Synthesis of Acetaminophen?

This is an acetylation of a primary aromatic amine. The mechanism is usually considered to involve a nucleophilic attack by the nitrogen lone pair at one of the anhydride carbonyls and the subsiquent displacement of acetic acid. The reaction is not reversible but the product can be hydrolysed back to the starting material. Note that at the conditions of the reaction there is no water present and any reversal will only occur in the work-up when the anhydrous mixture is usually drowned out into a volume of water to precipitate the product and remove unreacted anhydride.