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In a compound like acrolein (prop-2-enal or $\ce{CH2=CH-CHO}$, will the oxygen in the $\ce{-CHO}$ group always have a $\delta$- charge?

Acrolein structure

Acrolein does have three resonance structures as shown below, in which on one of them, oxygen has a $\delta$+ charge.

Acrolein's resonance structures

How stable is the third resonating structure? Is it viable to even consider it a resonating structure, since $\ce{-CHO}$, a decently strong minus I group has a $\delta$+ charge, and $\ce{-CH=CH2}$, a very weak minus I group has a $\delta$- charge?

This doubt stemmed from the following question, about the resonant structures of acrolein (apologies for the low quality):

textbook question

It's a multiple correct MCQ, and the answer given is $ABC$, but as per the resonating structures of acrolein, it should be $AC$, as B and D, both should be valid resonating structures of acrolein.

Mithoron
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Bongo Man
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  • Hmm, as per my knowledge, the third resonating is of much higher energy and thus doesn't contribute much to the overall stability of the molecule. The reason being empty orbital, $\ce{O^+}$, in $\ce{sp^3}$ hybridised oxygen, which is very unstable. – Laxmi Narayan Bhandari Mar 02 '24 at 06:39
  • That's a good logic, but is it really fair to say that the third structure entirely does not exist simply because it is unstable, it could still have a low resonance contribution. – Bongo Man Mar 02 '24 at 11:33

1 Answers1

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The problem with the third structure is not the positive formal charge on oxygen as such, but leaving the oxygen short of its octet. Electronegative elements generally have their octets filled in stable molecules or ions, both organic and inorganic.

On the other hand, once the octets are filled electronegative atoms often back-donate their "nonbonding" electron pairs and may develop a positive overall charge through this mesomeric effect. Examples with oxygen include carbon monoxide, pyrilium rings, and the cationic intermediate formed when a hydroxy or alkoxy group activates electrophilic aromatic substitution.

See this question for a rather unexpected example involving chlorine as the -I function.

Oscar Lanzi
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