[limonade]

Hi. 

 I was just wondering if the mechanism can be like I have drawn (but with calcium counter ions). or if  the hydroxide ion resulting from presence of CaCO3 in water is what is going to do the hydrolysis of the ester.

Thanks in advance. And sorry if this seems like a silly question.

[Babcock_Hall]

Would you care to propose an experiment to distinguish between the two possible mechanisms you suggested?

[orgopete]

I agree with the choice in the possible nucleophiles. If the key to predicting the course of which group becomes eliminated from the tetrahedral addition is the leaving group, then which will be a better leaving group, the CO3(-2) or the RO(-)? What might that suggest would be the group that adds to the C=O? I think the pKa of carbonate will result in hydroxide being in relatively low concentration.

[Babcock_Hall]

The carbonate may also play the role of a general base, removing a proton from water as it attacks the ester.