Claisen condensation involves nucleophilic substitution in esters by an enolate (formed by removing an alpha-H from an ester or a ketone) giving a beta-keto ester or a diketone.

My textbook says that for the reaction between ketone and ester, the ketone always forms the enolate by losing it's alpha-H . But why can't the ester form the enolate here? What will happen if the ester has alpha-Hs and the ketone dosen't?


Does it the O on the alcohol side of the enolate ester help in delocalizing the negative charge via an inductive effect making it less effecive as a nucleophile?

(I can't find anything on other sites I've seen:




https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Reactions/Organic_Reactions/Claisen_Condensation )

  • 2
    $\begingroup$ Wikipedia, at the very least, states that the combination is ketone plus non-enolisable ester. But to be fair, ketones (pKa ~ 20) are more acidic than esters (pKa ~ 25), so I am more curious about why the ketone doesn't just do an aldol with itself instead of a Claisen, unless the ester is used in large excess. $\endgroup$ Commented May 1, 2019 at 14:58
  • $\begingroup$ This link may help clarify the issue: chemistry.stackexchange.com/questions/93813/… $\endgroup$
    – user55119
    Commented May 4, 2019 at 0:09

1 Answer 1


The pKa of a ketone alpha hydrogen is of the order of 20, the pKa of an ester alpha hydrogen is of the order of 25 source. Bear in mind that this is a log scale, meaning the proportion of ketone enolate to ester enolate is 100000:1.

In answer to the comment by @orthocresol: the ketone does self-condense, but this is reversible. The condensation with the ester is irreversible due to formation of the anion of the product B-diketone.


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