In the given pair identify, most acidic compound in (A) and (B). Most basic in (C) and (D).

 

The ortho-effect : This is a special effect that is shorrrn by o-substituents of benzene and its derivatives, but is not necessarily just a steric effect, e. g., the basicities of some o-substituted anilines were explained in terms of steric effects and differences in crowding round the nitrogen atom. This ortho-effect also operates with the benzoic acids. Irrespectivg-of the polar tlpe, nearly all o-substituted benzoic acids are stronger than benzoic acid. As we have seen, benzoic acid is a resonance hybrid, and so the carboxyl groups is coplanar with the ring. An o-substituent tends to prevent this coplanarity. Thus resonance is diminished (or prevented), and so the O-atom of the OH groups has a greater positive c!1arge, resulting increased acid strength. It follows from this that the greater the steric inhibition to resonance, the stronger is the acid. Support for this is the following order of strengths of substituted benzoic acids.

$2,6\text{-di-Me }\left({\mathrm{pK}}_{\mathrm{a}}3.21\right)>2-\mathrm{t}-\mathrm{Bu}\left({\mathrm{pK}}_{\mathrm{a}}3.46\right)>2-\mathrm{Me}\left({\mathrm{pK}}_{\mathrm{a}}3.91\right)$

 

For options B and D, no ortho effect is valid and order of acidity and basicity is calculated by nearly examining the inductive effect.