Question

which is the stronger acid m nitrobenzoic acid or p nitrobenzoic acid? provide structures to support your choice?

Answer 1

Nitro group (-NO2), having -I and -R effect, is an electron withdrawing and deactivating group. Due to both these effects, it decreases electron density around the -COOH group of substituted(ortho, meta & para) benzoic acids and releases H+ ions, making these acidic. The nitrobenzoic acid which releases H+ group more easily is the most acidic.

Due to ortho effect, ortho acids are more acidic than all other substituted acids(even if an electron donating group is present at the ortho position. The only exception is -NH2 group, in which ortho- aminobenzoic acid is NOT the strongest acid).

Regarding acidity of meta and para acids, consider I and R effects. Inductive effects of meta and para acids reduce electron density around -COOH group, whereas resonance does not occur at meta position. It only occurs at para position, making the nitro group at para position a more strong withdrawer of electrons. Thus para-nitro benzoic acid is more acidic than meta-nitro benzoic acid.

In short, the higher acidity of p-nitrobenzoic acid compared to m-nitrobenzoic acid is attributed to its -I and -R effect.