Why can't esters or acids or their derivatives exhibit tautomerism? I mean if they did, then there would be a conjugation between lone pair electrons of oxygen and pi electrons adding to their stability. Then why don't they exhibit similar tautomerism?
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$\begingroup$ Though I do not have a reference, I believe that some esters would exhibit similar or the same behaviour as ketones. At least $\alpha{}$-protons of esters are far more acidic than their esterless counterparts, which proves a similar effect is applicable on esters. $\endgroup$– EljeeDec 13, 2014 at 14:12
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1 Answer
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Esters, amides and acids do exhibit tautomerization to the enol form, but the enol content in these compounds is just much less than that found with aldehydes and ketones. Why?
The keto-enol process is an equilibrium; to understand where an equilibrium lies, we must compare the relative stabilities of the products (enol) and reactants (carbonyl compound).
Let's compare some resonance structures for an ester and a ketone and their corresponding enols.
The ester is stabilized by resonance structures such as A, whereas in the case of the ketone, the analogous resonsnce structure A' contributes, but not nearly as much; so ester carbonyls are stabilized compared to ketone carbonyls.
In the enols, the ester resonance structure B is not particularly favorable due to the negative charge on carbon and positive charge on oxygen, just the reverse of what we would want based on electronegativities. At least in the ketone case resonance structure B' has a positive charge on hydrogen, so resonance structure B' contributes more than B (this is why carbon-carbon double bond stability increases with increasing alkylation). As a result the enol resulting from the ketone is stabilized compared to the enol resulting from the ester.
Just where the carbonyl-enol equilibrium lies will depend on the relative stabilities of the carbonyl compound and the enol. As we've just discussed, the ester carbonyl is stabilized relative to the ketone carbonyl, this will push the equilibrium further to the carbonyl side for the ester. Also, the enol from the ketone is somewhat more stabilized than the enol from the ester, this will push the equilibrium to the enol side for the ketone.
So both factors, starting material (the carbonyl compound) stability and product (enol) stability will work to increase the enol content for ketones and reduce the enol content for esters.
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$\begingroup$ In esters why can't the positive charge be on adjacent carbon instead of oxygen? That way the newly formed pi bond and the lone pair electrons on oxygen would be in conjugation adding to the stability of ester. Why isn't that possible? $\endgroup$ Dec 14, 2014 at 16:17
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$\begingroup$ To draw a resonance structure for the ester with the positive charge on the carbon adjacent to the carbonyl carbon you would need to have hydride ion ($\ce{H^{-}}$) as part of the resonance structure. Hydride ions are very high energy, consequently resonance structures involving them are very high energy and do not contribute significantly to the overall description of the molecule. BTW, if the above answer was helpful, please mark it as accepted. Thanks. $\endgroup$– ronDec 14, 2014 at 16:32
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$\begingroup$ Pardon me but I didn't understand why an hydride ion comes into the picture. Just like in keto-enol tautomerism a H+ ion will come out and then later form a bond with oxygen, right? $\endgroup$ Dec 17, 2014 at 9:28
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$\begingroup$ You commented, "Just like in keto-enol tautomerism a H+ ion will come out and then later form a bond with oxygen, right?". Yes, that is right. But in your first comment you asked, "In esters why can't the positive charge be on adjacent carbon instead of oxygen?" Try writing a mechanism to create such a structure, I think you will find that a hydride ion is required somewhere in the process. $\endgroup$– ronDec 17, 2014 at 14:27
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$\begingroup$ Yeah you are right about the +ve charge on the adjacent carbon issue. But can there be a double bond between ester carbon and the adjacent carbon due to tautomerism between those 2 carbons and oxygen? $\endgroup$ Dec 20, 2014 at 13:40