Final answer:
Freshly distilled acetic anhydride is critical in acetylation reactions, such as the esterification process in aspirin synthesis, due to its reactivity and efficiency as an acetylating agent. It must be fresh to avoid hydrolysis, which would reduce its effectiveness.
Step-by-step explanation:
Freshly distilled acetic anhydride is essential in an acetylation reaction as it is a highly reactive acyl substrate. In the context of synthesizing aspirin, acetic anhydride is a preferred acetylating agent over acetic acid due to its higher efficiency in facilitating the esterification process. This acyl substitution reaction involves transferring an acyl group to the oxygen atom of an alcohol, such as in the reaction between salicylic acid and acetic anhydride.
Moreover, acetic anhydride can react with water to form two equivalents of carboxylic acid, which is why it must be used fresh to prevent such hydrolysis from diminishing its reactivity. The use of fresh acetic anhydride ensures maximal efficiency and yield of the desired ester, as is necessary in the synthesis of aspirin under acid catalysis using sulfuric or phosphoric acids.