Question

Use orbitals to explain why bond rotation is possible around the single C‐C bonds in this molecule but not the double C=C bond. Note: To answer this question you may need to review prerequisite topics you covered in first‐semester general chemistry. Specifically, concepts about orbitals and bond formation should be reviewed (sigma bonds, pi bonds, s orbitals, p orbitals, etc.).

Answer 1

In the case of the C-C single bonds, the overlap between the TWO orbitals only takes place in a linear manner as there is no unhybridized orbital in the case of alkanes. All the orbitals or carbon are hybridized in this case. So there is no hindrance to rotation around the single sigma bond as no sideways parallel overlap is available!

In the case of a C=C bond, the situation is a bit interesting! As we know, that C=C bond carbons are hybridized, one p orbital (actually orbital) is left unhybridized which is actually perpendicular to the sigma bond(perpendicular to x-y plane as shown below). Now each of the two carbons has its own unhybridized p-orbital which goes to parallel or sideways overlap in both faces of the sigma bonded plane which is known as the pi-bond. Due to the presence of the pi-bond, the C=C system cannot rotate around as this will require the pi-bond to break!