Chapter 16.8, Problem 19CC

Interpretation Introduction

Interpretation:

The given [4+4] cycloaddition process should be explained with the use of MO theory whether it occurs through thermal or photochemical pathway.

Concept introduction:

According to MO theory,

• In a molecule, the atomic orbitals of atoms constituted in the molecule are combined to produce new orbitals are called Molecular Orbitals.
• The numbers of MO’s are equals to the number of atomic orbitals are combined in such a way that the linear combination of similar atomic orbitals to form equal numbers of bonding and anti-bonding MO’s.
• The bonding MO’s are lower in energy than the anti-bonding MO’s.
• HOMO is the highest energized occupied orbital in the MO’s.
• LUMO is the lowest energized orbital in the MO’s.

Frontier orbital theory says,

• In a cycloaddition reaction, the electron cloud flows from HOMO of the one molecule to LUMO of the other molecule.

According to conservation of orbital symmetry,

• The phases of MO’s (HOMO & LUMO) of reactants must be symmetric in order to reaction occur.
• The thermal pathway of cycloaddition reaction occurs in the ground state of 1,3-butadiene.
• The photochemical pathway of cycloaddition reaction occurs in the excited state of 1,3-butadiene.

To explain: the given [4+4] cycloaddition process with the use of MO theory whether it occurs through thermal or photochemical pathway.