The C2-C3 single bond in butane freely rotates at room temperature, see equation (1) below. In contrast, the C2-C3 single bond in the butenyl cation, equation (2), has a high energy barrier for rotation and does not rotate at room temperature. Use structures to explain why the C2-C3 rotational barrier in butenyl cation is so high.

Answer: Rotation occurs at single bonds that are sigma bonds. Rotational barrier is the amount of activation energy required to covert rotamer to another by rotation that occurs around the sigma bond (C-C single bond). Due to the presence of steric hindrance that is the nonbonding interaction effects the reactivity of ions and molecules, activation energy increases. So the rotational barrier in butenyl cation is high.