(a) Because -COOH group present in aromatic carboxylic acids is an electron withdrawing group causing deactivation of benzene ring. This results in the bonding of anhydrous AlCl3 with carboxyl group. Hence, electrophillic substitution i.e., Friedel-Crafts reaction does not occur in aromatic carboxylic acids.
(b) As 4-nitrobenzoic acid contains -NO2 group which is an electron withdrawing group resulting in higher acidity than benzoic acid. Greater is the acidic character lower is the pKa value. Thus, pK a value of 4-nikobenzoic acid is lower than that of benzoic acid.
