Acetanilide is formed by the acetylation of aniline under the presence of a strong base like pyridine. The lone pair of electrons on nitrogen of acetanilide interacts with oxygen atom due to resonance, resulting in less availability of the electron pair on nitrogen for reaction.

This way, activating effect of –NHCOCH₃ group is less than that of amino group and acetanilide is less basic than aniline.

In case of substituted aniline, electron releasing groups increase basic strength whereas electron withdrawing groups like decrease it.

The delocalization of electrons in the Acetanilide is show in below figure;