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Carbonyl has two resonance structures, where the $\pi$-bond between C and O breaks and the electrons "move" to the oxygen atom. This gives an empty p-orbital in carbon and a p-orbital in oxygen with two electrons, leading to a positive charge on carbon, and a negative charge on oxygen. To my understanding, oxygen gets the lone electron pair because it has a higher electronegativity than oxygen.

My question: is it possible that the two electrons "move" to carbon instead of oxygen, leading to a third resonance structure (I know that these resonance structures don't really exist, and that the real structure is somewhere in between), so that carbon has a lone electron pair? It would probably contribute little to the resonance energy compared to the other structures. Or is it not possible at all?

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    $\begingroup$ You can do that $\endgroup$ – ParaH2 Oct 22 '17 at 11:54
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    $\begingroup$ Yes, you can write it and it's not significant contributor. $\endgroup$ – Mithoron Oct 22 '17 at 12:37
  • $\begingroup$ Thank you both! I could find anything about it, but it seemed logical to me. $\endgroup$ – mitchbus Oct 22 '17 at 13:26
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Yes, resonance should never stop at the most likely resonance structure. If you’re really willing to play the game, you can take it even further and consider singlet diradical structures where the bond has been broken into a spin up and a spin down electron, each one of which resides in a different orbital. And more, and more.

(Side note: this is why it is useless to ‘count’ resonance structures. The only correct answer to ‘How many resonance structures does $\ce{X}$ have?’ is ‘Always one more.’.)

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