to its simple single sigma bond structure, highlighting how molecular bonding types influence rotational freedom.

Sigma and Pi Bonds

The types of bonds that connect atoms within a molecule are fundamental in determining a molecule's flexibility or rigidity. Sigma ( and pi (\(\)) bonds play crucial roles in this context. A single bond involves one sigma bond that is formed by the head-on overlap of atomic orbitals. This bond is strong and allows for the free rotation of atoms about the bond axis. Ethane is an example of a molecule with a purely sigma bond connecting the carbon atoms, making it very flexible.When dealing with multiple bonds, a pi bond accompanies a sigma bond, forming double bonds, and two pi bonds accompany a sigma bond in triple bonds. Pi bonds result from the sideways overlap of atomic orbitals. This type of overlap means pi bonds restrict any free rotation. Therefore, in molecules like ethylene and acetylene, the presence of pi bonds due to double and triple bonds limits their ability to rotate around the bond completely.

Steric Hindrance

Steric hindrance is another key factor affecting molecular rotation. It occurs when bulky groups within a molecule impede the free rotation or movement of other parts of the molecule, causing restricted motion overall. In the case of hexachloroethane, although it technically has single bonds indicating potential for rotation, the attached chlorine atoms are large and occupy more space around the carbon atoms. This spatial arrangement leads to steric interference, preventing the molecule from rotating freely like simple hydrocarbons with no bulky substituents. Steric hindrance can significantly influence the chemical behavior of molecules, impacting both their reactivity and the ease with which they can undergo conformational changes.

Molecular Flexibility

Molecular flexibility is a measure of how easily a molecule can adapt to different shapes or conformations. This property is primarily influenced by the type of chemical bonds within the molecule and steric factors. Free rotation around carbon-carbon sigma bonds contributes to a molecule's flexibility, as seen in ethane, where the atoms can pivot around the bond axis without constraint. However, when pi bonds are involved, as with double and triple bonds, this flexibility is notably reduced. The more pi bonds present, the less flexibility exists due to fixed structural rigidity. Furthermore, the size and positioning of other atoms or groups within a molecule can impact its flexibility. Large groups or substituents can create obstacles that hinder movement, as observed in molecules with significant steric hindrance. Understanding molecular flexibility helps chemists predict how molecules behave in different environments or reactions, providing a greater understanding of their functional possibilities.