= Solution
Propionic acid is a weak, relatively high-boiling acid: its normal boiling point is about $141^\circ\mathrm C$, compared with about $118^\circ\mathrm C$ for acetic acid. It provides mild proton catalysis while remaining in the hot orthoester mixture more effectively. This is useful because ketene-acetal formation and the thermal <Johnson-Claisen rearrangement> require heating, whereas strong acid would threaten the pyran <acetal>.
This is a practical catalyst choice, not an absolute mechanistic prohibition on acetic acid; acetic-acid-catalysed orthoester rearrangements are also possible.
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