Claisen Reaction

[Kstar007]

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Why are two alpha protons needed for a successful claisen reaction? After the initial enolate nucliopophile attacks an ester and does a addition elimination reaction what is the purpose of the second alpha proton?

 

[DAT Destroyer]

Why are two alpha protons needed for a successful claisen reaction? After the initial enolate nucliopophile attacks an ester and does a addition elimination reaction what is the purpose of the second alpha proton?

You have asked a very sophisticated question. The steps in the Claisen reaction are equilibrium reactions that lie far to the left, and do not go forward to any appreciable extent. In the next to last step, the removal of the second alpha proton is a HIGHLY EXOTHERMIC step and is needed for the reaction to drive to completion. Without this exothermic step, the reaction would simply reverse. If you need more clarity, the David Klein text does a fine job in explaining this as well.

Hope this helps.

Dr. Romano

 

[Kstar007]

Ok I think I understand. The Beta-keto ester has a much lower pKa then the alpha proton of the original ester so the base would much rather rip off that second proton, and form the resonance stabilized Beta-keto ester anion. This would lower the concentration of the Beta-keto ester, effectively driving the formation of it. This ties in nicely with your discussion on Le Chatelier.

Thanks for your help.