[ufpotato1]

Here is the reaction;



I will describe the mechanism i wrote out below;
The acetal is protonated to allow for the removal of a H-O-Me group. the ketone(O=C-R(2)) then attacks the O-Me group forming acetone+MeO=C-R(2) the MeO-R(2) is then attacked by the H-O-Me group. this forms the end product but with a hydrogen attached to the oxygen which is then lost to form the end product in the image.

Not sure if my description makes sense so here is the mechanism i followed but for my reaction;



Is this correct? any pointers on where i've gone wrong or how to tackle problems like this in the future would be greatly appreaciated. thanks!


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[mjc123]

I speak under correction as I learned my org chem a long time ago and have forgotten a lot of it, but as nobody else has replied here is what I think.
I would say that reaction proceeds by the standard acid-catalysed hydrolysis of one acetal and the standard acid-catalysed formation of the other (driven perhaps by distilling off the acetone). Characteristics of this mechanism, compared with yours, include:
1. The carbonyl oxygen arrives or departs as H₂O.
2. The carbonyl oxygen does not at any point (while it is in the form of a carbonyl oxygen) act as a nucleophile. (That's something I don't remember coming across at all.)
3. No Me-O bonds are formed or broken at any stage, only bonds between O and the carbonyl carbon.
March (again, 80s edition...) cites several pieces of evidence for this mechanism.
Possibly orthoesters may behave differently, and as the mechanism you show doesn't mention water, perhaps it is applicable to anhydrous conditions. But the question mentions water.
I would be interested to be contradicted.

[Mellitic]

That mechanism for the orthoester is also bothering me, but I haven't been able to find any journal articles that actually probe the mechanism.  I did find a mechanism I like better from Organic Mechanisms: Reactions, Stereochemistry and Synthesis (see attached image).  It makes more sense for the softer carbonyl nucleophile to attack the carbon center in my mind.

[orgopete]

I don't know whether the suggested mechanism is correct or not (though better than first). I had always assumed the reactions took place as though they were parallel ketalization and ketal hydrolysis reactions. The ketalization reaction generates water and the water formed converts the ketal back to to a ketone or ester. I always thought it would have been difficult to maintain scrupulously anhydrous conditions, so I thought some water was plausible. I thought this reaction was a casual reaction not needing carefully controlled conditions. The p-TsOH in my lab was the hydrate. So I assumed there would be a catalytic amount of free methanol present from hydrolysis. Whether these amounts are to enough to generate a different mechanism or not, I don't know. It also seems difficult to discern whether another mechanism might be competing with the suggested one.