Observe the molecular orbital picture of ethyne. In contrast to ethane, there is no free rotation around the triple bond of ethyne. Explain.

• A. Free rotation is allowed in ethane because the orbital overlap that connects the carbons are not affected by the rotation, while in ethyne, it is not allowed because the overlap of the of p-orbitals would be broken upon rotation.
• B. Free rotation is allowed in ethane because the greater amount of hydrogen atoms aids the rotation, while in ethyne, it is not allowed because the hydrogen atoms align with the axis.
• C. Free rotation is allowed in ethane because the orbital overlap that connects the carbons are not affected by the rotation, while in ethyne, it is not allowed because the overlap of the of sp-orbitals would be broken upon rotation.
• D. Free rotation is allowed in ethane because the single bond is weak, while in ethyne, it is not allowed because of the much stronger triple bond.

Answer: (A)