Figure 1A summarizes the first three steps of the Claisen condensation.
Notice that the product of the reaction contains a carbon atom that is a to two carbonyl groups. If that carbon is deprotonated, the resulting negative charge may be delocalized over both carbonyl groups by resonance. This double stabilization means that the a hydrogens are more acidic than hydrogens a to a single carbonyl group. In fact, the pKa of the hydrogens on the carbon atom flanked by two carbonyl groups in ethyl acetoacetate is approximately 11. Consequently, when the ethyl acetoacetate is formed it is immediately deprotonated by the ethoxide ion present in the reaction mixture. Figure 2A demonstrates these ideas.
The actual structure of the conjugate base of ethyl acetoacetate is shown below.It is called or sodioacetoacetic ester.
The negative charge is delocalized over the carbon and the two oxygen atoms. The sodium ion is coordinated to both oxygen atoms simultaneously in what is known as a chelated structure. The word chelate comes from the Greek chelos which means claw.
The equilibrium constant for the deprotonation of ethyl acetoacetate by ethoxide ion is approximately 105. The equilibrium constants for the steps leading up to the formation of ethyl acetoacetate range from 10-4 to 1. Coupling of the deprotonation step to the prior equilibria is what drives the Claisen condensation to completion. Experimentally the final step in the process is protonation of the chelated ion with aqueous acid during the work-up.