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Which would be more acidic, diethyl malonate $\left(\ce{CH2(CO2Et)2}\right)$ or ethyl acetoacetate $(\ce{CH3COCH2OOC2H5})$

In the above compounds, I have marked the hydrogen which I think will be the most acidic. Now removing that hydrogen would yield a carbocation. Both the carbocations would be equally stabilized due to resonance effect. In fact diethyl malonate acid would be more stabilized due to the extra inductive effect of the extra oxygen. Yet Wikipedia shows that ethyl acetoacetate has a p$K_\text{a}$ less than that of diethylmalonate acid.

What is making it more acidic?

  • $\begingroup$ That's diethyl malonate, not malonic acid... If you're looking at malonic acid on Wikipedia, that's going to be a completely different pKa value. $\endgroup$ – orthocresol May 15 '20 at 4:19
  • $\begingroup$ good catch.. edited it $\endgroup$ – gauri agrawal May 15 '20 at 8:25
  • $\begingroup$ chemistry.stackexchange.com/q/18987/16683 $\endgroup$ – orthocresol May 15 '20 at 9:22
  • $\begingroup$ Also carbanions not cations... $\endgroup$ – Mithoron May 15 '20 at 14:32

The hydrogens which you have chosen are correct but both the carbanions are NOT eqaully stablised via the resonance. In case of malonic ester, the elctrophilicity of the carbonyl(c=o) is reduced due to the resonance from the oxygen beside it. So the carbanion will not be pulled very effectively into resonance as the carbonyl becomes electron rich due to the oxygen of the ester.

However, in the ethyl acetoacetae only one of the carbonyl groups is affected by this reasonace. The other one is still highly elctrophillic. So it stablises the carbanion even more. So it has a lower pka(more acidic) than the malonic ester.


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