Structure of the Water Adduct of the Protonated Ester

The protonation of the tetrahedral intermediate can occur either at the methoxy-oxygen or at a hydroxy-oxygen. Protonation at the methoxy-O leads to the back-reaction to carboxylic acid and alcohol. Protonation at the hydroxyl-O leads to the water adduct of the protonated ester. Two items are noteworthy. First, the protonation at the hydroxyl-O leads to the more stable protonated species and the proton transfer from a hydronium ion to the tetrahedral intermediate is exothermic by about 32.8 kcal/mol. The other interesting item is a structural feature: The C-OH2 bond is rather long (2.47 Ang) and the product of protonation is best seen as an electrostatic complex between water and a protonated methyl formate.


Figure. The MP2(full)/6-31G* optimized structure of the water adduct of the protonated ester.