Is resonance always stabilizing

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. 


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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](https://i.stack.imgur.com/Pm7lx.png)](https://i.stack.imgur.com/Pm7lx.png)


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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