Arrange the following compounds in increasing order of their acidity.
(A) Phenol
(B) Ethanol
(C) Benzoic acid
(D) p-Nitrobenzoic acid
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B), (A), (C), (D)
An acid is stronger when its conjugate base is more stable, that is, when the negative charge left after losing \( \ce{H^+} \) is spread out more. Comparing the four:
(B) Ethanol: in the ethoxide ion \( \ce{C2H5O^-} \) the charge stays on one oxygen, and the ethyl group (+I) even increases it. Weakest acid (pKa about 15.9).
(A) Phenol: the charge of the phenoxide ion \( \ce{C6H5O^-} \) is delocalised into the benzene ring by resonance, so phenol is more acidic than ethanol (pKa 10.0).
(C) Benzoic acid: in the benzoate ion the charge is shared equally by two electronegative oxygen atoms, a more effective delocalisation than in phenoxide, where it goes onto less electronegative carbon atoms. So carboxylic acids are stronger than phenols (pKa 4.19).
(D) p-Nitrobenzoic acid: the electron-withdrawing nitro group (−I and −R) pulls further charge away from the carboxylate ion and stabilises it, so it is stronger than benzoic acid (pKa 3.41).
\( \displaystyle \ce{C6H5COOH <=> C6H5COO^- + H^+} \)
In increasing order of acidity:
\( \displaystyle \text{ethanol} \lt \text{phenol} \lt \text{benzoic acid} \lt p\text{-nitrobenzoic acid} \)
Shortcut: Ethanol is the weakest acid, and only Option (3) starts with (B).