Identify the correct pK_a value order A. Phenol B. p-Cresol C. Benzoi…

The correct answer is Option (1) → E < C < D < A < B

The stronger the acid, the smaller its pK_a (pK_a = −log K_a). Two ideas fix the order. Carboxylic acids are much stronger acids than phenols: the negative charge of a carboxylate ion is shared equally by two electronegative O atoms, while in a phenoxide ion it spreads partly onto less electronegative ring carbons. Within each class, an electron-withdrawing group lowers pK_a and an electron-donating group raises it.

• E. 4-Nitrobenzoic acid, pK_a 3.41 (first, lowest) — the –\( \ce{NO2} \) group withdraws electrons and stabilises the carboxylate ion.

• C. Benzoic acid, pK_a 4.19 (second) — the parent aromatic acid, with no substituent.

• D. 4-Methylbenzoic acid, pK_a 4.37 (third) — the –\( \ce{CH3} \) group donates electrons and destabilises the carboxylate ion, but it is still a carboxylic acid.

• A. Phenol, pK_a 10.0 (fourth) — a phenol, far weaker than any carboxylic acid.

• B. p-Cresol, pK_a 10.2 (last, highest) — the electron-donating –\( \ce{CH3} \) group makes it slightly weaker than phenol.

Shortcut: 4-nitrobenzoic acid (E) has the lowest pK_a and p-cresol (B) the highest; Options (1) and (3) both run from E to B. The three acids must all come before the two phenols, which rules out Option (3), where phenol A sits before the acid D.

NCERT (2026-27 edition):
Effect of Substituents on the Acidity of Carboxylic Acids – “Substituents may affect the stability of the conjugate base and thus, also affect the acidity of the carboxylic acids. Electron withdrawing groups increase the acidity of carboxylic acids by stabilising the conjugate base through delocalisation of the negative charge by inductive and/or resonance effects. Conversely, electron donating groups decrease the acidity by destabilising the conjugate base.”