Question

Account for the following:

(a) Aromatic carboxylic acids do not undergo Friedel-Crafts reaction.

(b) $p{K}_a$ value of 4-nitrobenzoic acid is lower than that of benzoic acid.

Answer 1

(a) This is because the -COOH group is deactivating and the catalyst $AlC{l}_3$ gets bonded to -COOH group.

(b) $-N{O}_2$ group is an electron withdrawing group owing to -I effect. So it stabilises the $CO{O}^{-}$ ion and strengthens the acid in 4-nitrobenzoic acid.