Account for the following:
(a). Aromatic carboxylic acids do not undergo Friedel-Crafts reaction.
b). pKa value of 4-nitrobenzoic acid is lower than that of benzoic acid.

a) This is because the -COOH group is deactivating and the catalyst AlCl₃ gets bonded to -COOH group.

b) -NO₂ group is an electron withdrawing group owing to -I effect. So, it stabilizes the COO^- ion and strengthens the acid in 4-nitrobenzoic acid.