Chapter 1 Worked Problem 4

Problem: Deocalized Bonding: Resonance & MO Theory

Write all major resonance structures for the following compound, and describe what these mean in terms of an MO description of the bonding.

Analysis

We clearly see there is a double bond, so there are σ electrons that might be delocalized if these carbons have either an adjacent atom with a lone pair or an adjacent atom with a vacant orbital.

As it turns out, both cases exist in this molecule, so there will be multiple resonance forms.
The options are:

Share the lone pair electrons with one of the carbons in the C=C σ bond, and push the σ electrons onto the other carbon. Don't exceed the octet at any atom! This creates a negative charge on the carbon with the lone pair (it gained electrons) and a positive charge on the oxygen (it lost electrons).
Share the σ electrons with the empty p orbital on B. This will create an electron deficiency--and a positive charge--on the remaining carbon.
In this case there is a fourth structure stemming from our third: the lone pair on O can form a σ bond to the electron deficient carbon.

info clear no info

Show the constituent p_z AOs.
Show the HOMO
Show the LUMO