vichu is correct...it is the steric hinderance....ortho effect like in orthonitro benzoic acid is not dependent on the kind of substituents whether e-withdrawing or donating.......in case of ortho-nitro benzoic acid......due to steric strain between nitro grp and carboxylate ion....the carboxylate ion is forced out of plane and that shift alters the charge into the ring and so it becomes easier to delocalize the charge in the ring easily.and that increases the acidity.

COOH is a bulky group. and any group at ortho position causes steric hinderence. due to this the planarity of COOH is lost.

We know that for resonanace the molecules under consideration shod be planar. ther4 resonance is inhibited.

U can easily see that Ph stabilises COOH by resonance.(in the canonical structures u can see that -ve charge comes on 1 O.

think if H is released(rather H+) it brings -ve on he other O also.

and it leads to a large repulsiv force(between 2 O-.) ther4 release of H+ is now prohibited

 

BUT when, planarity is lost i.e resonance is stopped. ad now -ve not comes on any O. and elease of H is facilitated.

 

 

I think now u ll hv no confusion..................

Kudos Ashish

 

Perfect ans!!!!!!