Why do aromatic carboxylic acids not undergo Friedel-Crafts reactions, and what is the role of the carboxyl group in their electrophilic substitution reactions?

Aromatic carboxylic acids do not undergo Friedel-Crafts reactions due to the deactivating effect of the carboxyl group. The carboxyl group withdraws electron density from the aromatic ring through resonance, creating a less reactive and less nucleophilic benzene ring. This deactivation hinders the electrophilic aromatic substitution characteristic of Friedel-Crafts reactions. In their electrophilic substitution reactions, the carboxyl group serves as an activating and directing group. It activates the ring by resonance, making it susceptible to electrophilic attack, and directs substitution to the ortho and para positions relative to the carboxyl group due to steric and electronic effects.