Question

If an electron is added to 1,3 -butadiene, into which molecular orbital does it go? If an electron is removed from 1,3 -butadiene, from which molecular orbital is it taken?

Short answer

Answer: In 1,3-butadiene, an electron will be added to the lowest unoccupied molecular orbital (LUMO), Ψ₃. An electron will be removed from the highest occupied molecular orbital (HOMO), Ψ₂.

Step-by-step solution

Understand molecular orbital theory

Molecular orbital theory explains the formation of molecular orbitals by the combination of atomic orbitals. In the case of conjugated dienes, such as 1,3-butadiene, the overlapping of atomic orbitals leads to the formation of molecular orbitals.

Identify the atomic orbitals involved

In 1,3-butadiene, there are four sp2 hybridized carbon atoms. Each sp2 carbon atom has one remaining unhybridized 2p atomic orbital (p-orbital). These p-orbitals will overlap side-by-side, participating in pi bonding.

Represent the energy levels of molecular orbitals

When the four 2p atomic orbitals combine, they form four discrete molecular orbitals at different energy levels. These molecular orbitals are often depicted in the form of an energy-level diagram with the lowest-energy molecular orbital at the bottom and the highest-energy molecular orbital at the top.

Fill electrons into molecular orbitals

Now, according to Hund's rule, we need to fill the electrons into the molecular orbitals. 1,3-butadiene has four pi electrons, so we fill up these electrons in the order of increasing energy, two electrons per orbital.

Determine the final molecular orbitals

After filling the electrons, we get the following molecular orbitals: - Lowest-energy molecular orbital (Ψ₁): 2 electrons - Next higher-energy molecular orbital (Ψ₂): 2 electrons - Next higher-energy molecular orbital (Ψ₃): 0 electrons - Highest-energy molecular orbital (Ψ₄): 0 electrons

Determine molecular orbital for electron addition and removal

When an electron is added to 1,3-butadiene, it will enter the lowest unoccupied molecular orbital (LUMO), which is Ψ₃. When an electron is removed from 1,3-butadiene, it will be taken from the highest occupied molecular orbital (HOMO), which is Ψ₂. So, an electron will be added to molecular orbital Ψ₃, and an electron will be removed from molecular orbital Ψ₂.