Ester 4:
Solution: Experiments conducted with isotopic oxygen 18 (18O; in red below) labeled ethanol demonstrated that the oxygen of water contains no label (16O) while the labeled oxygen is incorporated into the ester. The scheme below accounts for the reversiblity of the reaction, the label incorporation and the catalytic role of the acid. This experiment was conducted with a primary alcohol. Fischer esterification with tertiary alcohols proceeds by a different mechanism. Click here.