The nucleophilic acyl substitution reaction between acetic anhydride CH3CO 2O and aniline C6H5NH2 involves the following steps:1. Nucleophilic attack: The nitrogen atom in the aniline molecule, which has a lone pair of electrons, acts as a nucleophile and attacks the electrophilic carbonyl carbon atom in the acetic anhydride molecule. This results in the formation of a tetrahedral intermediate and the breaking of the bond between the carbonyl carbon and oxygen atoms.2. Proton transfer: The negatively charged oxygen atom in the tetrahedral intermediate abstracts a proton from the nitrogen atom in the aniline molecule, forming a new N-H bond and regenerating the carbonyl double bond C=O .3. Leaving group departure: The negatively charged oxygen atom in the acetate ion CH3COO- acts as a leaving group, breaking the bond between the carbonyl carbon and the oxygen atom. This results in the formation of an acylated aniline molecule C6H5NHC O CH3 and an acetate ion CH3COO- .4. Acid-base reaction optional : If the reaction is carried out in the presence of an acid, the acylated aniline molecule can undergo an acid-base reaction with the acid, resulting in the protonation of the nitrogen atom and the formation of a more stable ammonium salt.Overall, the reaction between acetic anhydride and aniline results in the formation of N-acetylaniline C6H5NHC O CH3 and an acetate ion CH3COO- . The mechanism is a classic example of a nucleophilic acyl substitution reaction, where the nucleophile aniline substitutes the leaving group acetate ion in the acyl compound acetic anhydride .