[trishasales714]

For a compound like DMSO or acetone where you can draw resonance structures, could you say that the electrons are delocalized? Does having a resonance = delocalized electrons or does the term delocalized electrons only apply to benzene where all the pi electrons are delocalized?

[Babcock_Hall]

Many compounds or metastable intermediates have more than one resonance form, not just benzene and its derivatives.  Would you try drawing resonance structures of acetone for us?

[trishasales714]

Many compounds or metastable intermediates have more than one resonance form, not just benzene and its derivatives.  Would you try drawing resonance structures of acetone for us?

There would be a - charge on the oxygen and a + charge on the carbon since the DB of the C=O bond is broken. I wasn't sure if the term delocalized electrons only applied to structures where all the pi bonds are delocalized.

[wildfyr]

Acetone and DMSO are an interesting choice, because they are isoelectronic, however their "identical" resonance forms contribute different amounts. Acetone is very much best drawn C=O, with minimal contribution from the zwitterion. However the zwitterionic form of DMSO (⁺S-O^-) is a decently strong contributor to the reactivity of DMSO. This is because S and O have poorer orbital overlap than C and O. This same effect explains the reactivity series of acid halides.

[Babcock_Hall]

I am not sure that I understand your question fully.  Some molecules could have a delocalized pi system, in addition to a localized pi system.  For example a derivative of benzene with a hydrocarbon chain that had a non-conjugated double bond distant from the aromatic ring.

[Irlanur]

I know this is an OC forum, but I would still like to emphasize that electrons are only ever localized in the chemist's head.

[Enthalpy]

[...] electrons are only ever localized in the chemist's head.

Sure! But the outdated and refuted theories with local bonds, resonances, electrons hopping during the reactions... are still fertile, in the sense that they permit predictions and syntheses. Apparently, no correct theory with molecular orbitals is enough manageable and elaborate to be fertile. So who will develop that?

[Irlanur]

My point is that these discussions only ever make sense when they are about actual "explanations", and about useful predictions. They don't make sense on a fundamental level.

[Enthalpy]

100% agreed! I just hoped that you would catch the suggestion and develop a new better theory, both correct and useful, as you have the proper spectrum of knowledge.
Or find someone who will. It would be about time.