CBSE Class 12 · 2018 · Aldehydes Ketones and Carboxylic AcidsShort Answer

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

Solution

(a) The –COOH group strongly deactivates the ring toward electrophiles, and its oxygen binds the Lewis acid AlCl

_{3}

, destroying the catalyst — so no Friedel–Crafts alkylation/acylation.
(b) The para-nitro group withdraws electron density (−I, −R) and stabilises the 4-nitrobenzoate anion; the acid ionises more → stronger acid → \textbf{lower pK

_{a}

CBSE Class 12 Board · 2018 · Subjective

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