For the given compounds, the correct order of increasing pKa value : [figure] Choose the correct answer from the options given below :
- (E) < (D) < (C) < (B) < (A)
- (B) < (D) < (A) < (C) < (E)
- (D) < (E) < (C) < (B) < (A)
- (E) < (D) < (B) < (A) < (C)
Answer
BONUS
Solution
An electron-withdrawing group stabilises the phenoxide ion, so the phenol is a stronger acid and has a lower pKa.
(B) p-nitrophenol: –R and –I of -NO2 at para, pKa 7.1
(D) m-nitrophenol: only –I of -NO2 at meta, pKa 8.3
(C) m-methoxyphenol: at meta only –I of -OCH3 acts, pKa 9.6
(A) phenol: pKa = 10.0
(E) p-methoxyphenol: +R of -OCH3 at para destabilises the phenoxide, pKa 10.2
∴ increasing pKa: (B) < (D) < (C) < (A) < (E)
This order is not among the options (option (2) interchanges (A) and (C)), so no option is correct and the question is dropped.
Answer: Bonus (full marks to all who attempted)
Note: The official NTA answer key gives option (2). Our faculty team finds that no option is fully correct, so this question should be dropped (bonus: full marks to everyone who attempted it).