When any group is present at ortho position to an  amide group (NH) in aniline then the basic character of that compound becomes weaker. For example, see the order of basicity of following substituted aniline:-
* p-Toluidine > m-Toluidine > Aniline > o-Toluidine
* Aniline > m-Nitroaniline > p-Nitroaniline > o-Nitroaniline
* Aniline > p-Haloaniline > m-Haloaniline > o-Haloaniline
* p-Aminophenol pKb=8.50 > o-Aminophenol pKb=9.28 > Aniline pKb=9.38 > m-Aminophenol pKb=9.80
The protonation of substituted aniline is inhibited by steric hindrance. When protonated, the nitrogen in the amino group changes its orbital hybridization from sp to sp, becoming non-planar. This leads to steric hindrance between the ortho-substituted group and the hydrogen atom of the amino group, reducing the stability of the conjugate acid and consequently decreasing the pH of substituted aniline.