Why does amino groups get acylated with acetic anhydride but not the hydroxy group?

Question

Why does amino groups get acylated with acetic anhydride but not the hydroxy group? Thank you!

Answer 1

The aromatic amine group is much more nucleophilic than the aromatic alcohol. This follows from the basicity of the amine. If only one equivalent of acetic anhydride is used, the amine group will be isolated preferentially. However, if two equivalents of acetic anhydride are used, and the reaction pushed, the diacylated product can be obtained.