What is the correct order of acidity of the following compounds ?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (C) > (B) > (A) > (D)
A carboxylic acid is acidic because its carboxylate ion is stabilised by resonance, the negative charge being shared by two oxygen atoms. Anything that further stabilises this anion increases acidity. Three factors decide the order here:
Phenyl group attached directly to –COOH: the sp2 carbon of the ring is electron withdrawing, so benzoic acid is stronger than acetic acid, which carries an electron releasing –\( \ce{CH3} \).
Distance of the ring: in phenylacetic acid a \( \ce{CH2} \) group separates the ring from –COOH, so the withdrawing effect is weaker; it lies between benzoic and acetic acid.
–NO2 at the para position: a strong electron withdrawing group (–I, –R) stabilises the anion further, so 4-nitrobenzoic acid is the strongest.
Combined order (pKa values in brackets; lower pKa = stronger acid):
4-nitrobenzoic acid (3.41) > benzoic acid (4.19) > phenylacetic acid (4.31) > acetic acid (4.76)
(C) \( \ce{NO2C6H4COOH} \): 4-nitrobenzoic acid
(B) \( \ce{C6H5COOH} \): benzoic acid
(A) \( \ce{C6H5CH2COOH} \): phenylacetic acid
(D) \( \ce{CH3COOH} \): acetic acid
Shortcut: The nitro acid (C) is the strongest. Only Option (3) starts with (C).