Resonance in organic chemistry ?

Question

What is the point of resonance. I understand that it is because there are different lewis structures for something like P03. The double bond oscillates between every PO bond. why not just one? Is it because of the negative formal charges. It tries to get away from them for stability?

Answer 1

Resonance dates back to the 1930's as an attempt to justify to several possible Lewis structures.  Consider SO2.

The Lewis structures are O-S=O <--> O=s****.>
We know today that there is no oscillation between the location of two double bonds in SO2.  The bonds are IDENTICAL and have bond orders of about 1.5.

Consider the nitrate ion.

O-N=O

... |

...O

Each of the bonds is IDENTICAL.  There is no double bond that "rotates" between the oxygen atoms.  What there is, is an unhybridized p-orbital on the nitrogen that overlaps THREE unhybridized p-orbitals on EACH of the oxygen atoms so that each bond has a bond order of around 1.3.

Valance bond theory does a much better job of dealing with this than does VSEPR theory and resonance structures.

And what's with PO3?  There is no PO3 molecule.  There is a PO3^3- polyatomic ion, the phosphite ion.  And there is no double bond in it. Each P-O bond is essentially the same. Each bond has a bond order of about 1.69.

See this link for a description of the phosphite ion.

http://www.colby.edu/chemistry/webmo/PO3...

You can also put formal charge on a shelf and drag it out when you need it to decide between several possible Lewis structures.  Formal charges are not ACTUAL charges.  Formal charge is a "mathematical construct", so to speak, that has a limited usefulness.  Far too often, people put far too much importance on formal charge.  Don't.

Answer 2

That is an organic chemist for you.  Resonance is a way to depict a molecule that can have bonds at different positions with them being the same in energy, the real molecule is the hybrid of resonance structures

Answer 3

My answer may not clarify matters for you,  but I'll try.

It isn't that the double bond oscillates among the PO bonds.   Rather,  there is a mathematical description of the bond between P and O.   It turns out that in PO3,  there is actually mathetically a 1 1/3 bond with each of the 3 PO links.

When one calculates the energy of a PO3 group,  a structure with the 3 1 and 1/3 bonds is more stable than if there were full P=O and P-O bonds.

So resonance forms are a way to try to help picture what is happening.   But it is an attempt.  Only when one can actually work the math does it really become pretty clear.   This came for me in graduate school.