In the reaction between aniline and acetic anhydride to form acetanilide, only one acyl group is placed on the aniline because the reaction proceeds via a nucleophilic acyl substitution mechanism. In this mechanism, the lone pair of electrons on the nitrogen atom of the aniline attacks the carbonyl carbon of the acetic anhydride, resulting in the formation of a new bond between the nitrogen and the carbon. This new bond formation is accompanied by the breaking of the bond between the carbonyl carbon and the oxygen atom of the acetic anhydride, resulting in the release of acetic acid as a byproduct.

Once the first acyl group has been attached to the aniline, the second equivalent of acetic anhydride is less likely to react with the aniline because the nitrogen atom is now less nucleophilic due to the positive charge on the nitrogen from the first acyl group. The positive charge on the nitrogen decreases the electron density on the nitrogen atom, making it less likely to donate its lone pair of electrons to attack the carbonyl carbon of the second equivalent of acetic anhydride.

Furthermore, the steric hindrance caused by the first acyl group also contributes to the decreased reactivity of the aniline towards the second equivalent of acetic anhydride. The presence of the first acyl group on the nitrogen atom creates a bulky environment around the nitrogen, making it more difficult for the second equivalent of acetic anhydride to approach and react with the nitrogen atom.

As a result of these factors, the reaction between aniline and acetic anhydride typically results in the formation of acetanilide with only one acyl group attached to the aniline.