Question

结构化学的选择题

We learned that four new molecular orbitals are created from the p orbitals for each carbon atom in the butadiene molecule (the axes perpendicular to the page). Which statement about these molecular orbitals is NOT true?

A The four molecular p orbitals of the four carbon atoms can be lined up parallel in order to create four new molecular orbitals.

B The electrons in the new molecular orbitals benefit because they are not confined to just one atom.

C The electrons, because of the new molecular orbits, are spread out over the entire molecule.

D As a result of the new molecular orbitals, the bond between the second and third carbon atoms is not really a single bond. In fact, the bond should be shorter.

求高手帮忙了啊~~顺便能不能说下理由呢~~不胜感激

Answer 1

D is not true, because in the newly formed 4 MO's, first two are occupied (bonding orbitals), and the 3rd and 4th are unoccupied.
The lowest (in energy) orbital has equal electron density distribution between any two adjacent C atoms, while the next filled orbital has a nodal plane between the second and third carbon atoms. So the "bond order" between the second and third carbon atoms should be lower than those between the first carbon and the second carbon, the 3rd and 4th carbon. And hence, the bond should be LONGER.

there is a typo in C. "the new molecular orbits" should be "the new molecular ORBITALS"

Everything seems right except for C, where "the new orbits" should be the new orbitals.