Correct Option: Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
Step 1 (Conceptual Basis): [0.5 Mark]
The Friedel-Crafts reaction (alkylation or acylation) requires an electrophile to attack the benzene ring. Aromatic carboxylic acids (e.g., benzoic acid, $C_6H_5COOH$) contain a strongly electron-withdrawing carboxyl group ($-COOH$). This group is strongly deactivating, which significantly reduces the electron density of the benzene ring, making it unsuitable for electrophilic aromatic substitution.
Step 2 (Mechanism of Inhibition): [0.25 Mark]
The catalyst used in Friedel-Crafts reactions is a Lewis acid, typically anhydrous aluminium chloride ($AlCl_3$). The oxygen atoms of the $-COOH$ group possess lone pairs of electrons. Consequently, the $AlCl_3$ acts as a Lewis acid and coordinates with the carboxyl group:
$$C_6H_5COOH + AlCl_3 \rightarrow C_6H_5COOH \cdot AlCl_3$$This coordination creates a strong positive charge on the substituent, which further intensifies the deactivation of the ring and prevents the formation of the necessary electrophilic complex.
Step 3 (Final Conclusion): [0.25 Mark]
Because the ring is deactivated and the catalyst is consumed/blocked by the formation of the $C_6H_5COOH \cdot AlCl_3$ complex, the Friedel-Crafts reaction fails to proceed. Thus, both the Assertion and the Reason are scientifically accurate, and the Reason correctly explains the failure of the reaction.
💡 EL10 Concept Insight: Think of the benzene ring as a party that needs to attract a guest (the electrophile). The $-COOH$ group is like a "party pooper" that sucks all the energy out of the room, and the $AlCl_3$ catalyst is like a security guard who gets stuck to the party pooper instead of helping the guest get in!