Does the existence of resonance structures always translate as stability?

I've been taught that resonance structures generally "spread out" charge in order to compensate for deficiency or excess of electrons. So one could assume that as the number of those structures increases, so does the stability of the molecule, since nature favours it.

However, considering an ester and a ketone and comparing them as to the acidity of their α-hydrogens, things seem to me a bit complicated.

We can draw the structures shown below if I'm not mistaken: enter image description here

So, couldn't someone suggest that it should be easier for the ester to lose its hydrogen since there is one more structre, compared to ketone? I know that ketones are more acidic and I do understand that the last ester enolate structure is not stable at all compared to the others. But doesn't the existence of it, however brief, affect favourably the stability of the ester enolate?

  • $\begingroup$ Resonance is a concept that is necessary to describe some observed phenomena due to the incompleteness of the model (the Lewis structure). From these diagrams alone it is impossible to judge even on a qualitative level how pronounced resonance is; it does not tell you how important a single structure of the set is to the description. Related: chemistry.stackexchange.com/q/51632/4945 $\endgroup$ – Martin - マーチン Oct 21 '18 at 16:19
  • $\begingroup$ I see. So regarding the question of acidity? $\endgroup$ – Αντώνιος Κελεσίδης Oct 21 '18 at 16:30
  • $\begingroup$ Your last structure illustrates exactly why it's weaker not stronger - it's impeding delocalisation of negative charge from carbon. BTW "existence of it, however brief" is wrong - it on't exist, but is contributor to resonance hybrid. $\endgroup$ – Mithoron Oct 22 '18 at 17:29

The number of Resonance structures can be a way of describing stability of some molecule relative to the other ones, as it is found to be helpful in understanding many basic ideas about natural phenomena. But higher number of resonance structures can't be the ultimate inference for describing every phenomenon we observe.

For example, when we are concerned about acidity of the esters and the ketones, resonating structures do contribute to the stability of the conjugate base. But we ultimately are bothered about how much the negative charge generated in the conjugate base after losing the proton is delocalised. More resonating structures in case of esters don't simply mean that the negative charge is more delocalised. It is the cross-conjugation which plays the role of prohibiting the electronic delocalisation in esters and also that is absent in ketones resulting in their increasing acidity.

So, In many cases, it may appear that higher number of resonance structure will help in favour of the phenomenon, but it's always not the case. We may actually have to take in account of the real effect of Resonance to judge the ultimate result.


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