Consider the ortho-effect in the basicity of anilines (also known as Steric Inhibition of Protonatio — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

The ortho-effect in anilines dictates that any substituent located at the ortho position, regardless of its electronic nature (electron-donating or withdrawing), significantly decreases the basic strength relative to unsubstituted aniline. This occurs because the ortho substituent sterically hinders the approach of a proton (Steric Inhibition of Protonation) and destabilizes the bulky tetrahedral anilinium ion formed upon protonation due to intense steric crowding. Thus, o-toluidine is the weakest base.