Question

The molecular orbitals associated with the π-bonding MOs in square cyclobutadiene (C4H4) are, like benzene, form by overlap of a single p-orbital on each carbon atom that is not involved in the sigma-bonding network of the molecule. These p-orbitals point perpendicularly with respect to the plane defined by the carbon atoms. There is no central atom, so the only interactions that matter are between the p-orbitals among themselves.

i) draw the four π-orbitals of C4H4

ii) Use the idea that the energy of a MO increases with number of new nodes made during bonding (i.e. nodes not already present in initial AO functions) to sketch a molecular orbital energy level diagram for the π-molecular orbitals of C4H4. Remember that, by definition, degenerate orbitals are identical except for trivial rotation in space, and have identical energy.

iii) Fill the MOs with the electrons from the atomic p-orbitals that you used to form the π-MOs of C4H4. Use your diagram to determine the overall bond order by considering each doubly occupied bonding MO to contribute 1, each nonbonding orbital 0 and each antibonding orbital -1. The result would be divided by 4 C-C bonds to get the average contribution to each C-C bond from π-bonding, which then should be added to a σ-bonding contribution of 1.0

iv) Repeat step (iii) for the dication C4H4 2+ to determine the net bond order for this species. Why do you think you got this value?

v) Predict which of C4H4 and C4H4 2+ could be observed by ESR spectroscopy

vi) Which of C4H4 2+ and C4H4 is more likely to be susceptible to electrophilic attack.

Just need a basic, but ACCURATE, run through. Will thumbs up. Please and Thank you.

Answer 1

Will use MO to answer this.