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

Explanation

(i) The carboxyl (-COOH) group present on the benzene ring deactivates the ring due to its electron withdrawing nature to very large extent. As a result, the Friedel Craft reaction involving the electrophile attack of alkyl carbocation from R-X in the presence of anhydrous `AlCl_(3)`, is not feasible. Moreover, anhydrous `AlCl_(3)` is a Lewis acid. It co-ordinates with the carboxyl group and fails to release the alkyl / acyl carbocation nedded for electrophile attack.
(i) For answer, consult section 17.