[Shikimic]

Thanks to all the people who replied to my first question in the other thread, you were all very helpful and I appreciated it very much. I was meaning to also ask this question in that thread, but I forgot to.

In a Claisen condensation of two esters, a beta-keto ester is formed that is deprotonated by an expelled alkoxide ion from the ester substitution, so the beta-keto ester has to be reprotonated with acid. In the McMurry textbook, aqueous acid (H₃O⁺) is used to do this. Why doesn't this hydrolyse the ester?

I'm thinking that a stoichiometric amount of acid is added and that H⁺ would protonate the enolate first before protonating the ester carbonyl oxygen and setting it up for hydrolysis (the enolate negative charge would be delocalised over the ester carbonyl anyway, so it would be necessary to get rid of the enolate first). Does this make sense?

Thanks in advance.

[stewie griffin]

When a reaction is simply being quenched with acid, like this example where you simply need to protonate the enolate, we often just write H₃O⁺. However this doesn't mean we're actually adding in some concentrated HCl or sulfuric, but rather we are adding some slightly acidic water (like a saturated solution of NH₄Cl). This way it's acidic enough to quench the enolate, but not so acidic as to carry out further unwanted reactions.
Furthermore, after we quench a reaction we work it up and purify it relatively quickly. It's not like we would let this stir for days with some dilute acid. Instead we quench, extract within a few minutes, and probably run a column on it.

[orgopete]

Yes, the enolate is protonated first. There is little danger of hydrolyzing the ester (within reason). If you consider the thermodynamics of the steps involved, it is difficult to hydrolyze a beta-ketoester under acidic conditions. I have included the pKa values for the intermediates (http://research.chem.psu.edu/brpgroup/pKa_compilation.pdf). Because of the keto group inductively pulling electron density away from the ester, it should be more difficult to protonate than an isolated ester. Therefore, an acid catalyzed hydrolysis is relatively difficult.

On the practical side of actually doing this reaction, there is a greater danger of hydrolyzing the ester under basic conditions than acidic. Because the equilibria for ester enolization is small and one equivalent of base is consumed in forming a more acidic product than the starting material, an excess of base is commonly used. If water is carelessly added to this reaction, this excess base will form hydroxide and begin hydrolysis. It is actually safer to add the reaction mixture to (cold) acid so that an excess of acid is present.