First aniline is dissolved in water using one equivalent of hydrochloric acid:
Then 1.2 equivalents of acetic anhydride is added followed by 1.2 equivalents of aqueous sodium acetate solution. Aniline attacks acetic anhydride followed by deprotonation of the ammonium ion:
Acetate then acts as a leaving group:
The acetanilide product is insoluble in water and can therefore be filtered off as crystals.