Question

Which of the following will have least hindered rotation about carbon-carbon bond? (A) Ethylene (B) Hexachloroethane (C) Ethane (D) Acetylene

Short answer

Based on bond type and steric hindrance, ethane (option C) has the least hindered rotation about the carbon-carbon bond. It has a single bond that allows for free rotation and minimal steric hindrance due to the presence of only hydrogen atoms.

Step-by-step solution

Analyze the bond type

In ethylene, the carbon-carbon double bond restricts rotation compared to a single bond, because the sigma bond and the pi bond need to break for rotation to occur. In acetylene, the carbon-carbon triple bond restricts rotation even more, as there are two pi bonds and a sigma bond needed to be broken to rotate. Ethane and hexachloroethane have single bonds, which allow for free rotation.

Analyze steric hindrance

Ethane has only hydrogen atoms attached to the carbon atoms, so there is minimal steric hindrance as hydrogen atoms are relatively small. Hexachloroethane has six chlorine atoms attached to the carbon atoms, leading to significant steric hindrance, as chlorine atoms have a larger atomic radius than hydrogen atoms.

Determine the least hindered rotation

Based on bond type and steric hindrance, ethane (option C) has the least hindered rotation about the carbon-carbon bond. It has a single bond that allows for free rotation and minimal steric hindrance due to the presence of only hydrogen atoms.

Key concepts

Steric Hindrance

Steric hindrance refers to the inhibition of bond rotation due to the physical presence of adjacent groups or atoms. When bulky substituents are located around a bond, their electron clouds can overlap, causing repulsion that makes it more difficult for the bond to rotate freely. The concept is critical when analyzing molecules and their reactions, as it influences the reactivity and the possible conformations of a molecule.

For instance, in the context of the exercise, ethane has minimal steric hindrance due to the small size of hydrogen atoms. In contrast, hexachloroethane experiences significant steric hindrance because chlorine atoms are much larger, thus making it harder for the carbon-carbon bond to rotate. This concept is essential when evaluating which compound would have the least hindered carbon-carbon bond rotation, indicating why ethane is the correct answer.

Sigma and Pi Bonds

In chemical bonding, sigma (\f\text{\(\text{sigma}\)}\f) and pi (\f\text{\(\text{pi}\)}\f) bonds are types of covalent bonds that arise from the overlap of atomic orbitals. Sigma bonds are formed by head-on overlapping and are the first bond formed between two atoms; they allow for free rotation around the bond axis. Pi bonds, on the other hand, result from the side-to-side overlap of p-orbitals, positioned above and below the axis of the bonded atoms.The differences between sigma and pi bonds are crucial in understanding molecular shape and bond rotation. For instance, in ethylene and acetylene, the presence of pi bonds in addition to sigma bonds significantly restricts rotation due to the parallel alignment required by the p-orbitals. Ethane, however, solely has a sigma bond between its carbon atoms, enabling free rotation and making it the molecule with the least hindered rotation as mentioned in the exercise solution.

Ethane Structure

The structure of ethane (\f\text{C\(_2\)H\(_6\)}\f) is characterized by a sigma bond between its two carbon atoms, with each carbon also bonded to three hydrogen atoms. The simplicity of its structure allows for a thorough discussion about bond rotations. Since the molecule has a single bond between carbons, this bond can rotate freely, resulting in an infinite number of conformations, which are different spatial arrangements of atoms that can be converted into one another by rotation around single bonds.

Moreover, ethane's small-sized hydrogen atoms provide minimal steric hindrance, facilitating the bond rotation further. Understanding the ethane structure elucidates why ethane experiences the least hindered rotation among the options provided in the exercise, as its atomic and molecular properties allow for greater freedom of movement than in molecules with larger substituents or with multiple bonds that limit rotation.