JEE Main · 2025 · 03 Apr 2025 · Shift 1 · General Organic Chemistry

The least acidic compound, among the following is:

(A)3-ethoxyphenol (m-EtO-C6H4-OH)
(B)benzenesulfonic acid (C6H5-SO3H)
(C)butanoic acid (CH3CH2CH2-COOH)
(D)ethyl propiolate (EtO2C-C#C-H)
✓ Correct

Answer: (D)

Solution

The least acidic species is the one whose conjugate base is least stabilised. Benzenesulfonic acid (

p K_{a} \approx - 2.5

) and butanoic acid (

p K_{a} \approx 4.8

) give resonance-stabilised oxy-anions, and 3-ethoxyphenol (

p K_{a} \approx 10

) a phenoxide delocalised into the ring. Ethyl propiolate loses its terminal alkynyl proton to give an

s p

-hybridised carbanion that is localised and gains no resonance stabilisation, so it is the weakest acid of the four. The least acidic compound is ethyl propiolate.

JEE Main · 2025 · 03 Apr 2025 · Shift 1 · Single correct option

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