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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?
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.
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.
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.
Resonance only occurs when the result is beneficial. Anti-aromatic compounds exist since the molecular bonding releases energy from the state of individual atoms. $\endgroup$