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Does resonance always stabilize a molecule or can resonance effects destabilize molecules?

The only example I can think of in which resonance (more accurately, conjugation) destabilizes molecules is in the cases of anti-aromatic compounds. Is this a valid example, and are there any more such examples?

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    $\begingroup$ If resonance is not stabilizing, then why would it occur? $\endgroup$
    – LDC3
    Commented Nov 1, 2014 at 18:43
  • $\begingroup$ Couldn't you say that about a lot of things? If anti-aromatic compounds were unstable, why do they exist? I think the answer to your question lies with meta-stability. $\endgroup$
    – Dissenter
    Commented Nov 1, 2014 at 18:53
  • $\begingroup$ What I was trying to point out is stated by Ron, Resonance only occurs when the result is beneficial. Anti-aromatic compounds exist since the molecular bonding releases energy from the state of individual atoms. $\endgroup$
    – LDC3
    Commented Nov 1, 2014 at 19:06

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Resonance only occurs when the result is beneficial, that is when a more-stabilized molecule will result. If resonance (delocalization) occurred in a compound such as cyclobutadiene to produce a square molecule with equivalent bond lengths, an antiaromatic molecule would be the result. Therefore, resonance does not occur and a square molecule with equivalent bond lengths is not produced.

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  • $\begingroup$ As you say that a square molecule with equivalent bond length will not result therefore the rectangular molecule will be called anti aromatic or non aromatic? $\endgroup$
    – Lalit
    Commented Feb 1, 2022 at 10:12
  • $\begingroup$ The double bonds will be localized, the molecule will be non-aromatic. $\endgroup$
    – ron
    Commented Feb 1, 2022 at 18:44
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Yes, resonance can definitely destabilize.

One example you can think of is destabilization of carbanions, which is something we often consider when trying to determine the relative acidities of some given species.

Consider the relative acidities of o-Methoxytoluene, m-Methoxytoluene and p-Methoxytoluene. Destabilization of their respective conjugate bases due to resonance plays an important role here, as mentioned in the renowned book Solomons|Frhyle|Snyder.

solomon

As clear in the attached figure, +R effect, where a substituent increases the electron density in a ring, destabilizes the conjugate base. Hence, the order of relative acidities here is: B > A > C

I encourage the reader to try and draw the resonance canonical forms for the three species mentioned, and realize why this happens for themselves.

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